Allen Wrench Size Chart
Hex key sizes in order with SAE and metric equivalents, how close each pair is, and what each size is normally used on.
Quick answer
The genuinely interchangeable hex key pairs are 5/32" ↔ 4 mm (0.03 mm apart) and 3/32" ↔ 2.5 mm. 1/8" is 3.18 mm against a 3 mm key, and 3/16" is 4.76 mm against 5 mm — both loose enough to round a hardened set screw. Electrical lug set screws are usually 5/32", 3/16" or 7/32".
Hex key sizes — SAE and metric
| SAE | Decimal (in) | Exact (mm) | Nearest metric | Difference (mm) | Typical use |
|---|---|---|---|---|---|
| .050" | 0.0500 | 1.27 | 1.3 mm | 0.03 | Small set screws, electronics |
| 1/16" | 0.0625 | 1.59 | 1.5 mm | 0.09 | Set screws on knobs and collars |
| 5/64" | 0.0781 | 1.98 | 2 mm | 0.02 | Small machine screws |
| 3/32" | 0.0938 | 2.38 | 2.5 mm | 0.12 | Cabinet and furniture hardware |
| 7/64" | 0.1094 | 2.78 | 2.5 mm | 0.28 | Small socket screws |
| 1/8" | 0.1250 | 3.17 | 3 mm | 0.18 | Lugs, terminal set screws |
| 9/64" | 0.1406 | 3.57 | 3.5 mm | 0.07 | Panel and enclosure screws |
| 5/32" | 0.1563 | 3.97 | 4 mm | 0.03 | Common electrical lug set screws |
| 3/16" | 0.1875 | 4.76 | 5 mm | 0.24 | Mechanical lugs, busbar hardware |
| 7/32" | 0.2188 | 5.56 | 5.5 mm | 0.06 | Larger lugs and connectors |
| 1/4" | 0.2500 | 6.35 | 6 mm | 0.35 | Structural socket cap screws |
| 5/16" | 0.3125 | 7.94 | 8 mm | 0.06 | Heavy socket cap screws |
| 3/8" | 0.3750 | 9.52 | 10 mm | 0.47 | Large mechanical connectors |
| 1/2" | 0.5000 | 12.70 | 12 mm | 0.70 | Heavy machinery hardware |
Published standard — source: ASME B18.3 hex key nominal sizes with exact metric conversion
Source & method: Green differences are safe substitutions, amber are usable with care, red should not be substituted.
Frequently asked
- What metric Allen key replaces 5/32 inch?
- 4 mm is closest to 5/32 inch. 5/32 inch is 3.97 mm, so a 4 mm key is only 0.03 mm larger — effectively interchangeable, which matters because 5/32 is the common electrical lug set-screw size.
- Are Allen key SAE and metric sizes interchangeable?
- A few pairs are: 5/32 and 4 mm, 3/16 and 5 mm are close, and 1/8 and 3 mm are close. Most other pairs differ enough to strip the socket, especially on hardened set screws.
- What size Allen key for electrical lugs?
- Mechanical lugs on panels and disconnects commonly use 5/32, 3/16 or 7/32 inch hex set screws, with 1/4 and 3/8 inch on large feeder and busbar connectors. Always torque to the label value.
- What is the difference between an Allen key and a hex key?
- Nothing. Allen is a brand name that became the common term for a hexagonal socket-drive key, standardised as ASME B18.3 hex keys.
- Why do hex keys strip so easily?
- A slightly undersized key contacts only the corners of the socket. Using an SAE key on a metric screw, or a worn key, rounds the socket after very little torque.
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