Tools reference
Allen key size guide for socket-head screws
Common metric and inch internal-hex drive sizes, separated by head profile so a button head is not mistaken for a socket-head cap screw.
Reviewed against GaugeFox data and the sources below · Updated August 4, 2026
Metric internal-hex drives
| Thread | Socket-head cap | Button head | Flat / countersunk |
|---|---|---|---|
| M2 | 1.5 mm | — | — |
| M2.5 | 2 mm | — | — |
| M3 | 2.5 mm | 2 mm | 2 mm |
| M4 | 3 mm | 2.5 mm | 2.5 mm |
| M5 | 4 mm | 3 mm | 3 mm |
| M6 | 5 mm | 4 mm | 4 mm |
| M8 | 6 mm | 5 mm | 5 mm |
| M10 | 8 mm | 6 mm | 6 mm |
| M12 | 10 mm | 8 mm | 8 mm |
| M14 | 12 mm | — | — |
| M16 | 14 mm | 10 mm | 10 mm |
Inch internal-hex drives
| Thread | Socket-head cap | Button head | Flat / countersunk |
|---|---|---|---|
| #4 | 3/32 in | 1/16 in | 1/16 in |
| #6 | 7/64 in | 5/64 in | 5/64 in |
| #8 | 9/64 in | 3/32 in | 3/32 in |
| #10 | 5/32 in | 1/8 in | 1/8 in |
| 1/4 in | 3/16 in | 5/32 in | 5/32 in |
| 5/16 in | 1/4 in | 3/16 in | 3/16 in |
| 3/8 in | 5/16 in | 7/32 in | 7/32 in |
| 1/2 in | 3/8 in | — | — |
| 5/8 in | 1/2 in | — | — |
| 3/4 in | 5/8 in | — | — |
How to check the recess without stripping it
- Clean dirt or paint from the recess before judging the size.
- Seat the key all the way to the bottom.
- Try the largest key that enters without force.
- Reject a key that rocks noticeably before torque is applied.
- If neither metric nor inch fits cleanly, inspect for wear, Torx, or another drive type.
Reference trail
Sources and review method
These source pages identify the standards or industry references used for this guide. Full standards may require purchase. The guide is an identification aid, not a substitute for a calibrated inspection or the complete applicable standard.
Read the editorial standardUse it at the bench
Put these references on your Android screen.
GaugeFox adds calibrated on-screen gauges, searchable nearby sizes, and a local Parts Bin to the reference data in this guide.