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Hex Key (Allen) Size Chart
Quick answer: Hex keys are measured across the flats. 5/64″ and 2 mm are effectively identical (0.6 thou apart) and 5/32″ and 4 mm are 1.2 thou apart. Most other pairs are too far off to substitute safely.
SAE and metric hex keys side by side with the exact gap between them — and a straight answer on which substitutions will round out the socket.
Hex key (Allen wrench) size chart
Hex keys are measured across the flats, the same as a wrench opening. The chart pairs each SAE key with its nearest metric neighbour and gives the gap in thousandths.
| SAE | Decimal (in) | Metric | Decimal (in) | Gap (thou) | Verdict |
|---|---|---|---|---|---|
| 0.028 | 0.0280 | 0.7 mm | 0.0276 | 0.4 | Interchangeable |
| 0.035 | 0.0350 | 0.9 mm | 0.0354 | 0.4 | Interchangeable |
| 1/16 | 0.0625 | 1.5 mm | 0.0591 | 3.4 | Interchangeable |
| 5/64 | 0.0781 | 2 mm | 0.0787 | 0.6 | Interchangeable |
| 3/32 | 0.0938 | 2.5 mm | 0.0984 | 4.6 | Snug — care |
| 7/64 | 0.1094 | 2.5 mm | 0.0984 | 11.0 | Do not substitute |
| 1/8 | 0.1250 | 3 mm | 0.1181 | 6.9 | Snug — care |
| 9/64 | 0.1406 | 3.5 mm | 0.1378 | 2.8 | Interchangeable |
| 5/32 | 0.1563 | 4 mm | 0.1575 | 1.2 | Interchangeable |
| 3/16 | 0.1875 | 5 mm | 0.1969 | 9.4 | Do not substitute |
| 7/32 | 0.2188 | 5.5 mm | 0.2165 | 2.3 | Interchangeable |
| 1/4 | 0.2500 | 6 mm | 0.2362 | 13.8 | Do not substitute |
| 5/16 | 0.3125 | 8 mm | 0.3150 | 2.5 | Interchangeable |
| 3/8 | 0.3750 | 10 mm | 0.3937 | 18.7 | Do not substitute |
Hex sockets round out far more readily than external fasteners because the driving faces are small and the tool is often short. Treat “snug” here as a last resort, not a substitution.
Where you will meet each system
- Metric hex — flat-pack furniture, bicycles, most machinery built outside the US, and nearly all electronics enclosures.
- SAE hex — US-built equipment, older American machinery, some plumbing and HVAC fittings.
- Ball-end keys — let you drive at up to about 25° off axis. Convenient, but they contact less metal, so break them out for access and switch to a straight key for final torque.
Torx is not hex
A six-lobed Torx socket looks superficially similar in a dark recess but has curved lobes rather than flats. Forcing a hex key into a Torx socket destroys both. Torx sizes run T6 to T60 in common use; if the recess has six rounded points rather than six flat sides, you need a Torx driver.
Getting a rounded hex socket out
- Try the next size up. A metric key tapped into an SAE socket, or vice versa, sometimes bites where the correct size no longer does.
- Torx trick. A Torx bit one size larger than the hex opening, tapped in, often grips the damaged flats.
- Screw extractor. Drill a pilot into the fastener head and use a left-hand extractor.
- Drill the head off. Last resort — drill until the head separates, remove the part, then grip the exposed shank with pliers.
Frequently asked questions
What size Allen key is closest to 1/4 inch?
6 mm. A 1/4 in key is 0.2500 in and 6 mm is 0.2362 in, a gap of 13.8 thousandths — too loose to be safe. Use the correct 1/4 in key; substituting here is a common way to round the socket.
Is 5mm the same as 3/16 inch?
Very close. 3/16 in is 0.1875 in and 5 mm is 0.1969 in, 9.4 thousandths apart. The 5 mm will not enter a 3/16 in socket, and a 3/16 in key in a 5 mm socket is loose enough to be risky under torque.
How are hex keys measured?
Across the flats — the distance between two opposite faces of the hexagon, exactly as a wrench opening is measured. It is not the point-to-point distance.
What do I do if the hex socket is rounded?
Try tapping in a Torx bit one size larger than the hex opening — the lobes often bite on the damaged flats. Failing that, a screw extractor, or drilling the head off entirely and backing the shank out with pliers.