Across-flats size, short arm and long arm lengths for inch hex keys (Allen wrenches, ASME B18.3) and metric hex keys (ISO 2936), and which key fits which socket screw.
A hex key (Allen wrench, L-key) is a six-sided bar bent at a right angle. Its size is the distance across the flats of the hex, the same as the socket in the screw it turns. Inch keys come in a short series and a long series: the same hex and short arm, with a longer long arm for reach. Metric keys come in standard, long and extra-long lengths.
The letters match the table columns. Not to scale.
| Key size | Across flats W | Across corners Y | Short arm B | Long arm C, short series | Long arm C, long series |
|---|---|---|---|---|---|
| .028″ | .0280 / .0275 | .0314 / .0300 | .312 / .125 | 1.312 / 1.125 | 2.688 / 2.500 |
| .035″ | .0350 / .0345 | .0393 / .0378 | .438 / .250 | 1.312 / 1.125 | 2.766 / 2.578 |
| .050″ | .0500 / .0490 | .0560 / .0540 | .625 / .438 | 1.750 / 1.562 | 2.938 / 2.750 |
| 1/16″ | .0625 / .0615 | .0701 / .0680 | .656 / .469 | 1.844 / 1.656 | 3.094 / 2.906 |
| 5/64″ | .0781 / .0771 | .0880 / .0859 | .703 / .516 | 1.969 / 1.781 | 3.281 / 3.094 |
| 3/32″ | .0937 / .0927 | .1058 / .1035 | .750 / .562 | 2.094 / 1.906 | 3.469 / 3.281 |
| 7/64″ | .1094 / .1079 | .1238 / .1210 | .797 / .609 | 2.219 / 2.031 | 3.656 / 3.469 |
| 1/8″ | .1250 / .1235 | .1418 / .1390 | .844 / .656 | 2.344 / 2.156 | 3.844 / 3.656 |
| 9/64″ | .1406 / .1391 | .1593 / .1566 | .891 / .703 | 2.469 / 2.281 | 4.031 / 3.844 |
| 5/32″ | .1562 / .1547 | .1774 / .1745 | .938 / .750 | 2.594 / 2.406 | 4.219 / 4.031 |
| 3/16″ | .1875 / .1860 | .2135 / .2105 | 1.031 / .844 | 2.844 / 2.656 | 4.594 / 4.406 |
| 7/32″ | .2187 / .2172 | .2490 / .2460 | 1.125 / .938 | 3.094 / 2.906 | 4.969 / 4.781 |
| 1/4″ | .2500 / .2485 | .2845 / .2815 | 1.219 / 1.031 | 3.344 / 3.156 | 5.344 / 5.156 |
| 5/16″ | .3125 / .3110 | .3570 / .3531 | 1.344 / 1.156 | 3.844 / 3.656 | 6.094 / 5.906 |
| 3/8″ | .3750 / .3735 | — / .4238 | 1.469 / 1.281 | 4.344 / 4.156 | 6.844 / 6.656 |
| 7/16″ | .4375 / .4355 | — / .4944 | 1.594 / 1.406 | 4.844 / 4.656 | 7.594 / 7.406 |
| 1/2″ | .5000 / .4975 | — / .5650 | 1.719 / 1.531 | 5.344 / 5.156 | 8.344 / 8.156 |
| 9/16″ | .5625 / .5600 | — / .6356 | 1.844 / 1.656 | 5.844 / 5.656 | 9.094 / 8.906 |
| 5/8″ | .6250 / .6225 | — / .7080 | 1.969 / 1.781 | 6.344 / 6.156 | 9.844 / 9.656 |
| 3/4″ | .7500 / .7470 | — / .8512 | 2.219 / 2.031 | 7.344 / 7.156 | 11.344 / 11.156 |
| 7/8″ | .8750 / .8720 | — / .9931 | 2.469 / 2.281 | 8.344 / 8.156 | 12.844 / 12.656 |
| 1″ | 1.0000 / .9970 | 1.1470 / 1.1350 | 2.719 / 2.531 | 9.344 / 9.156 | 14.344 / 14.156 |
| 1-1/4″ | 1.2500 / 1.2430 | — | 3.250 / 2.750 | 11.500 / 11.000 | — |
| 1-1/2″ | 1.5000 / 1.4930 | — | 3.750 / 3.250 | 13.500 / 13.000 | — |
| 1-3/4″ | — | — | — | — | — |
| 2″ | — | — | — | — | — |
| 2-1/4″ | — | — | — | — | — |
| 2-3/4″ | — | — | — | — | — |
| 3″ | — | — | — | — | — |
Each cell: maximum / minimum, inches. Short series and long series keys have the same hex and short arm; only the long arm differs. A dash: our sources differ, or only one lists it — give us a call.
