Allen Wrench (Hex Key) Size Chart
The hex key that fits a socket head cap screw of a given size, standard imperial and metric.
Imperial (ASME B18.3)
| Screw size | Hex key size |
|---|---|
| #0 | 0.050" |
| #1 | 1/16" |
| #2, #3 | 5/64" |
| #4, #5 | 3/32" |
| #6 | 7/64" |
| #8 | 9/64" |
| #10 | 5/32" |
| 1/4" | 3/16" |
| 5/16" | 1/4" |
| 3/8" | 5/16" |
| 7/16" | 3/8" |
| 1/2" | 3/8" |
| 9/16" | 7/16" |
| 5/8" | 1/2" |
| 3/4" | 5/8" |
| 7/8" | 3/4" |
| 1" | 3/4" |
Metric (DIN 912)
| Screw size | Hex key size |
|---|---|
| M2 | 1.5 mm |
| M2.5 | 2 mm |
| M3 | 2.5 mm |
| M4 | 3 mm |
| M5 | 4 mm |
| M6 | 5 mm |
| M8 | 6 mm |
| M10 | 8 mm |
| M12 | 10 mm |
| M14 | 12 mm |
| M16 | 14 mm |
| M20 | 17 mm |
This is for socket head cap screws specifically
These pairings apply to standard socket head cap screws (ASME B18.3 imperial, DIN 912 metric) — button head, flat head, and low-head socket screws are usually the same hex size as the standard socket head at a given thread size, but set screws and some specialty fasteners can differ. Check the fastener's own spec if it's not a standard socket head cap screw.
Why the jump from 1/2" to 5/8" isn't proportional
Notice 7/16" and 1/2" screws share the same 3/8" hex key — hex key sizes don't scale in perfect lockstep with screw size, since the standard was set to balance a reasonable number of distinct wrench sizes against the range of screws in common use. Always check a chart rather than assuming a pattern continues linearly.
Ball-end vs. standard hex keys
A ball-end hex key uses the same across-flats size as a standard hex key for a given screw — the ball tip just allows driving at an angle, at the cost of significantly reduced torque capacity compared to a fully seated standard hex key. Don't reach for a ball-end key when breaking a stubborn or highly-torqued screw loose; switch to a standard key seated straight in.