Product Guides
Sandpaper Grit Guide: Types, Sizes & Selection
Walk into any hardware store and you'll find sandpaper labelled 40, 80, 120, 240, 400, 2000 — sometimes with a P in front of the number, sometimes without. This guide explains the FEPA P-grade and ANSI/CAMI systems, grit-to-micron conversion, abrasive minerals, backing materials, and the grit sequences that produce the right result for timber, metal, and automotive work.
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Flap Disc & Abrasive Sanding Guide
Every angle grinder operator has stood in front of an abrasive display wondering which disc to grab. Flap disc or grinding disc? Aluminium oxide or zirconia? Type 27 or Type 29? 1.0 mm or 1.6 mm cutting disc? The choices look arbitrary until you understand what each product is designed to do — then they become obvious. This guide covers every abrasive disc type used with angle grinders and bench grinders in Australian workshops: how each works, when to use it, which abrasive mineral to choose, how to match grit to job, what causes discs to fail early, and how to use them without injuring yourself or destroying the workpiece. It covers mild steel, stainless, aluminium, and masonry applications. Types of Abrasive Discs: What Each One Does Abrasive discs are not interchangeable. Each product type has a specific construction, a specific backing, a specific abrasive geometry, and a specific application. Using the wrong type — particularly a cutting disc for grinding, or a standard disc on aluminium — is both ineffective and dangerous. Flap discs are constructed from overlapping abrasive-coated cloth flaps bonded radially to a fibreglass or phenolic resin backing plate. As the flaps wear, fresh abrasive is continuously exposed. The result is a disc that grinds and finishes in a single operation, with less heat generation, less gouging, and a smoother surface than a bonded grinding disc. Flap discs are the most versatile angle grinder accessory in a general workshop — they remove welds, blend seams, prep for paint, and remove rust without switching tools. Grinding discs (also called depressed-centre grinding wheels) are solid bonded abrasive wheels — abrasive grains bonded into a rigid matrix with resin or vitrified bond. They remove metal faster than a flap disc and handle heavier, sustained stock removal. The tradeoff is a rougher surface, more heat, and a higher risk of gouging the workpiece. Use grinding discs when you need maximum material removal rate and surface finish is not the priority. Cutting discs are thin (1.0–3.0 mm) bonded abrasive wheels designed exclusively for parting cuts — cutting bar stock, angle iron, pipe, sheet, and structural sections. They are NOT grinding discs. A cutting disc is not rated for side load (lateral grinding). Applying side force to a cutting disc causes it to flex and can cause catastrophic disc failure. This distinction is non-negotiable: cut only with cutting discs, grind only with grinding or flap discs. Fibre discs (resin fibre discs — see our dedicated Sanding Disc & Abrasive Disc Guide for the grain golden rule, hook-and-loop vs PSA, and backing pad selection) have a heavy fibreglass-reinforced paper backing and require a backing pad to use — they cannot be mounted directly to the grinder. With a backing pad, they conform slightly to the surface and provide very aggressive flat-area stock removal. Fibre discs give a consistent removal rate over their full life, whereas flap discs change character as the flaps wear. Common in 24–120 grit for weld grinding, rust removal, and surface prep on flat stock. Flap wheels are the bench grinder and die grinder equivalent of a flap disc. Abrasive-coated cloth segments are arranged radially around a hub — available in arbor-mount versions for bench grinders and straight-shank or tapered-shank versions for die grinders and pneumatic tools. They are designed for deburring, edge rounding, contouring, and finishing on complex profiles where a flat disc cannot reach. Sanding discs (hook-and-loop and PSA discs) are used with random orbital sanders and angle grinder backing pad attachments. They are lighter-duty finishing tools — not designed for weld grinding or heavy stock removal. Their application is surface preparation, paint removal, and finish work. Non-woven flap discs look like a standard flap disc but use flaps of non-woven (Scotch-Brite®-style) material instead of coated-abrasive cloth, giving a softer, more conformable cut for blending and finishing rather than aggressive grinding. See AIMS's dedicated Non-Woven Abrasive Pad & Disc Guide for the full breakdown of non-woven formats, grades and stainless-steel handling. Flap Disc vs Grinding Disc: When to Use Each This is the most frequently asked question in the angle grinder category, and the answer depends on two factors: how much metal you need to remove, and what surface condition you need to leave behind. A grinding disc wins on raw material removal rate. The rigid bonded abrasive cuts aggressively and handles sustained pressure without rapid wear. Use a grinding disc when you are grinding down heavy weld runs, removing thick rust scale or surface defects, or profiling thick stock where surface finish is irrelevant. The downside: grinding discs concentrate heat, gouge easily if the angle is wrong, and leave a rough, directional scratch pattern that requires further finishing work. A flap disc wins on versatility and finish quality. The self-renewing flap construction cuts efficiently with less heat than a bonded wheel. It leaves a smoother, more consistent surface because the cloth backing conforms slightly to the workpiece. A 40–60 grit flap disc will remove most welds and heavy surface defects, and a subsequent pass with 80–120 grit on the same or a fresh disc will bring the surface to a paint-ready finish — without switching tools. For most general fabrication and maintenance welding, a flap disc replaces both the grinding disc and the finishing steps. Use a grinding disc when: the volume of material to remove is very large, sustained heavy pressure is required, or the job is purely preparatory — see our full Grinding Disc Guide for spec codes and material selection. Use a flap disc for almost everything else — especially when the next step is painting, coating, or inspection of the surface. ⚠️ Never use a cutting disc for grinding. Cutting discs are thin and engineered for straight parting cuts only. They are not rated for lateral side load. Applying side force to a cutting disc — even briefly — can cause the disc to crack or shatter during use. Australian WorkSafe authorities (SafeWork NSW, QLD, WA, SA) all specifically cite this as a recurring cause of serious injury. Always use a dedicated grinding disc or flap disc for stock removal. Abrasive Mineral Types: Aluminium Oxide, Zirconia and Ceramic The abrasive mineral is the working element of the disc. It determines cutting speed, heat generation, disc life, and cost per unit of material removed. Three minerals dominate the angle grinder market in Australia: Aluminium oxide (AO) is the standard entry-level abrasive mineral. It is manufactured by fusing bauxite at high temperature. Aluminium oxide cuts by fracturing — exposing new cutting edges as it wears. It is effective for light-duty finishing on mild steel and is the dominant mineral in budget-range flap discs and grinding discs. The limitation is longevity: aluminium oxide dulls faster than engineered minerals and does not self-sharpen under sustained pressure. For occasional use or light jobs, aluminium oxide is adequate. For production grinding or sustained heavy use, it is not economical. Zirconia alumina is a blended mineral (typically 25–40% zirconia, balance aluminium oxide) that is harder, tougher, and self-sharpening under load. Under the pressure of grinding, zirconia grains fracture to expose fresh sharp edges — maintaining cut rate far longer than straight aluminium oxide. The result is a disc that stays aggressive longer, generates less heat, and removes significantly more material per disc. Zirconia flap discs typically cost 30–50% more than aluminium oxide but last 3–5 times longer in sustained grinding. For anyone doing more than occasional weld grinding, zirconia delivers lower cost per metre ground. Zirconia performs particularly well on hard ferrous metals including carbon steel, stainless steel, and cast iron, but requires moderate-to-firm pressure to trigger the self-sharpening fracture mechanism — very light pressure will not fully activate it. Ceramic alumina (also labelled "SG", "ceramic", or "precision-shaped grain" in premium lines such as 3M Cubitron II, Pferd Ceramo, and Norton Quantum) is the highest-performance abrasive mineral available. Ceramic grains are precision-engineered with sharp, consistent cutting points that fracture in a controlled manner to continuously expose fresh edges. Ceramic abrasives cut faster, cooler, and longer than zirconia. On stainless steel and high-tensile alloys, the cool-running characteristic of ceramic is especially valuable — it minimises heat discolouration (heat tint) and reduces the risk of work-hardening the surface. A ceramic flap disc on stainless steel will typically last 4–8 times longer than an aluminium oxide disc on the same application. The premium per unit is significant, but the cost per unit of material removed is often lower than zirconia on high-volume or difficult-to-machine materials. Mineral Cutting Speed Disc Life Best For Cost Tier Aluminium Oxide Moderate Standard Mild steel, occasional use, light finishing $ (Budget) Zirconia Alumina High 3–5× AO Sustained weld grinding, production use, stainless, carbon steel $$ (Mid) Ceramic Alumina Very High 4–8× AO Hard alloys, stainless, high-tensile, titanium, production $$$ (Premium) For most Australian workshop and maintenance use, zirconia is the pragmatic choice: meaningfully better than aluminium oxide, substantially cheaper than ceramic, and available from all major suppliers (Pferd, Flexovit, Weiler, Tyrolit, Walter). Reserve ceramic for stainless steel, high-tensile alloy work, or high-volume production where disc change time is a cost factor. Grit Selection Guide Grit number refers to the mesh size used to sort abrasive particles — lower numbers are coarser, higher numbers are finer. For angle grinder discs and flap discs, the working range is roughly 24 to 120 grit. Grit Range Classification Typical Applications 24–36 