Allen Wrench Size Chart
Quick answer: measure a hex key across its flat sides and use the matching inch or metric size. The conversion columns are mathematical equivalents, not substitute tool sizes; a close-fitting wrong key can damage the fastener.
SAE Allen Key Sizes
This selected inch-size reference includes common fractional sizes and smaller decimal sizes. Metric values are calculated from the exact inch-to-millimeter conversion and rounded to 0.01 mm.
| 0.028 in | 0.028 | 0.71 mm |
|---|---|---|
| 0.035 in | 0.035 | 0.89 mm |
| 0.050 in | 0.050 | 1.27 mm |
| 1/16 in | 0.0625 | 1.59 mm |
| 5/64 in | 0.0781 | 1.98 mm |
| 3/32 in | 0.0938 | 2.38 mm |
| 7/64 in | 0.1094 | 2.78 mm |
| 1/8 in | 0.1250 | 3.18 mm |
| 9/64 in | 0.1406 | 3.57 mm |
| 5/32 in | 0.1563 | 3.97 mm |
| 3/16 in | 0.1875 | 4.76 mm |
| 7/32 in | 0.2188 | 5.56 mm |
| 1/4 in | 0.2500 | 6.35 mm |
| 5/16 in | 0.3125 | 7.94 mm |
| 3/8 in | 0.3750 | 9.53 mm |
15 rows
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Metric Allen Key Sizes
The inch values below are decimal conversions rounded to four places. They help compare dimensions but do not indicate that the nearest SAE key is interchangeable.
| 0.7 mm | 0.0276 in |
|---|---|
| 0.9 mm | 0.0354 in |
| 1.3 mm | 0.0512 in |
| 1.5 mm | 0.0591 in |
| 2 mm | 0.0787 in |
| 2.5 mm | 0.0984 in |
| 3 mm | 0.1181 in |
| 4 mm | 0.1575 in |
| 5 mm | 0.1969 in |
| 6 mm | 0.2362 in |
| 8 mm | 0.3150 in |
| 10 mm | 0.3937 in |
12 rows
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Choose the Right Fit
- Seat the key fully and confirm it fits snugly before applying force.
- Do not force a nearby metric or inch size into the socket; a poor fit can round the key or fastener.
- Use a ball-end key for angled access, not to break a tight fastener loose.
- Replace a key with rounded or damaged corners.
For socket-head cap screw dimensions, see the metric socket-head cap screw size chart.
Sources and Conversion
Selected available inch and metric hex-key sizes are shown in Bondhus hex-key product ranges and MISUMI’s Allen wrench chart. Inch/mm conversions use the exact 25.4 mm-per-inch definition from NIST.
