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.

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.

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.

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.

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.

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.

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.

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.

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.