Very Coarse Heavy weld grinding, aggressive stock removal, rapid rust scale removal, thick surface defects 40–60 Coarse Weld grinding to flush, bevel preparation, heavy rust removal, general stock removal 60–80 Medium Blending weld zones, removing coarse scratch patterns, rust removal on thinner material 80–120 Fine Pre-paint surface prep, finishing after blending, light rust and oxidation removal 120+ Very Fine Final finishing — generally better handled with a random orbital sander at this grit level A critical and frequently broken rule: never skip more than two grit grades in sequence. Going directly from 40 grit to 120 grit will cause the finer disc to clog immediately — it cannot remove the deep scratches left by the coarser grade without excessive load and heat. The correct sequence for weld removal and finishing: 40 grit to remove the weld proud, 60–80 grit to blend, 80–120 grit to finish. Each pass removes the scratch pattern from the previous grade, and the finish work proceeds cleanly. On stainless steel, start no coarser than 60 grit — coarser grades leave deep scratches that are very difficult to remove from stainless without extensive additional passes, and the risk of embedding iron contamination increases with heavier cutting. Type 27 (Flat) vs Type 29 (Conical) Flap Discs Type 27 and Type 29 refer to the profile of the flap disc backing plate — the geometry that controls the angle at which the abrasive flaps contact the workpiece. This is one of the most consistently misunderstood distinctions in the abrasive category. Type 27 flap discs have a flat (depressed-centre) profile. The flaps are arranged in a flat plane. When used on an angle grinder, a Type 27 disc works most efficiently at a low presentation angle — typically 0–15° to the workpiece surface. At this shallow angle, a large contact area of flap is engaged, delivering blending and finishing performance. Type 27 is the standard choice for surface blending, pre-paint finishing, and light weld blending where the priority is a smooth, consistent result. Type 29 flap discs have a conical profile — the backing plate is shaped so that the flap pack sits at an angle. This geometry is optimised for working at a steeper presentation angle (15–35° to the workpiece), which concentrates abrasive pressure at the leading edge of the disc contact zone. The result is a more aggressive cutting action and higher stock removal rate per pass. A common mistake with Type 27 discs used at a steep angle is premature edge wear — the outer flap edges take all the load at angles they are not designed for. If you consistently find yourself grinding at 15–35°, Type 29 is the right choice. Practical rule: Type 27 for surface blending and finishing (flat, 0–15°). Type 29 for aggressive weld grinding and stock removal (steeper, 15–35°). If you only stock one type for general use, Type 27 is the more versatile — it can be worked at steeper angles if needed, though with reduced efficiency. Type 27 is significantly more widely stocked in Australia. Cutting Disc Selection: Thickness, Material and Application Cutting discs are specified by diameter, thickness, bore, and material rating. Thickness is the most critical variable for cutting performance. Thickness and cutting speed: A thinner disc removes less material per cut and generates less heat — cuts are faster and cleaner. Thin discs (1.0–1.6 mm) are the choice for fast, clean cuts on sheet, tube, and small-section material. Thicker discs (2.0–3.0 mm) are more durable and handle vibration and deflection better on longer cuts through heavy sections. For most workshop cutting on mild steel bar, angle iron, pipe, and tube, a 1.6 mm disc is a good default. On thin sheet (below 3 mm), 1.0–1.2 mm is faster and cleaner. On heavy sections (above 12 mm) or structural cutting, 2.0–3.0 mm handles the job better. Material ratings: Cutting discs are rated for specific materials. A disc rated for steel will load up on aluminium — molten aluminium fills the abrasive pores, the disc becomes ineffective and heats dangerously. Always use an aluminium-rated cutting disc when cutting aluminium, and a masonry disc for concrete and stone. Using a steel cutting disc on aluminium is both dangerous and produces poor results. ⚠️ Aluminium disc loading warning. Aluminium melts at a low temperature and clogs abrasive pores within seconds on standard discs. The disc loads up, generates heat, and in severe cases can shatter. Always use aluminium-rated or multi-material abrasives (labelled "inox/aluminium" or "multi") when working on aluminium. For grinding aluminium, use a disc with an anti-loading (stearate) coating — see below. Grinding Aluminium: Anti-Loading Coatings and Why They Matter Aluminium presents a specific grinding challenge that standard abrasives cannot handle: loading. Aluminium is soft and has a low melting point — under the heat of grinding, the metal particles become semi-molten and embed themselves in the abrasive pores, turning the disc into a useless, smooth surface within seconds. This is why standard grinding and flap discs fail rapidly on aluminium even when fresh. The solution is a disc with an anti-loading coating — typically calcium stearate, applied to the abrasive surface. Calcium stearate functions as a dry lubricant: under the heat of grinding, it liquefies into a microscopic film that prevents aluminium chips from adhering to the abrasive grains. The result is a disc that stays open and cutting for a fraction of the aluminium work instead of loading within the first few strokes. When buying discs specifically for aluminium grinding, look for products labelled "aluminium", "for aluminium", or "with stearate coating". Some products label this as "non-loading" or "anti-load". Standard discs — even premium zirconia grades — will not perform adequately on aluminium without this coating. At lower speeds and light pressure, an uncoated disc will survive longer, but for any sustained aluminium grinding, specify anti-loading products. A practical tip from workshop experience: keep a block of paraffin wax (or purpose-made abrasive dressing wax) nearby when grinding aluminium. Touching the running disc lightly to the wax provides a temporary lubrication layer that extends disc life between disc changes — particularly useful when switching between aluminium and steel in the same session. Glazing and Loading: Why Your Disc Stops Cutting One of the most common workshop questions is "why has my disc gone smooth?" or "my flap disc isn't cutting anymore — is it worn out?" In most cases, the disc has either glazed or loaded — two distinct failure modes with different causes and solutions. Glazing occurs when the abrasive grains become dull without fracturing. Instead of micro-fracturing to expose sharp new cutting edges, the grains wear flat under excessive heat or insufficient pressure. The disc surface develops a shiny, glazed appearance and stops cutting efficiently — forcing the operator to apply more pressure, which generates more heat and accelerates the glazing. The most common cause is applying too little pressure on self-sharpening abrasives (zirconia and ceramic) — these minerals require meaningful pressure to trigger the fracture mechanism that keeps them sharp. Running a zirconia disc very lightly will glaze it prematurely. Loading occurs when swarf (metal particles) embed in the abrasive pores rather than being expelled. This is most common on soft metals (aluminium, copper, brass), on soft steel at low speeds, or when the grit is too fine for the material removal rate. The disc surface appears shiny and compacted rather than open and gritty. Loading is distinct from glazing — the grains may still be sharp, but they are buried under embedded material. Restoring a glazed or loaded disc: A glazed or lightly loaded disc can often be restored with an abrasive dressing stick (also called a disc cleaning stick or abrasive conditioning stick) — a stick of compressed abrasive that removes the glazed surface layer or embedded material and re-opens the abrasive pores. Touch the running disc briefly to the dressing stick; fresh abrasive is exposed and cutting performance typically restores immediately. This is a standard tool in any production grinding operation and extends disc life significantly. A heavily loaded disc (particularly from aluminium) may be beyond restoration — discard and fit a fresh anti-loading disc. Pressure rules by mineral type: Aluminium oxide — moderate pressure works. Zirconia — requires firm, consistent pressure to self-sharpen; too light will glaze. Ceramic — moderate pressure is sufficient; the precision-shaped grains are extremely efficient and do not need heavy force. In all cases: consistent, controlled pressure outperforms intermittent heavy pressing. Stainless Steel: Cross-Contamination and Heat Tint Stainless steel requires more care than mild steel in abrasive operations, and two specific problems catch operators by surprise. Cross-contamination: Never use an abrasive disc on stainless steel that has previously been used on carbon steel or cast iron. Even a brief pass on carbon steel embeds microscopic iron particles in the abrasive cloth. When that disc is then used on stainless, these iron particles are transferred into the stainless surface. The result is surface rust — visible within days of grinding — on what should be a corrosion-resistant material. This is the most common cause of rust spots on freshly fabricated stainless steel assemblies. The solution is simple but must be enforced consistently: dedicate specific discs to stainless steel and mark them clearly. A piece of green tape on the disc packet, or a separate storage rack, prevents cross-contamination. Inox-rated (stainless-rated) discs are manufactured without the iron, sulfur, or chlorine additives that contaminate stainless — look for the "INOX" label, which confirms the disc meets this manufacturing standard. Heat tint (blue/purple discolouration): When the surface of stainless steel turns blue, purple, or yellow during grinding, the metal has been overheated — the oxide layer has thickened due to excessive temperature. Heat tint on stainless is not merely cosmetic; it indicates a zone where the chromium oxide passive layer has been compromised, which can initiate corrosion. If you see heat tint developing, do not stop the disc on the hot spot — stopping concentrates heat in one location. Instead, reduce pressure and increase your