Allen Wrench Size Chart

Quick answer: Allen wrench sizes are measured across the flats of the hex key. Metric and SAE sizes are close in a few places, but they are not reliably interchangeable. If the key feels loose, use the exact size instead of forcing the nearest conversion.

SAE Allen Wrench Size Chart

SAE hex key	Decimal inches	Approx. mm	Typical use note
0.028 in	0.028	0.71 mm	Precision/electronics
0.035 in	0.035	0.89 mm	Precision/electronics
0.050 in	0.050	1.27 mm	Small set screws
1/16 in	0.0625	1.59 mm	Small hardware
5/64 in	0.0781	1.98 mm	Small hardware
3/32 in	0.0938	2.38 mm	Small hardware
7/64 in	0.1094	2.78 mm	Small hardware
1/8 in	0.1250	3.18 mm	Common shop size
9/64 in	0.1406	3.57 mm	Common shop size
5/32 in	0.1563	3.97 mm	Close to 4 mm, but verify fit
3/16 in	0.1875	4.76 mm	Common shop size
7/32 in	0.2188	5.56 mm	Medium hardware
1/4 in	0.2500	6.35 mm	Medium hardware
5/16 in	0.3125	7.94 mm	Larger hardware
3/8 in	0.3750	9.53 mm	Larger hardware

Metric Allen Wrench Size Chart

Metric hex key	Approx. inches	Common note
0.7 mm	0.0276 in	Precision size
0.9 mm	0.0354 in	Precision size
1.3 mm	0.0512 in	Precision size
1.5 mm	0.0591 in	Small metric hardware
2 mm	0.0787 in	Small metric hardware
2.5 mm	0.0984 in	Small metric hardware
3 mm	0.1181 in	Common metric size
4 mm	0.1575 in	Common metric size
5 mm	0.1969 in	Common metric size
6 mm	0.2362 in	Common metric size
8 mm	0.3150 in	Larger metric hardware
10 mm	0.3937 in	Larger metric hardware

See also O Ring Size Chart from Parker: AS568 Dash Sizes and Dimensions

Allen Key Fit Tips

Seat the key fully before applying torque.
Use ball-end keys for access, not for breaking tight fasteners loose.
Switch to a fresh key if the corners are rounded.

For socket-head cap screws, also check the metric socket head cap screw size chart.

Source Notes

Sizes and conversions were checked against Allen/hex-key references including Misumi’s Allen wrench size chart, hand-tool hex key charts, and RS Components hex key guidance. Close conversions are approximate.