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Allen Wrench Sizes: Metric and SAE Hex Key Chart

By Md. Asrafuzzaman (MA Zaman) · Updated

An Allen wrench, or hex key, is sized by the distance across the flats of its six-sided tip. Metric sets run from 1.5 to 10 mm and SAE (inch) sets from 1/16 to 1/2 in. An M6 socket cap screw takes a 5 mm key, and a 1/4-20 socket cap screw takes a 3/16 in key. Metric and SAE keys only match closely at a few sizes, so use the exact size whenever you can.

Allen wrench size for metric screws

ScrewHex key size
M32.5 mm
M43 mm
M54 mm
M65 mm
M86 mm
M108 mm
M1210 mm

These sizes follow the standard for socket head cap screws (ISO 4762). Button head and flat head screws can use a smaller key for the same thread, so check the screw head if it is not a socket head cap screw.

Allen wrench size for inch screws

ScrewHex key size
#43/32 in
#67/64 in
#89/64 in
#105/32 in
1/4 in3/16 in
5/16 in1/4 in
3/8 in5/16 in
7/16 in3/8 in
1/2 in3/8 in
5/8 in1/2 in

Metric to SAE hex key conversion chart

MetricNearest SAEGap
1.5 mm1/16 in (1.59 mm)SAE 0.09 mm larger
2 mm5/64 in (1.98 mm)SAE 0.02 mm smaller
2.5 mm3/32 in (2.38 mm)SAE 0.12 mm smaller
3 mm1/8 in (3.18 mm)SAE 0.18 mm larger
4 mm5/32 in (3.97 mm)SAE 0.03 mm smaller
5 mm3/16 in (4.76 mm)SAE 0.24 mm smaller
6 mm1/4 in (6.35 mm)SAE 0.35 mm larger
8 mm5/16 in (7.94 mm)SAE 0.06 mm smaller
10 mm3/8 in (9.53 mm)SAE 0.47 mm smaller

A key that is larger than the socket will not go in, and a key that is smaller is loose and rounds the socket. The pairs within about 0.1 mm are 1.5 mm and 1/16 in, 2 mm and 5/64 in, 4 mm and 5/32 in, and 8 mm and 5/16 in. Even there, the key that is a hair too big will not go in, and the one that is a hair small fits loosely. Every other pair is further apart.

Types of hex keys

  • L-keys are the standard bent key, with a short arm for torque and a long arm for reach.
  • Ball-end keys let you turn the screw at an angle, roughly 25 degrees off axis,, but a ball end gives less contact and less torque than a straight tip.
  • T-handle keys are easier to turn and give a better grip.
  • Hex bits fit a drill chuck or a ratchet, which is faster for repeated work.

How to avoid stripping a hex screw

  1. Pick the exact size, and push the key fully into the socket.
  2. Clean dirt and paint out of the socket first.
  3. Turn with steady pressure, not a jerk.
  4. Use the short arm of an L-key for the final tightening, so you do not overstress the key.

Frequently asked questions

What size Allen wrench fits an M8 screw?

A 6 mm key fits an M8 socket head cap screw. M3 takes 2.5 mm, M4 takes 3 mm, M5 takes 4 mm, M6 takes 5 mm, M10 takes 8 mm and M12 takes 10 mm.

Is an Allen wrench the same as a hex key?

Yes. Allen is a brand name that became the common term. Both mean a hex key with a six-sided tip.

What is the difference between an Allen wrench and a Torx key?

An Allen wrench has a six-sided hex tip. A Torx key has a six-pointed star tip. They are not interchangeable, and a hex key in a Torx screw will strip it.

Why does my Allen key slip?

Usually the key is a size too small, or the socket is worn or full of debris. Try the next size up, clean the socket, and replace worn keys.

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