stroke speed across the surface, allowing air to circulate between the flaps and cool both the disc and the workpiece. Switch to a ceramic abrasive if available — ceramic runs significantly cooler than zirconia or aluminium oxide and is the preferred choice for stainless applications where heat tint is a concern. Fibre Discs: Construction, Applications and How to Use Them Fibre discs are a distinct product class that many tradespeople overlook or confuse with sanding discs. A fibre disc (resin fibre disc) is constructed from layers of vulcanised fibreglass-reinforced paper impregnated with abrasive grain. Critically, fibre discs must be used with a rubber or plastic backing pad — they cannot be mounted directly to the grinder spindle. The backing pad supports the disc uniformly and allows the slight flex that makes fibre discs effective. Without a backing pad, a fibre disc will fail rapidly and unpredictably. Compared to flap discs, fibre discs provide a more consistent removal rate over their working life — a flap disc changes character as the flaps wear down, whereas a fibre disc maintains a similar cutting action until it is consumed. This consistency makes fibre discs predictable for production flat-surface work. On flat plate and sheet, a 24–40 grit fibre disc with a firm backing pad removes material very aggressively and efficiently — faster than a comparable flap disc on the same surface. The main limitation of fibre discs is their inability to work on contoured or concave surfaces — for those applications, a flap disc or flap wheel is more appropriate. On flat surfaces, however, a coarse fibre disc is one of the most efficient stock removal tools available. Available in 24–120 grit in aluminium oxide and zirconia. Disc Sizes and RPM Ratings Every abrasive disc has a maximum operating speed stamped on its label in RPM. Every angle grinder has a rated free-speed in RPM. Before fitting any disc, these two numbers must be checked — the disc maximum RPM must be equal to or greater than the grinder free-speed. ⚠️ Never exceed disc rated speed — this is not a guideline, it is a hard safety limit. Running an abrasive disc above its rated maximum RPM can cause disc failure. A reinforced grinding wheel or cutting disc can shatter explosively, ejecting fragments at velocities exceeding 80 m/s. This has caused fatalities on Australian worksites. The Queensland WorkSafe fatal incident report (2021) from a Brisbane construction site identified an unguarded angle grinder as a primary contributing factor. SafeWork NSW, SafeWork QLD, SafeWork SA, and WorkSafe WA have all issued specific alerts on angle grinder disc safety. Checking the disc RPM rating takes five seconds and is not optional. Disc Diameter Typical Max RPM Max Surface Speed Common Grinder RPM 100 mm (4 inch) 15,200 RPM 80 m/s 11,000–15,000 RPM 115 mm (4½ inch) 13,300 RPM 80 m/s 10,000–12,000 RPM 125 mm (5 inch) 12,200 RPM 80 m/s 10,000–12,000 RPM 180 mm (7 inch) 8,500 RPM 80 m/s 6,000–8,500 RPM 230 mm (9 inch) 6,650 RPM 80 m/s 6,000–6,650 RPM Grinder free-speed (no-load RPM) is always higher than operating speed under load — the disc rating must meet or exceed the free-speed, not the under-load speed. Always use the guard supplied with the grinder. Guards are a legally required safety device under AS/NZS 60745 and Australian WHS regulations — never remove the guard to improve visibility. Safe Use of Abrasive Discs Angle grinders are associated with a disproportionate number of serious workshop injuries — lacerations, eye injuries, hand injuries, and disc-fragment injuries. Safe use is not a bureaucratic formality. Pre-use inspection — the ring test: Before mounting any bonded abrasive disc (grinding disc or cutting disc), hold it at the centre hole and tap the face gently with the handle of a screwdriver. A sound disc produces a clear ring. A cracked disc produces a dull thud — discard immediately. Also check the disc expiry date; bonded abrasive wheels have a shelf life (typically 3 years from manufacture) printed on the label. Do not use expired discs. For flap discs, inspect the backing plate and flap bonding visually for cracks or delamination. Storage: Abrasive discs are sensitive to moisture, impact, and temperature cycling. Store flat, dry, away from chemicals. A disc dropped edge-on onto a concrete floor should be discarded — the impact may have initiated a crack even with no visible external damage. Cutting discs are particularly vulnerable to moisture; some production users vacuum-seal their supply. PPE requirements: A full face shield — not safety glasses alone — is the minimum. Disc fragments travel at 60–80 m/s and can penetrate the eye orbit past safety glasses. Hearing protection is required for sustained use. Heavy gloves, long sleeves, and an apron are appropriate for grinding operations. Grinding sparks are incandescent metal particles and can ignite flammable material up to 10 metres away — clear the area before starting. Body position: Never position yourself in the plane of disc rotation. If a disc fails, fragments travel primarily in the plane of rotation. Position yourself to the side of the disc plane and secure the workpiece in a vice or clamp — a moving workpiece is a major disc-breakage risk. Material-Specific Selection Guide Material Recommended Abrasive Grit Key Considerations Mild Steel AO or zirconia flap disc; standard grinding disc 40–80 grinding; 80–120 finishing Most forgiving material. Any standard abrasive works. Zirconia justified for production volumes. Stainless Steel INOX-rated flap disc (zirconia or ceramic); stainless-rated cutting disc 60–120 (avoid coarse) Dedicate discs — cross-contamination from carbon steel causes rust. Ceramic runs cooler, reduces heat tint. Never use discs previously used on carbon steel. Aluminium Anti-loading (stearate-coated) flap disc or cutting disc rated for aluminium 60–120 for grinding; 1.0–1.6 mm for cutting Standard discs load immediately. Use stearate-coated or aluminium-rated products only. Paraffin wax on the disc face extends life further. Concrete / Masonry Diamond cutting disc (dry or wet); silicon carbide grinding disc N/A for diamond; coarse (16–24) for SiC Never use metal cutting discs on masonry. High silica dust — use P2 respirator minimum. Wet cutting dramatically reduces dust. Cast Iron AO or zirconia grinding disc or flap disc 40–80 Cast iron is brittle — secure firmly. Graphite dust from grinding is conductive; keep clear of electrical equipment. Flap Wheels: Bench Grinders and Die Grinders Flap wheels are a separate product to flap discs, though they use the same basic construction. The key difference is mount type and application geometry. Bench grinder flap wheels are arbor-mounted and provide a softer, more controlled action than a bonded wheel — excellent for deburring, edge rounding, and light shaping work on small components. A 120-grit flap wheel on a bench grinder is one of the most efficient tools for deburring machined parts without removing excessive material. For precision hole-edge deburring and small-part edge breaking where a flap wheel can't reach, see the Deburring Tool Guide covering swivel-blade hand deburrers. Die grinder flap wheels are available in straight-shank versions for inline die grinders and angle-head versions for pneumatic right-angle tools. They are ideal for accessing internal bores, contoured surfaces, slots, and die cavities that a flat disc cannot reach. (For aggressive cutting and stock removal in those same areas — weld bead, port work, deburring castings — see our Carbide Burr & Rotary Burr Guide.) Available in 40–320 grit in aluminium oxide and zirconia. On stainless steel components, zirconia or ceramic flap wheels deliver significantly longer life than aluminium oxide. The same RPM rules apply — check the wheel rated speed against the grinder spindle speed before fitting. Die grinder spindle speeds vary from 6,000 to 30,000 RPM depending on tool type. Disc Life, Cost-Per-Use and Buying Strategy The temptation with abrasives is to buy on price — cheapest disc per unit. This calculation almost always produces higher total cost when disc life and productivity are factored in. A rough example: an aluminium oxide 125 mm flap disc at $4 lasting 20 minutes of active grinding vs a zirconia disc at $7 lasting 60–90 minutes. The zirconia costs 75% more per unit but delivers 3–4.5 times the useful life. At an operator cost of $60/hour, frequent disc changes are themselves a significant cost — quite apart from the consumable price. The practical buying strategy: stock zirconia as the standard flap disc for weld grinding and stock removal; aluminium oxide for light prep and finishing where disc life is not a factor; ceramic for stainless and high-tensile production work. Buy from established manufacturers — Pferd, Flexovit, Weiler, Tyrolit, 3M, and Walter are the major brands available through Australian industrial suppliers. Discount abrasives from unknown manufacturers carry undergrading risk (the marked grit differs from actual particle size) and poor bonding quality that can lead to premature failure. Frequently Asked Questions What is the difference between a flap disc and a grinding disc? A flap disc has overlapping abrasive-coated cloth flaps bonded to a backing plate — it grinds and finishes in one operation, producing a smoother surface with less gouging and less heat. A grinding disc is a solid bonded abrasive wheel that removes metal faster but leaves a rougher surface and generates more heat. Use a flap disc when surface finish matters after grinding; use a grinding disc when maximum material removal rate is the priority and further finishing will follow separately. What grit flap disc do I need for weld grinding? 40–60 grit for grinding welds flush with the base material. 