Drill Bit Size Chart: Metric, Imperial & Fractional

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.

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.

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 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.
• Socket Size Chart — match metric and imperial socket sizes with drive size guide.
• 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 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 on this site, which lists the correct drill size for each thread specification.

How do I convert a metric drill bit size to imperial?

Find your metric size in the millimetres column in the chart above and read across to the fractional or decimal inches columns. For example, 6mm is approximately 15/64" or 0.2362 inches.

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.

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, you would typically use a spade bit, hole saw, or annular cutter.

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.