How to Identify a Drill Bit
- Check the size stamped or engraved on the shank first.
- Metric bits are normally marked in millimetres, such as 6 mm or 6.5 mm.
- Imperial bits are commonly marked as fractions of an inch, such as 1/4" or 3/8". Some specialist sets also use number or letter sizes.
- If the marking is missing, measure the shank or undamaged cutting diameter with suitable calipers.
- When comparing metric and imperial bits, convert both to millimetres or decimal inches rather than judging by appearance.
Important: A close metric/imperial size is not automatically the correct substitute. Pilot holes, tapping holes, wall plugs and precision work may require a specific diameter.
Metric & Imperial Drill Size Comparison
Use this table to compare common metric drill bits with nearby fractional-inch sizes. The rows are ordered approximately by diameter so close sizes are easy to spot.
| Metric size | Metric in inches | Imperial size | Imperial in mm |
|---|---|---|---|
| 1.0 mm | 0.0394" | 3/64" | 1.1906 mm |
| 1.5 mm | 0.0591" | 1/16" | 1.5875 mm |
| 2.0 mm | 0.0787" | 5/64" | 1.9844 mm |
| 2.5 mm | 0.0984" | 3/32" | 2.3813 mm |
| 3.0 mm | 0.1181" | 1/8" | 3.1750 mm |
| 3.5 mm | 0.1378" | 9/64" | 3.5719 mm |
| 4.0 mm | 0.1575" | 5/32" | 3.9688 mm |
| 4.5 mm | 0.1772" | 11/64" | 4.3656 mm |
| 5.0 mm | 0.1969" | 13/64" | 5.1594 mm |
| 5.5 mm | 0.2165" | 7/32" | 5.5563 mm |
| 6.0 mm | 0.2362" | 15/64" | 5.9531 mm |
| 6.5 mm | 0.2559" | 1/4" | 6.3500 mm |
| 7.0 mm | 0.2756" | 9/32" | 7.1438 mm |
| 7.5 mm | 0.2953" | 19/64" | 7.5406 mm |
| 8.0 mm | 0.3150" | 5/16" | 7.9375 mm |
| 8.5 mm | 0.3346" | 21/64" | 8.3344 mm |
| 9.0 mm | 0.3543" | 23/64" | 9.1281 mm |
| 9.5 mm | 0.3740" | 3/8" | 9.5250 mm |
| 10.0 mm | 0.3937" | 25/64" | 9.9219 mm |
| 10.5 mm | 0.4134" | 13/32" | 10.3188 mm |
| 11.0 mm | 0.4331" | 7/16" | 11.1125 mm |
| 11.5 mm | 0.4528" | 29/64" | 11.5094 mm |
| 12.0 mm | 0.4724" | 15/32" | 11.9063 mm |
| 12.5 mm | 0.4921" | 1/2" | 12.7000 mm |
| 13.0 mm | 0.5118" | 33/64" | 13.0969 mm |
Quick conversion: 1 inch = 25.4 mm exactly. To convert inches to millimetres, multiply by 25.4. To convert millimetres to inches, divide by 25.4.
Choosing the Correct Drill Size
| Task | What to check |
|---|---|
| Wall plug / anchor | Use the drill diameter specified by the plug or anchor manufacturer. Do not select the bit only from the screw size. |
| Pilot hole in timber | Choose the pilot size for the screw, timber hardness and thread design. Hard timber commonly needs a larger pilot than soft timber. |
| Clearance hole | The hole must be large enough for the fastener shank to pass through as required. |
| Thread tapping | Use the specified tapping drill for the exact thread diameter and pitch. Do not simply use the nearest-looking drill. |
| Masonry | Use a masonry bit suitable for the substrate and the fixing manufacturer's specified diameter. |
| Metal | Use a suitable metal-cutting bit and select speed, lubrication and technique for the material. |
Metric & Imperial Examples
6 mm versus 15/64": 15/64" is 5.9531 mm, only about 0.047 mm smaller than 6 mm. They are very close, but whether substitution is acceptable depends on the job.
6.5 mm versus 1/4": 1/4" is 6.35 mm, so it is 0.15 mm smaller than 6.5 mm.
10 mm versus 25/64": 25/64" is 9.9219 mm, about 0.078 mm smaller than 10 mm.
Practical Notes
- Confirm hidden electrical cables, pipes and other services before drilling into walls, floors or ceilings.
- Use appropriate eye protection and follow the drill, bit and fixing manufacturer's instructions.
- Secure the workpiece where appropriate rather than holding small material by hand.
- Replace damaged, bent or badly worn drill bits.
- For precision holes, confirm the required finished diameter rather than relying on a nearby metric/imperial equivalent.