60–80 grit for blending the weld zone and removing the coarse scratch pattern from the first pass. 80–120 grit for pre-paint or pre-coat finishing. Never skip more than two grit grades — going directly from 40 grit to 120 grit will cause the fine disc to clog immediately on the deep scratches left by the coarse grade. On stainless, start no coarser than 60 grit and use inox-rated discs throughout. What is the difference between aluminium oxide and zirconia flap discs? Aluminium oxide is the standard lower-cost mineral adequate for light finishing on mild steel but wears relatively quickly under sustained grinding. Zirconia alumina is self-sharpening under load — it maintains cut rate significantly longer and generates less heat. In sustained weld grinding, zirconia discs typically last 3–5 times longer than aluminium oxide, making them less expensive per unit of material removed despite the higher per-disc price. For anything more than occasional light use, zirconia is the more economical choice. Can I use the same flap disc on stainless steel and mild steel? No. Once a disc has been used on carbon (mild) steel, it must not be used on stainless. Carbon steel particles embed in the abrasive cloth during grinding. When that disc is then applied to stainless steel, those iron particles are transferred into the stainless surface — causing rust spots within days, on what should be a corrosion-resistant material. Dedicate specific discs to stainless steel and mark them clearly. Use only INOX-rated discs on stainless — these are manufactured without iron, sulfur, or chlorine additives that contaminate stainless surfaces. What is a Type 27 vs Type 29 flap disc? Type 27 has a flat backing plate profile — best for blending and finishing at a low angle (0–15°) to the surface. Type 29 has a conical profile — designed for more aggressive stock removal at a steeper angle (15–35°). If you grind Type 27 discs at too steep an angle, the outer flap edges take all the load and the disc wears prematurely on one edge. For general surface blending and finishing: Type 27. For aggressive weld removal and edge bevelling: Type 29. Type 27 is significantly more widely stocked in Australia. Why does my flap disc stop cutting and go smooth? Two distinct causes: glazing and loading. Glazing occurs when the abrasive grains dull without fracturing — the disc surface goes shiny and slick. With self-sharpening minerals (zirconia, ceramic), glazing is usually caused by insufficient pressure — these minerals need meaningful load to fracture and self-sharpen. Too light a touch will glaze them. Loading occurs when soft metal (especially aluminium) fills the abrasive pores. A glazed disc can often be restored by briefly touching it to an abrasive dressing stick while running — this removes the glazed layer and re-opens the pores. A loaded aluminium disc is generally not recoverable; discard and fit an anti-loading (stearate-coated) disc. What disc do I use to cut or grind aluminium? For cutting aluminium, use an aluminium-rated cutting disc (labelled "inox/aluminium" or "for aluminium"). Standard steel cutting discs load up within seconds on aluminium, generating dangerous heat. For grinding aluminium, use a flap disc with an anti-loading (stearate) coating — the calcium stearate liquefies under heat to prevent aluminium chips adhering to the abrasive. Without this coating, standard discs will load and stop cutting almost immediately. What is the maximum RPM of a 125 mm angle grinder disc? Most standard 125 mm abrasive discs are rated to 12,200 RPM (80 m/s surface speed). Most 125 mm angle grinders run at 10,000–12,000 RPM free speed — within this rating. Always verify the disc maximum RPM on its label and check it against your grinder's nameplate RPM before fitting. Never mount a disc with a lower maximum RPM than the grinder's free speed — disc failure at overspeed has caused fatalities on Australian worksites. How do I inspect an abrasive disc before use? For bonded grinding and cutting discs, perform the ring test: hold the disc at the centre hole and tap the face with a screwdriver handle. A clear ring = sound disc. A dull thud = cracked — discard immediately. Also check: chips or damage on the grinding face, expiry date (typically 3 years from manufacture for bonded wheels), and that the disc has not been stored in damp conditions or dropped. For flap discs, inspect the backing plate and flap bonding for cracks or delamination. Never use a disc showing any sign of damage. Can I use a cutting disc for grinding? No. Cutting discs are thin (1.0–2.0 mm) and designed for straight parting cuts only. They are not rated for lateral side load. Applying side force to a cutting disc causes it to flex, crack, and potentially shatter. Australian WorkSafe authorities across multiple states have issued specific safety alerts on this. Use a dedicated grinding disc (6–8 mm thick) or flap disc for stock removal, and a cutting disc only for cutting. What PPE do I need when using angle grinders? A full face shield — not safety glasses alone — is essential. Disc fragments travel at 60–80 m/s and can penetrate the eye orbit past safety glasses. Hearing protection is required for sustained grinding (angle grinders typically produce 95–105 dB). Heavy leather or cut-resistant gloves, long sleeves, and an apron protect against grinding sparks. Sparks are incandescent metal particles that can ignite flammable material up to 10 metres away. Always keep the guard fitted — it is a legal requirement under Australian WHS regulations, not an optional accessory. How long does a flap disc last? Disc life varies significantly with abrasive mineral, material, pressure, and technique. On mild steel under active grinding: aluminium oxide — typically 15–30 minutes. Zirconia — 30–60 minutes. Ceramic — 45–90 minutes or more. Applying consistent moderate pressure and working at the correct angle (nearly flat for Type 27, 15–35° for Type 29) are the two habits that most extend disc life. Letting the abrasive do the work rather than forcing the disc is more effective and less tiring. For a complete overview of angle grinder types, disc speed ratings, guard requirements, and safe grinding technique, see the AIMS Angle Grinder Guide. Shop Abrasive Discs at AIMS Industrial AIMS Industrial stocks a full range of angle grinder discs for Australian workshops — flap discs, grinding wheels, cutting discs, fibre discs, and more from leading brands including Klingspor, Pferd, and Flexovit. Shop Flap Discs Shop Grinding Wheels Browse All Abrasives Shop Angle Grinders For the drive-ratio formula and worked RPM examples, see our Pulley Speed Ratio Calculator guide.
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Grinding Discs & Grinding Wheels: Types, Spec Code & Selection Guide
Grinding discs and wheels: how to read the spec code, abrasive types, grit and grade selection, speed ratings, and AS1788 safe use for Australian industry.
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Spanner Size Chart: Metric & Imperial Wrench Sizes
Use this spanner size chart to find the right spanner for the fastener or fitting in front of you — whether metric, imperial (AF), or BSP. Spanner size refers to the across-flats (AF) measurement of the fastener head, which is the same dimension the spanner jaw must match. Getting it right avoids rounded heads and stripped fittings. This guide is part of AIMS Industrial's curated Engineering Reference Charts library — 78 reference articles across fasteners, threading, bearings, lubrication and safety standards. Spanner Size Selector — Match Spanner to Bolt This chart is a working spanner selector — every size row links to the AIMS spanner range or a covering set. Use the scenarios below to find the right AF spanner fast, or scroll to the chart tables below. How to use: 1. Identify bolt size or AF spanner size needed 2. Click the size in the chart for the matching range 3. Open the right AIMS spanner set M4–M8 Light Workshop 7–13mm AF — small fasteners 7–13mm View → M10–M16 General 17–24mm AF — workshop default 17–24mm View → M18–M24 Heavy Bolts 27–36mm AF — structural / engine 27–36mm View → Imperial AF (SAE) Inch sizes — older / US specs AF View → BSP Fittings Hydraulic & pneumatic fittings BSP View → Ratcheting Spanners Speed in restricted spaces Ratchet View → Adjustable Spanner One-tool catch-all (shifter) Shifter View → Browse Stahlwille / Ko-Ken / Bahco Premium European + Asian range Brands View → Quick rule: spanner size is the AF (across-the-flats) measurement — the bolt head's outside dimension, not the bolt thread. M10 bolt = 17mm AF spanner. M12 = 19mm AF. AIMS stocks Stahlwille, Ko-Ken, Bahco and Trax spanner sets covering 6–32mm metric and 1/4"–1-1/4" imperial. Need help? Call (02) 9773 0122. Jump to: How Sizes Work Bolt → Spanner Metric Range Imperial AF Conversion BSP Fittings Open / Ring / Combo Related Selectors How Spanner Sizes Work Spanner size is measured across the flats (AF) of the fastener head — the distance between two parallel faces of the hex. A 19mm spanner fits any fastener that measures 19mm across the flats, regardless of whether the fastener thread is metric or imperial. Bolt thread diameter (M8, M12 etc.) and spanner size are different measurements. The tables below show the relationship between bolt size and spanner size. An M8 bolt has a 13mm hex head — so you need a 13mm spanner, not an 8mm one. Open-end spanners engage two flats and are faster to use. Ring spanners (12-point) engage all six flats and are preferred for high-torque work, as they're less likely to round a fastener head. Combination spanners give you both in one tool — open end for speed, ring end for torque. Metric Spanner Size Chart — Bolt Thread to Spanner Size This table shows the spanner size required for each metric bolt thread size. Sizes follow ISO standard hex dimensions. Always confirm against the actual fastener if in doubt — some manufacturers use non-standard hex sizes. Bolt Size Spanner Size (AF) Common Application M4 7mm Small fasteners, electronics, thin sheet M5 8mm Small fasteners, covers, guards M6 10mm Most common — engines, brackets, interior panels M7 11mm Less common metric size M8 13mm General engineering, structural fasteners M10 17mm General engineering, machinery M12 19mm Automotive, structural, machinery M14 22mm Suspension components, driveline M16 24mm Heavy structural fasteners M18 27mm Heavy fasteners, industrial equipment M20 30mm Large structural and machinery fasteners M22 32mm Heavy machinery, plant equipment M24 36mm Large bolts, plant and structural M27 41mm Heavy plant and infrastructure M30 46mm Large plant, civil infrastructure M33 50mm Very large structural fasteners M36 55mm Heavy infrastructure, mining Metric Spanner Size Chart — Full Range The table below covers the full common metric spanner range from 6mm to 50mm, showing typical fastener applications for each size. Useful when you know which spanner you have and need to identify what it fits. Spanner Size (mm) Typical Bolt / Fastener Notes 6 M3.5 bolt head Uncommon — small precision fasteners 7 M4 bolt head Electronics, small