How do I convert metric hex keys to SAE Allen wrench sizes?

Convert the metric hex key size from millimeters to inches by dividing by 25.4, then compare it with common SAE Allen wrench fractions. This converter shows the closest practical match and the size difference.

What SAE Allen wrench is closest to 4 mm?

5/32 inch is the closest common SAE Allen wrench to 4 mm. It equals 3.969 mm, so it is about 0.031 mm smaller than 4 mm.

Can I use a metric hex key in an SAE socket screw?

Only when the match is very close and the screw is not tight, damaged, or high torque. Hex sockets strip easily, so use the exact marked key whenever possible.

Allen wrench size converter

Hex Key Size Converter

Convert hex key and Allen wrench sizes between metric millimeters and SAE inch fractions. Enter the size stamped on the key and see the nearest substitute, exact decimal size, and fit warning.

Convert a hex key size

Use metric sizes like 4 mm or SAE sizes like 5/32. Results update instantly.

Starting unit Hex key size Fastener condition

Closest match 4 mm ≈ 5/32 in

5/32 inch is 0.031 mm smaller than 4 mm.

Exact decimal 0.1575 in

Difference 0.031 mm

4 mm is very close to 5/32 inch, but the sizes are not identical.

Metric to SAE Allen wrench basics

Hex keys are measured across the flats of the hex shape. Metric keys use millimeters, while SAE Allen keys use inch fractions. The math is exact: 1 inch = 25.4 mm, but whether a substitute works depends on the clearance inside the screw socket.

3 mm is closest to 1/8 inch, but 1/8 inch is larger and may not fit every socket.
4 mm is closest to 5/32 inch and is a very close match.
5 mm is closest to 3/16 inch, but the difference is noticeable.

When a substitute hex key is risky

Allen screws and set screws can strip quickly when the key is undersized or loose. A close substitute may be fine for a quick check, but exact fit is best for tiny screws, tight screws, bicycles, furniture hardware, machinery, and anything you need to torque properly.

The fit note explains whether the nearest substitute is larger or smaller so you can avoid turning a small repair into a goblin-grade extraction quest.

Metric to SAE hex key size chart

Metric hex key	Closest SAE	SAE in mm	Difference

SAE to metric Allen key chart

SAE Allen key	Exact mm	Closest metric	Difference

Hex key size conversion FAQ

Is an Allen wrench the same thing as a hex key?

Yes. “Allen wrench” usually means the same L-shaped or T-handle tool as a hex key. Allen is a common name for hex socket tools.

What SAE size is closest to 4 mm?

5/32 inch is the closest common SAE Allen wrench. It equals about 3.969 mm, which is 0.031 mm smaller than 4 mm.

What metric size is closest to 3/16 inch?

5 mm is the closest common metric hex key. 3/16 inch equals 4.763 mm, so 5 mm is about 0.238 mm larger.