assemblies 8 M5 bolt head Light fasteners, covers 9 General use Less common in metric sets 10 M6 bolt head Most common metric spanner size 11 M7 bolt head Less common metric size 12 General use Some fittings and M7 fine thread 13 M8 bolt head Common workshop size 14 1/8" BSP fittings Hydraulic and pneumatic fittings 15 General use Some M9 fasteners, brake fittings 16 General use Some M10 fine thread 17 M10 bolt head (standard) Common automotive and machinery size 18 General use Some hydraulic fittings 19 M12 bolt head Also close to 3/4" AF (19.05mm) 21 General use Some wheel nuts and couplings 22 M14 bolt head / 3/8" BSP Common fitting and fastener size 24 M16 bolt head Heavy structural applications 27 1/2" BSP fittings Most common BSP fitting size 27 M18 bolt head Industrial and heavy equipment 30 M20 bolt head Large structural fasteners 32 M22 bolt head / 3/4" BSP Heavy machinery and plant 36 M24 bolt head Large bolts, plant equipment 41 M27 bolt head / 1" BSP Heavy plant and large fittings 46 M30 bolt head Large plant and infrastructure 50 M33 bolt head / 1-1/4" BSP Very large structural and fittings Imperial (AF) Spanner Size Chart Imperial spanners are sized in fractions of an inch and are common on American-manufactured vehicles and equipment, agricultural machinery, and older plant. The sizing follows the across-flats (AF) convention — the same measurement system as metric, just in inches. Spanner Size (inch) Decimal (inch) Metric Equivalent (mm) Typical Use 1/4" 0.250" 6.35 Very small fasteners 5/16" 0.313" 7.94 Small fasteners 3/8" 0.375" 9.53 Light fasteners 7/16" 0.438" 11.11 General use 1/2" 0.500" 12.70 General use 9/16" 0.563" 14.29 General use 5/8" 0.625" 15.88 General use 11/16" 0.688" 17.46 General use 3/4" 0.750" 19.05 Common — close to 19mm metric 13/16" 0.813" 20.64 General use 7/8" 0.875" 22.23 Common — close to 22mm metric 15/16" 0.938" 23.81 General use 1" 1.000" 25.40 General use 1-1/16" 1.063" 26.99 Close to 27mm metric 1-1/8" 1.125" 28.58 General use 1-3/16" 1.188" 30.16 Close to 30mm metric 1-1/4" 1.250" 31.75 General use 1-5/16" 1.313" 33.34 General use 1-3/8" 1.375" 34.93 General use 1-7/16" 1.438" 36.51 Close to 36mm metric 1-1/2" 1.500" 38.10 General use Metric to Imperial Spanner Conversion Chart No exact metric-to-imperial match exists for most sizes — the measurement systems are independent. The table below shows the closest imperial spanner to each common metric size. Where the difference is large, the fit will be too loose for torqued fasteners. Always use the correct size where precision matters. Metric Size (mm) Closest Imperial Imperial in mm Difference 7 9/32" 7.14 +0.14mm 8 5/16" 7.94 -0.06mm (tight) 10 3/8" 9.53 -0.47mm (won't fit) 11 7/16" 11.11 +0.11mm 13 1/2" 12.70 -0.30mm (won't fit) 14 9/16" 14.29 +0.29mm 17 11/16" 17.46 +0.46mm 19 3/4" 19.05 +0.05mm ✓ 22 7/8" 22.23 +0.23mm 24 15/16" 23.81 -0.19mm (tight) 27 1-1/16" 26.99 -0.01mm ✓ 30 1-3/16" 30.16 +0.16mm 32 1-1/4" 31.75 -0.25mm (tight) 36 1-7/16" 36.51 +0.51mm 41 1-5/8" 41.28 +0.28mm 46 1-13/16" 46.04 +0.04mm ✓ BSP Fitting Spanner Sizes BSP (British Standard Pipe) sizes are nominal pipe bore sizes — not the actual across-flats measurement of the fitting. This catches people out: a 1/2" BSP fitting requires a 27mm spanner, not a 1/2" (12.7mm) one. The table below shows the spanner size needed for common BSP male threaded fittings. Sizes may vary slightly between fitting types and manufacturers. BSP Size Spanner Size (AF) Common Application 1/8" BSP 14mm Small fittings, gauges, bleed nipples 1/4" BSP 19mm Air fittings, small hydraulic connectors 3/8" BSP 22mm General plumbing, pneumatic lines 1/2" BSP 27mm Most common BSP size — hydraulic and pneumatic fittings 3/4" BSP 32mm General industrial plumbing 1" BSP 41mm Larger hydraulic and plumbing fittings 1-1/4" BSP 50mm Large pipe and industrial fittings 1-1/2" BSP 55mm Large pipe fittings 2" BSP 65mm Very large industrial fittings Open-End, Ring and Combination Spanners Choosing the right type of spanner matters as much as choosing the right size. Each type suits different situations. Spanner Type How It Grips Use When Limitation Open-end 2 flats Access is tight, fastener is in good condition, speed matters More likely to round worn fasteners Ring (box-end) All 6 flats (12-point) High torque, corroded or tight fasteners, precision work Must be dropped over the fastener — needs clearance above Combination Open one end, ring other end General use — ring to break loose or torque, open to run down Both ends are the same size Flare nut (crow's foot) 5 flats — slotted ring Brake and fuel lines — allows the spanner to pass over the line Lower torque rating than a solid ring spanner Ratchet spanner Ring with ratchet mechanism Tight spaces where a full swing arc isn't possible Not suited to very high torque Related AIMS Selectors This selector pairs with AIMS's other fastener & tool guides: Socket Size Chart Selector — when you need socket + ratchet not open-end spanner. Types of Spanners — open-end / ring / combo / flare / podger reference. Ratchet Spanner Guide — flex-head vs reversible vs gear count. Adjustable Spanner Guide — shifter selection and use. Choosing Socket Drive Size — when to step up or down a drive size. Metric Bolt Size Guide — bolt thread to head size reference. Metric Bolt Torque Chart — torque values per grade and size. BSP vs NPT vs UNC Thread Standards — fitting thread identification. Or browse the full spanners & wrenches range, ring spanners, ratcheting spanners, or by brand: Stahlwille, Ko-Ken, Bahco, Trax. Next-day Australia-wide dispatch from our Milperra warehouse.Frequently Asked Questions What is the most common spanner size?In metric, 10mm is the most frequently used spanner size — it fits M6 bolt heads, which appear on engines, brackets, and interior components across virtually every vehicle and machine. In imperial, 3/4" and 7/8" are among the most common SAE sizes. What size spanner fits an M8 bolt?An M8 bolt has a 13mm hex head, so you need a 13mm spanner. The bolt diameter (8mm) and the spanner size (13mm) are different measurements — the spanner fits the hex head, not the thread shank. This is a common source of confusion. What's the difference between AF and metric spanners?Both metric and AF (across flats) spanners measure the jaw opening in the same way — across the flats of the fastener. The difference is the unit: metric spanners are sized in millimetres, AF spanners in fractions of an inch. An AF spanner will be labelled in fractions (3/4", 7/8" etc.), while a metric spanner will be labelled in whole millimetres (19mm, 22mm etc.). Can I use a metric spanner on an imperial fastener?In some cases yes — where the metric size is very close to the imperial size. The best match is 19mm and 3/4" (19.05mm), where the difference is only 0.05mm. However, for torqued fasteners always use the correct spanner to avoid rounding. The conversion chart above shows the closest matches and their differences. What spanner do I need for BSP fittings?BSP fittings require a larger spanner than the pipe size suggests. The most common size — 1/2" BSP — requires a 27mm spanner, not a 1/2" (12.7mm) one. Always refer to the BSP fitting spanner chart above, as the pipe bore size and the fitting hex size are completely different measurements. What does AF mean on a spanner?AF stands for Across Flats — the distance between two parallel faces of a hex fastener. Both metric and imperial spanners are sized by this measurement. When you see a spanner marked "3/4" AF, it means the jaw opens to 3/4 of an inch across the flats. Metric spanners don't usually carry the AF label but are sized the same way. What's the difference between a ring spanner and an open-end spanner?A ring spanner has a closed circular end that fits over the fastener and engages all six flats. This reduces the risk of rounding and allows more torque to be applied safely. An open-end spanner has a U-shaped jaw that engages only two flats — it can be inserted sideways, which is useful in tight spaces, but it's more likely to slip or round a worn fastener. For any high-torque application, use the ring end. What is a flare nut spanner used for?A flare nut spanner (also called a crow's foot spanner) has a ring end with a slot cut into it, allowing it to pass over a brake line or fuel line before engaging the fitting nut. It grips five of the six flats rather than two, giving better purchase than an open-end spanner while still allowing it to be slid onto a fitting with a line attached. They are essential for brake and fuel line work where a standard ring spanner cannot be dropped over the top. Got the size? Get the spanner. Shop our full range of metric & imperial spanners From open-end to ring and combination spanners — AIMS Industrial stocks sizes across metric, AF, and BSP standards, ready to ship Australia-wide. Browse spanners Talk to a specialist People Also Ask — Spanner Size Chart: Metric & Imperial Wrench Sizes Q: What spanner size fits an M10 bolt? An M10 coarse bolt (1.5 mm pitch) uses a 17 mm spanner across the flats. M10 fine (1.25 mm pitch) also uses 17 mm. This is one of the most common sizes in Australian trade and maintenance work — a 17 mm open-end or combination spanner is considered a toolbox essential. Q: How do I convert spanner sizes from metric to imperial? Divide the metric jaw size in mm by 25.4 to get inches. A 13 mm spanner equals approximately 1/2" (actually 12.7 mm, so there's a slight mismatch — never substitute if the fit is loose). Common near-equivalents: 11 mm ≈ 7/16", 13 mm ≈ 1/2", 17 mm ≈ 11/16", 19 mm ≈ 3/4". Q: What size spanner is needed for BSP fittings? BSP (British Standard Pipe) fitting sizes are not the same as their thread diameter — a 1/2" BSP fitting uses a 27 mm spanner across the hex, while a 3/4" BSP uses 32 mm. Always check the fitting's hex size, not its pipe thread designation, to choose the right spanner. Q: What is the difference between an open-end and combination spanner? An open-end spanner engages two faces of the fastener and suits confined spaces where a ring won't fit. A combination spanner has an open end on one side and a ring (box) end on the other — the ring end gives better grip and is less likely to round off a fastener, making it the better choice when access allows. See AIMS's full adjustable hand reamers range — trade pricing and Australia-wide despatch. For open end wrenches, see our open end wrenches range stocked across Australia.
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Drill Bit Size Chart: Metric, Imperial & Fractional
Drill Bit Selector — Most-Asked Sizes This page is a working selector tool — not just a reference. Use it to get the right drill bit into your hand in one click. The 10 most-asked drill sizes at AIMS are below. For less common sizes, scroll to the full conversion chart further down. How to use: 1. Find your drill size 2. Click Buy drill 3. Confirm material match using the "By Material" section below 3 mm HSS jobber drill Buy drill → 4 mm HSS jobber drill Buy drill → 5 mm HSS jobber drill Buy drill → 6 mm HSS jobber drill Buy drill → 6.5 mm HSS jobber drill Buy drill → 8 mm HSS jobber drill Buy drill → 10 mm HSS jobber drill Buy drill → 12 mm HSS jobber drill Buy drill → 13 mm HSS jobber drill Buy drill → 16 mm HSS jobber drill Buy drill → Default recommendation: Sutton D101 Silver Bullet HSS jobber drill — the workshop standard for occasional drilling in mild steel, aluminium, plastics and timber. For production steel work switch to D102 Blue Bullet (steam oxide finish). For stainless steel or hardened material, use D108 or D109 cobalt drill bits. For concrete and masonry, use D600 / D601 TCT masonry drill bits (linked in the Anchor Bolt sections below). Need help? Call us on (02) 9773 0122. Jump to: Quick Reference Full Conversion Chart By Material Anchor Bolt Sizes Related Selectors Imperial Drill Bit Sizes in Millimetres — Quick Reference The fastest way to convert a fractional or imperial drill bit size to millimetres is the constant 25.4 mm per inch. So 1/8" = 3.175 mm, 5/32" = 3.969 mm, 3/16" = 4.763 mm, 7/32" = 5.556 mm, 1/4" = 6.350 mm, 5/16" = 7.938 mm, 3/8" = 9.525 mm and 15/64" = 5.953 mm. The full metric, imperial and fractional cross-reference chart sits in the body below. What size is a 1/8" drill bit in mm? 1/8" equals 3.175 mm exactly (1 ÷ 8 × 25.4). The closest standard metric drill bit is 3.2 mm. What size is a 3/16" drill bit in mm? 3/16" equals 4.763 mm (3 ÷ 16 × 25.4). The closest standard metric drill bits are 4.8 mm or 5.0 mm. What is the standard mm size of a 5/32" drill bit? 5/32" equals 3.969 mm (5 ÷ 32 × 25.4). The closest standard metric drill bit is 4.0 mm. Use this drill bit size chart to cross-reference metric (mm), imperial (fractional and decimal inches), and gauge sizes — including number and letter — in a single reference table. Whether you're sizing a pilot hole for a screw or bolt, converting between measurement systems, or identifying an unmarked bit, find your size below and read across the row for the equivalent. Quick conversion — most-searched sizes Imperial to metric: 1/8" = 3.175mm · 5/32" = 3.97mm · 3/16" = 4.76mm · 7/32" = 5.56mm · 1/4" = 6.35mm · 5/16" = 7.94mm · 3/8" = 9.525mm · 7/16" = 11.11mm · 1/2" = 12.7mm · 9/16" = 14.29mm · 5/8" = 15.88mm · 3/4" = 19.05mm · 1" = 25.4mm Metric to imperial: 3mm ≈ 1/8" · 4mm ≈ 5/32" · 5mm ≈ 13/64" · 6mm ≈ 15/64" · 8mm ≈ 5/16" · 10mm ≈ 25/64" · 12mm ≈ 15/32" Full metric / fractional / number / letter cross-reference table below. This guide is part of AIMS Industrial's curated Engineering Reference Charts library — 78 reference articles across fasteners, threading, bearings, lubrication and safety standards. Drill Bit Size Chart — Quick Reference Quick conversion between the most common metric (mm) and imperial (fractional inch) drill bit sizes. Full cross-reference table with number and letter gauge sizes is below. Metric (mm) Fractional Inch Decimal Inch 3.0 mm 1/8" 0.1181 4.0 mm 5/32" 0.1575 5.0 mm 13/64" 0.1969 6.0 mm 15/64" 0.2362 8.0 mm 5/16" 0.3150 10.0 mm 25/64" 0.3937 12.0 mm 15/32" 0.4724 16.0 mm 5/8" 0.6299 20.0 mm 25/32" 0.7874 25.4 mm 1" 1.0000 How to Use This Chart The table covers drill bit sizes from 0.010mm to 25.4mm (1 inch). Find your required hole size in the metric or imperial column and read across to identify the equivalent gauge number, letter, fractional, or decimal size. A few things to keep in mind. Not all metric drill bits have imperial equivalents and vice versa. Some metric values are close enough to share the same imperial counterpart. Values are rounded to the nearest thousandth millimetre or ten-thousandth decimal inch. Need help finding your size? Call AIMS on (02) 9773 0122 — the team can confirm stock and pick the right drill type for your material. Or browse all jobber drill bits. Gauge Sizing Metric / SI Imperial / US Number / Letter Size Millimeters Fractional Inches Fractional Inches (Reduced) Decimal Inches --- 0.010 --- --- 0.0004 104 0.080 --- --- 0.0031 103 0.090 --- --- 0.0035 102 --- --- --- 0.0039 --- 0.100 --- --- 0.0039 101 0.110 --- --- 0.0043 100 0.120 --- --- 0.0047 99 0.130 --- --- 0.0051 98 0.140 --- --- 0.0055 97 0.150 --- --- 0.0059 96 0.160 --- --- 0.0063 95 0.170 --- --- 0.0067 94 0.180 --- --- 0.0071 93 0.190 --- --- 0.0075 --- 0.198 1/128 --- 0.0078 92 0.201 --- --- 0.0079 91 0.210 --- --- 0.0083 90 0.220 --- --- 0.0087 89 0.230 --- --- 0.0091 88 0.240 --- --- 0.0095 --- 0.250 --- --- 0.0098 87 0.254 --- --- 0.0100 --- 0.260 --- --- 0.0102 86 0.270 --- --- 0.0105 85 0.280 --- --- 0.0110 84 0.290 --- --- 0.0115 --- 0.300 --- --- 0.0118 83 0.305 --- --- 0.0120 82 0.318 --- --- 0.0125 81 0.330 --- --- 0.0130 80 0.343 --- --- 0.0135 79 0.368 --- --- 0.0145 --- 0.397 1/64 --- 0.0156 78 0.406 --- --- 0.0160 77 0.457 --- --- 0.0180 --- 0.500 --- --- 0.0197 76 0.508 --- --- 0.0200 75 0.533 --- --- 0.0210 74 0.572 --- --- 0.0225 73 0.610 --- --- 0.0240 72 0.635 --- --- 0.0250 71 0.660 --- --- 0.0260 70 0.710 --- --- 0.0280 69 0.742 --- --- 0.0292 --- 0.750 --- --- 0.0295 68 0.787 --- --- 0.0310 --- 0.794 2/64 1/32 0.0312 67 0.813 --- --- 0.0320 66 0.838 --- --- 0.0330 65 0.889 --- --- 0.0350 64 0.914 --- --- 0.0360 63 0.940 --- --- 0.0370 62 0.965 --- --- 0.0380 61 0.991 --- --- 0.0390 --- 1.000 --- --- 0.0394 60 1.016 --- --- 0.0400 59 1.041 --- --- 0.0410 58 1.067 --- --- 0.0420 57 1.092 --- --- 0.0430 56 1.181 --- --- 0.0465 --- 1.191 3/64 --- 0.0469 --- 1.250 --- --- 0.0492 55 1.321 --- --- 0.0520 54 1.397 --- --- 0.0550 --- 1.500 --- --- 0.0591 53 1.511 --- --- 0.0595 --- 1.588 4/64 1/16 0.0625 52 1.613 --- --- 0.0635 51 1.702 --- --- 0.0670 --- 1.750 --- --- 0.0689 50 1.778 --- --- 0.0700 --- 1.800 --- --- 0.0709 --- 1.850 --- --- 0.0728 49 1.854 --- --- 0.0730 48 1.930 --- --- 0.0760 --- 1.984 5/64 --- 0.0781 47 1.994 --- --- 0.0785 --- 2.000 --- --- 0.0787 46 2.057 --- --- 0.0810 45 2.083 --- --- 0.0820 44 2.184 --- --- 0.0860 --- 2.250 --- --- 0.0886 43 2.261 --- --- 0.0890 42 2.375 --- --- 0.0935 --- 2.381 6/64 3/32 0.0938 41 2.438 --- --- 0.0960 --- 2.450 --- --- 0.0965 40 2.489 --- --- 0.0980 --- 2.500 --- --- 0.0984 39 2.527 --- --- 0.0995 38 2.578 --- --- 0.1015 37 2.642 --- --- 0.1040 36 2.705 --- --- 0.1065 --- 2.750 --- --- 0.1083 --- 2.778 7/64 --- 0.1094 35 2.794 --- --- 0.1100 34 2.819 --- --- 0.1110 33 2.870 --- --- 0.1130 32 2.946 --- --- 0.1160 --- 3.000 --- --- 0.1181 31 3.048 --- --- 0.1200 --- 3.175 8/64 1/8 0.1250 --- 3.250 --- --- 0.1280 30 3.264 --- --- 0.1285 29 3.454 --- --- 0.1360 --- 3.500 --- --- 0.1378 28 3.569 --- --- 0.1405 --- 3.572 9/64 --- 0.1406 27 3.658 --- --- 0.1440 26 3.734 --- --- 0.1470 --- 3.750 --- --- 0.1476 25 3.797 --- --- 0.1495 24 3.861 --- --- 0.1520 23 3.912 --- --- 0.1540 --- 3.969 10/64 5/32 0.1562 22 3.988 --- --- 0.1570 --- 4.000 --- --- 0.1575 21 4.039 --- --- 0.1590 20 4.089 --- --- 0.1610 19 4.216 --- --- 0.1660 --- 4.250 --- --- 0.1673 18 4.305 --- --- 0.1695 --- 4.366 11/64 --- 0.1720 17 4.394 --- --- 0.1730 16 4.496 --- --- 0.1770 --- 4.500 --- --- 0.1772 15 4.572 --- --- 0.1800 14 4.623 --- --- 0.1820 13 4.469 --- --- 0.1850 --- 4.750 --- --- 0.1870 --- 4.763 12/64 3/16 0.1875 12 4.800 --- --- 0.1890 11 4.850 --- --- 0.1910 10 4.915 --- --- 0.1935 9 4.978 --- --- 0.1960 --- 5.000 --- --- 0.1969 8 5.055 --- --- 0.1990 7 5.105 --- --- 0.2010 --- 5.159 13/64 --- 0.2031 6 5.182 --- --- 0.2040 5 5.220 --- --- 0.2055 --- 5.250 --- --- 0.2067 4 5.309 --- --- 0.2090 3 5.410 --- --- 0.2130 --- 5.500 --- --- 0.2165 --- --- 14/64 7/32 0.2188 2 5.613 --- --- 0.2210 --- 5.750 --- --- 0.2264 1 5.791 --- --- 0.2280 A 5.944 --- --- 0.2340 --- 5.953 15/64 --- 0.2344 --- 6.000 --- --- 0.2362 B 6.045 --- --- 0.2380 --- 6.100 --- --- 0.2402 C 6.147 --- --- 0.2420 --- 6.200 --- --- 0.2441 D 6.248 --- --- 0.2460 --- 6.250 --- --- 0.2461 E 6.350 16/64 1/4 0.2500 --- 6.500 --- --- 0.2559 F 6.528 --- --- 0.2570 --- 6.600 --- --- 0.2598 G 6.629 --- --- 0.2610 --- 6.747 17/64 --- 0.2656 --- 6.750 --- --- 0.2657 H 6.756 --- --- 0.2660 --- 6.800 --- --- 0.2677 I 6.909 --- --- 0.2720 --- 7.000 --- --- 0.2756 J 7.036 --- --- 0.2770 --- 7.100 --- --- 0.2795 K 7.137 --- --- 0.2810 --- 7.144 18/64 9/32 0.2813 --- 7.250 --- --- 0.2854 --- 7.300 --- --- 0.2874 L 7.366 --- --- 0.2900 --- 7.400 --- --- 0.2913 M 7.493 --- --- 0.2950 --- 7.500 --- --- 0.2953 --- 7.541 19/64 --- 0.2969 --- 7.600 --- --- 0.2992 N 7.671 --- --- 0.3020 --- 7.750 --- --- 0.3051 --- 7.938 20/64 5/16 0.3125 --- 8.000 --- --- 0.3150 O 8.026 --- --- 0.3160 --- 8.100 --- --- 0.3189 --- 8.200 --- --- 0.3228 P 8.204 --- --- 0.3230 --- 8.250 --- --- 0.3248 --- 8.300 --- --- 0.3268 --- 8.334 21/64 --- 0.3281 Q 8.433 --- --- 0.3320 --- 8.500 --- --- 0.3346 R 8.611 --- --- 0.3390 --- 8.700 --- --- 0.3425 --- 8.731 22/64 11/32 0.3438 --- 8.750 --- --- 0.3445 S 8.839 --- --- 0.3480 --- 9.000 --- --- 0.3543 T 9.093 --- --- 0.3580 --- 9.128 23/64 --- 0.3594 --- 9.250 --- --- 0.3642 U 9.347 --- --- 0.3680 --- 9.500 --- --- 0.3740 --- 9.525 24/64 3/8 0.3750 V 9.576 --- --- 0.3770 --- 9.600 --- --- 0.3780 --- 9.700 --- --- 0.3819 --- 9.750 --- --- 0.3839 --- 9.800 --- --- 0.3858 W 9.804 --- --- 0.3860 --- 9.922 25/64 --- 0.3906 --- 10.000 --- --- 0.3937 X 10.080 --- --- 0.3970 --- 10.084 --- --- 0.4016 Y 10.260 --- --- 0.4040 --- 10.490 26/64 13/32 0.4063 Z 10.490 --- --- 0.4130 --- 10.500 --- --- 0.4134 --- 10.716 27/64 --- 0.4219 --- 11.000 --- --- 0.4331 --- 11.112 28/64 7/16 0.4375 --- 11.500 --- --- 0.4528 --- 11.509 29/64 --- 0.4531 --- 11.906 30/64 15/32 0.4688 --- 12.000 --- --- 0.4724 --- 12.200 --- --- 0.4803 --- 12.303 31/64 --- 0.4844 --- 12.500 --- --- 0.4921 --- 12.700 32/64 1/2 0.5000 --- 12.800 --- --- 0.5039 --- 13.000 --- --- 0.5118 --- 13.097 33/64 --- 0.5156 --- 13.494 34/64 17/32 0.5312 --- 13.500 --- --- 0.5315 --- 13.891 35/64 --- 0.5469 --- 14.000 --- --- 0.5512 --- 14.288 36/64 9/16 0.5625 --- 14.500 --- --- 0.5709 --- 14.684 37/64 --- 0.5781 --- 15.000 --- --- 0.5906 --- 15.081 38/64 19/32 0.5938 --- 15.478 39/64 --- 0.6094 --- 15.500 --- --- 0.6102 --- 15.875 40/64 5/8 0.6250 --- 16.000 --- --- 0.6299 --- 16.272 41/64 --- 0.6406 --- 16.500 --- --- 0.6496 --- 16.669 42/64 21/32 0.6563 --- 17.000 --- --- 0.6693 --- 17.066 43/64 --- 0.6719 --- 17.462 44/64 11/16 0.6875 --- 17.500 --- --- 0.6890 --- 17.859 45/64 --- 0.7031 --- 18.000 --- --- 0.7087 --- 18.256 46/64 23/32 0.7188 --- 18.500 --- --- 0.7283 --- 18.653 47/64 --- 0.7344 --- 19.000 --- --- 0.7480 --- 19.050 48/64 3/4 0.7500 --- 19.447 49/64 --- 0.7656 --- 19.500 --- --- 0.7677 --- 19.844 50/64 25/32 0.7813 --- 20.000 --- --- 0.7874 --- 20.241 51/64 --- 0.7969 --- 20.500 --- --- 0.8071 --- 20.638 52/64 13/16 0.8125 --- 21.000 --- --- 0.8268 --- 21.034 53/64 --- 0.8281 --- 21.431 54/64 27/32 0.8438 --- 21.500 --- --- 0.8465 --- 21.828 55/64 --- 0.8594 --- 22.000 --- --- 0.8661 --- 22.225 56/64 7/8 0.8750 --- 22.500 --- --- 0.8858 --- 22.622 57/64 --- 0.8906 --- 23.000 --- --- 0.9055 --- 23.019 58/64 29/32 0.9062 --- 23.416 59/64 --- 0.9219 --- 23.500 --- --- 0.9252 --- 23.812 60/64 15/16 0.9375 --- 24.000 --- --- 0.9449 --- 24.209 61/64 --- 0.9531 --- 24.500 --- --- 0.9646 --- 24.606 62/64 31/32 0.9688 --- 25.000 --- --- 0.9843 --- 25.003 63/64 --- 0.9844 --- 25.400 64/64 1 1.0000 Understanding Gauge Sizing Gauge sizing directly determines drill bit diameter and affects your selection in three key ways. Precision: the number and letter gauge system offers a high degree of precision for smaller drill bits — critical when exact hole dimensions matter. Compatibility: many fasteners and components are specified by gauge size, so the drill bit must match the gauge of the intended component for a proper fit. Standardisation: gauge sizing provides a consistent method for specifying drill bit dimensions across industries, making communication and cross-referencing straightforward. Selecting the Right Drill Bit The right drill size is not always fixed — it depends on several variables. The material being drilled (steel, aluminium, plastic, timber) affects which size and type of bit to use. The desired thread depth determines the clearance needed for the tap. The required thread fit — close, medium, or free — influences the drill size. Different tap types (taper, plug, bottoming) also require slightly different drill sizes. For fluteless or thread-forming taps specifically, the Engineers Black Book recommends using a good cutting oil or lubricant rather than coolant. If you need complete size data including metric and thread sizes from 0.100mm up to 101.6mm (4 inches), the Engineers Black Book (3rd Edition — Metric) is worth having on hand. Note: this table covers a broad range of sizes but may not include every available drill bit size. Always consult a detailed chart for precision applications. Drill Bit Selection by Material Drilling the right hole isn't just about the diameter — it's about matching the drill bit material and geometry to what you're cutting. The wrong combination means broken bits, oversized holes, work-hardened steel, or burnt edges. Here's what the AIMS workshop crew reaches for: Mild steel (most common) Drill: Sutton D101 Silver Bullet HSS jobber for occasional / hobby work. Sutton D102 Blue Bullet (steam-oxide finish) for production drilling — the oxide finish helps swarf release. Geometry: Standard 118° split point. Lubricant: Any general-purpose cutting fluid or Tap Magic Original. Stainless steel (304, 316) Drill: Cobalt drill bit — Sutton D108 (cobalt jobber, colour-temp tempered) or D109 (heavy duty cobalt bulk pack). The 5-8% cobalt content keeps the drill sharp through stainless's work-hardening. HSS bright drills will glaze the hole, work-harden the steel, and snap. Geometry: 130° to 135° split point preferred (sharper attack angle handles work-hardening better). Lubricant: Tap Magic for Stainless — specifically formulated. Aluminium, brass, copper, plastics Drill: Standard HSS Sutton D101 is fine for most. Sutton D113 Blue Bullet Long Drill for deeper holes. Some users prefer a higher helix angle drill for soft non-ferrous materials to evacuate long stringy chips. Geometry: 130° to 140° point geometry for soft materials. Lubricant: Tap Magic Aluminium variant — prevents the gummy build-up aluminium causes on standard cutting fluids. Cast iron Drill: HSS Sutton D102 Blue Bullet (steam-oxide). Cast iron is brittle and abrasive — the steam oxide finish prolongs drill life. Lubricant: Generally drilled DRY — chips are powder, not strings, so no cutting fluid needed. Hardened steel (above ~30 HRC) For anything above mild steel hardness — pre-hardened tool steels, heat-treated parts, hardfaced surfaces — call AIMS before you start. Solid carbide drills (Sutton D323/D326/D332 series) are the right answer, not HSS or cobalt. Contact us or call (02) 9773 0122 — we'll save you broken drills and damaged workpieces. Concrete, brick, masonry Drill: Tungsten-carbide-tipped (TCT) masonry drill — Sutton D600 (standard fixing, 3mm-13mm) or Sutton D601 (single brick, higher quality, up to 25mm). NOT HSS — HSS bits glaze and dull instantly on concrete. Use: Hammer drill mode required for concrete and brick. Regular rotary drilling on glazed tile only. Lubricant: None — drilled dry. Use a vacuum or compressed air to clear dust as you go. The AIMS workshop rule: Drill speed (RPM) matters more than most beginners realise. Big drill bits need SLOW speeds; small bits need fast. Wrong RPM for the size + material is the #1 cause of drill bit failure. The lathe RPM formula guide covers the maths; the cutting speeds & feeds reference gives you the Vc per material. Anchor Bolt Drill Size Chart Anchor bolts require a hole drilled into concrete, masonry, or brick before installation. The required drill bit size depends on the anchor type — sleeve anchors, wedge anchors, and drop-in anchors each have different requirements. Use a hammer drill or SDS drill with a masonry bit for all concrete anchor applications. Sleeve Anchors & Wedge Anchors (Dynabolts) For sleeve anchors and wedge anchors — including Ramset Dynabolts and equivalent brands — the drill bit diameter matches the anchor diameter. Drill the hole to the specified depth, clear the dust, then drive the anchor. Need help finding your size? Call AIMS on (02) 9773 0122 — the team can confirm stock and pick the right drill type for your material. Or browse masonry drill bits. Anchor Size Drill Bit Size Minimum Hole Depth Typical Application M6 6.0mm 40mm Light fixings, brackets M8 8.0mm 50mm Shelving, light structures M10 10.0mm 60mm Equipment bases, handrails M12 12.0mm 75mm Machinery, structural steel M16 16.0mm 90mm Heavy machinery, posts M20 20.0mm 110mm Structural anchoring M24 24.0mm 125mm Heavy structural anchoring Drop-In Anchors (Internal Thread) Drop-in anchors are flush-mount anchors used where a bolt or threaded rod is inserted after installation. The drill bit size matches the outer body diameter of the anchor — not the internal thread size. Always check the manufacturer's specification for the specific anchor being used, as outer diameters can vary slightly by brand. Internal Thread Drill Bit Size Minimum Hole Depth M6 8.0mm 30mm M8 10.0mm 40mm M10 12.0mm 50mm M12 16.0mm 60mm M16 20.0mm 80mm M20 25.0mm 100mm Chemical Anchors (Resin) Chemical anchors use a two-part resin to bond a threaded rod or rebar into concrete. The drill bit is typically 2–4mm larger than the bar diameter to allow the resin to flow and fully encapsulate the rod. Always follow the anchor manufacturer's specification for hole diameter and depth, as embedment requirements vary by resin system and load rating. Rod / Bar Diameter Drill Bit Size Notes M8 10.0mm Confirm with resin manufacturer M10 12.0mm Confirm with resin manufacturer M12 14.0mm Confirm with resin manufacturer M16 20.0mm Confirm with resin manufacturer M20 24.0mm Confirm with resin manufacturer M24 28.0mm Confirm with resin manufacturer Note: All anchor bolt applications in concrete require a hammer drill or SDS drill with a quality masonry bit. A standard drill will not achieve the required penetration depth or accuracy. Always blow or vacuum dust from the hole before installing the anchor — debris left in the hole reduces holding strength significantly. Related AIMS Selectors & Guides This page sits at the centre of AIMS's drilling tool cluster. The related selectors and selection guides below go deeper on the choices around it: Tap & Drill Bit Selector — Tap Drill Sizes — for threading work, pair this chart with the tap drill sizes Drill Bit Selection Guide — HSS vs cobalt vs carbide, jobber vs stub vs reduced shank vs annular cutter Cobalt Drill Bit Guide — when cobalt is worth the upgrade from HSS HSS vs Carbide End Mill — same material principles apply to drilling decisions Cutting Speeds & Feeds Reference — RPM and feed rate by material and drill diameter Cutting Tool Materials — HSS, cobalt, carbide, PCBN, PCD compared Cutting Tool Coatings — TiN, TiAlN, AlCrN, when each matters for drills Cutting Tool Troubleshooting — walking drills, poor finish, oversize holes, snapped tips Metric / Imperial / Gauge Conversion Master Chart — full drill bit + thread size cross-reference Or browse the full jobber drill bits range + cobalt drill bits — Sutton primary stock, Bordo and P&N alternates, masonry TCT, annular cutters and specialty drilling all in stock for next-day Australia-wide dispatch from our Milperra warehouse.Related Reference Charts Fastener Reference Chart — cross-reference bolt, nut, and screw measurements across metric, Unified Thread Standard, and British Thread Standard values, with thread pitch identification guides. Loctite Grade Selection Guide — identify the right Loctite product for your materials and surfaces. Spanner Size Chart — find the correct spanner or wrench size for your bolt or nut head, metric and imperial. Tapping Drill Size Chart — find the right pilot hole drill size before cutting threads. Covers metric coarse and fine, BSP, UNC, and UNF threads. Frequently Asked Questions What is 3/8 inch drill bit in mm? A 3/8 inch drill bit is 9.525mm. This is one of the most common sizes used for medium pilot holes in steel and timber. What is 1/2 inch drill bit in mm? A 1/2 inch drill bit is 12.7mm exactly. Used for clearance holes on M12 bolts and general large-bore drilling. What is 1/4 inch drill bit in mm? A 1/4 inch drill bit is 6.35mm. Common pilot size for 1/4-20 UNC fasteners and a workshop default for general drilling. What is 5/16 inch drill bit in mm? A 5/16 inch drill bit is 7.94mm — close enough to 8mm for most practical applications, but always use the exact size for precision work. What is 5/8 inch drill bit in mm? A 5/8 inch drill bit is 15.875mm. Used for M16 clearance holes and large structural fixings. What is 9/16 inch drill bit in mm? A 9/16 inch drill bit is 14.288mm. Often used as a clearance hole for 1/2 inch bolts. What is 3/16 inch drill bit in metric? A 3/16 inch drill bit is 4.763mm — close to 4.75mm. Common for light pilot holes in timber and sheet metal. What is 6mm drill bit in imperial? A 6mm drill bit is approximately 15/64 inch (0.2362"). It sits between 15/64" (5.953mm) and 1/4" (6.35mm) — use 6mm where the exact metric size is needed. What is 8mm drill bit in imperial? An 8mm drill bit is approximately 5/16 inch (7.94mm) — they're functionally interchangeable for most general drilling work. What is the order of drill bit sizes? Drill bit sizes run from very small to very large across four overlapping systems: number drills (#80 smallest = 0.343mm, up to #1 = 5.79mm), letter drills (A = 5.94mm up to Z = 10.49mm), fractional inches (1/64" up to 1" and beyond), and metric (typically 0.5mm up to 25mm+). Use the full chart above to cross-reference. What size drill bit do I need for a pilot hole? It depends on the screw or bolt diameter and the material being drilled. As a general rule, size the pilot hole at around 75-85% of the screw's outer diameter for timber, and closer to 90% for metal. For threaded pilot holes, refer to the Tapping Drill Size Chart, which lists the correct drill size for each thread specification. What do number and letter drill bit sizes mean? Number drills (1-80) and letter drills (A-Z) are gauge-based sizing systems used primarily in the USA and in precision engineering. Number sizes run from very small (#80 = 0.343mm) up to #1 (5.791mm), then letter sizes continue from A (5.944mm) through to Z (10.490mm). What are the most common drill bit sizes for general workshop use? For general use in Australia, the most commonly used metric sizes are 3mm, 4mm, 5mm, 6mm, 8mm, and 10mm. These cover most standard fastener pilot holes and everyday drilling tasks. A 13-piece metric set from 1.5mm to 6.5mm in 0.5mm increments is the typical starter kit. Can I substitute a fractional imperial drill bit for a metric size? In most cases yes, provided the sizes are close enough for the application. The chart shows metric and imperial sizes side by side so you can identify the nearest equivalent. For precision work — tight tolerances or threaded holes — always use the exact specified size. What is the largest drill bit size covered in this chart? This chart covers drill bit sizes up to 25.4mm (1 inch / 64/64"). For larger bore sizes, use a spade bit, hole saw, or annular cutter instead of a standard twist drill. Where can I buy drill bits in Australia? AIMS Industrial stocks HSS, cobalt, and carbide drill bits for metal, timber, masonry, and general use, available online with Australia-wide delivery. For complete metric bolt sizing (M3-M24) with thread pitch and head dimensions, see our Metric Bolt Size Guide. People Also Ask — Drill Bit Size Chart: Metric, Imperial & Fractional Q: What size drill bit do I need for an M8 tap? For an M8 × 1.25 coarse thread, use a 6.8 mm drill bit. For M8 × 1.0 fine thread, use a 7.0 mm bit. Always check the specific pitch before drilling — the wrong pilot hole size is the most common cause of tap breakage or stripped threads. Q: How do I convert metric drill bits to imperial? Divide the metric diameter by 25.4 to get inches, or multiply inches by 25.4 to get millimetres. For example, a 10 mm bit equals 0.394" (closest imperial equivalent is 25/64"). A drill bit conversion chart saves calculation errors on the job. Q: What drill bit size is closest to 1/4 inch in metric? A 1/4" drill bit is exactly 6.35 mm. The closest standard metric size is 6.0 mm (slightly under) or 6.5 mm (slightly over). For clearance holes, 6.5 mm is usually the better choice; for tap pilot holes, refer to the thread specification instead. Q: What are the standard drill bit sizes in mm? Standard metric drill bit sets typically cover 1.0–13.0 mm in 0.5 mm steps, with jobber sets often including 0.5–13 mm in 0.1 mm increments. Common sizes for general trade work are 3, 4, 5, 6, 6.5, 8, 10, and 12 mm. Fractional sets cover 1/16" to 1/2" in 1/64" steps. See AIMS's full carbide drill bits range — trade pricing and Australia-wide despatch. Need machining? Browse the AIMS range at machining.
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