| Key size | Across flats s | Across corners e | Short arm l2 | Long arm l1, standard | l1, long | l1, extra-long |
|---|---|---|---|---|---|---|
| 0.7 mm | 0.71 / 0.70 | 0.79 / 0.76 | 7 / 5 | 33 / 31 | — | — |
| 0.9 mm | 0.89 / 0.88 | 0.99 / 0.96 | 11 / 9 | 33 / 31 | — | — |
| 1.3 mm | 1.27 / 1.24 | 1.42 / 1.37 | 13 / 11 | 41 / 39 | 63.5 / 61.5 | 81 / 79 |
| 1.5 mm | 1.50 / 1.48 | 1.68 / 1.63 | 15.5 / 13.5 | 46.5 / 44.5 | 63.5 / 61.5 | 91.5 / 89.5 |
| 2 mm | 2.00 / 1.96 | 2.25 / 2.18 | 18 / 16 | 52 / 50 | 77 / 75 | 102 / 100 |
| 2.5 mm | 2.50 / 2.46 | 2.82 / 2.75 | 20.5 / 18.5 | 58.5 / 54.5 | 87.5 / 83.5 | 114.5 / 110.5 |
| 3 mm | 3.00 / 2.96 | 3.39 / 3.31 | 23 / 21 | 66 / 62 | 93 / 89 | 129 / 125 |
| 3.5 mm | 3.50 / 3.45 | 3.96 / 3.91 | 25.5 / 23.5 | 69.5 / 65.5 | 98.5 / 94.5 | 140 / 136 |
| 4 mm | 4.00 / 3.95 | 4.53 / — | 29 / 27 | 74 / 70 | 104 / 100 | 144 / 140 |
| 4.5 mm | 4.50 / 4.45 | 5.10 / 5.04 | 30.5 / 28.5 | 80 / 76 | 114.5 / 110.5 | 156 / 152 |
| 5 mm | 5.00 / 4.95 | 5.67 / — | 33 / 31 | 85 / 81 | 120 / 116 | 165 / 161 |
| 6 mm | 6.00 / 5.95 | 6.81 / — | 38 / 36 | 96 / 92 | 141 / 137 | 186 / 182 |
| 7 mm | 7.00 / 6.94 | — / 7.85 | 41 / 39 | 102 / 96 | 147 / 141 | 197 / 191 |
| 8 mm | 8.00 / 7.94 | 9.09 / 8.97 | 44 / 42 | 108 / 102 | 158 / 152 | 208 / 202 |
| 9 mm | 9.00 / 8.94 | 10.23 / 10.10 | 47 / 45 | 114 / 108 | 169 / 163 | 219 / 213 |
| 10 mm | 10.00 / 9.94 | 11.37 / 11.23 | 50 / 48 | 122 / 116 | 180 / 174 | 234 / 228 |
| 11 mm | 11.00 / 10.89 | 12.51 / 12.31 | 53 / 51 | 129 / 123 | 191 / 185 | 247 / 241 |
| 12 mm | 12.00 / 11.89 | 13.65 / 13.44 | 57 / 55 | 137 / 131 | 202 / 196 | 262 / 256 |
| 13 mm | 13.00 / 12.89 | 14.79 / — | 63 / 60 | 145 / 138 | 213 / 206 | 277 / 270 |
| 14 mm | 14.00 / 13.89 | 15.93 / 15.70 | 70 / 67 | 154 / 147 | 229 / 222 | 294 / 287 |
| 15 mm | 15.00 / 14.89 | 17.07 / 16.83 | 73 / 70 | 161 / 154 | 240 / 233 | 307 / 300 |
| 16 mm | 16.00 / 15.89 | 18.21 / — | 76 / 73 | 168 / 161 | 240 / 233 | 307 / 300 |
| 17 mm | 17.00 / 16.89 | 19.35 / 19.09 | 80 / 77 | 177 / 170 | 262 / 255 | 337 / 330 |
| 18 mm | 18.00 / 17.89 | 20.49 / — | 84 / 81 | 188 / 181 | 262 / 255 | 358 / 351 |
| 19 mm | 19.00 / 18.87 | 21.63 / 21.32 | 89 / 86 | 199 / 192 | — | — |
| 21 mm | 21.00 / 20.87 | 23.91 / 23.58 | 96 / 91 | 211 / 199 | — | — |
| 22 mm | 22.00 / 21.87 | 25.05 / 24.71 | 102 / 97 | 222 / 210 | — | — |
| 23 mm | 23.00 / 22.87 | — | 108 / 103 | 233 / 221 | — | — |
| 24 mm | 24.00 / 23.87 | 27.33 / 26.97 | 114 / 109 | 248 / 236 | — | — |
| 27 mm | 27.00 / 26.87 | 30.75 / 30.36 | 127 / 122 | 277 / 265 | — | — |
| 29 mm | 29.00 / 28.87 | 33.03 / — | 141 / 136 | 311 / 299 | — | — |
| 30 mm | 30.00 / 29.87 | 34.17 / 33.75 | 142 / 137 | 315 / 303 | — | — |
| 32 mm | 32.00 / 31.84 | 36.45 / 35.98 | 157 / 152 | 347 / 335 | — | — |
| 36 mm | 36.00 / 35.84 | 41.01 / 40.50 | 176 / 171 | 391 / 379 | — | — |
Each cell: maximum / minimum, mm. The short arm is the same for every length. A dash: our sources differ, or the standard doesn’t make that length — give us a call.
| Key size | Socket head cap | Button head | Flat head | Set screw | Shoulder (shoulder dia) |
|---|---|---|---|---|---|
| .028″ | — | — | — | #0 | — |
| .035″ | — | — | #0 | #1, #2 | — |
| .050″ | #0 | — | #1, #2 | #3, #4 | — |
| 1/16″ | #1 | #4 | #3, #4 | #5, #6 | — |
| 5/64″ | #2, #3 | #6 | #5, #6 | #8 | — |
| 3/32″ | #4, #5 | #8 | #8 | #10 | — |
| 7/64″ | #6 | — | — | — | — |
| 1/8″ | — | #10 | #10 | 1/4″ | 1/4″ |
| 9/64″ | #8 | — | — | — | — |
| 5/32″ | #10 | 1/4″ | 1/4″ | 5/16″ | 5/16″ |
| 3/16″ | 1/4″ | 5/16″ | 5/16″ | 3/8″ | 3/8″ |
| 7/32″ | — | 3/8″ | 3/8″ | 7/16″ | — |
| 1/4″ | 5/16″ | — | 7/16″ | 1/2″ | 1/2″ |
| 5/16″ | 3/8″ | 1/2″ | 1/2″ | 5/8″ | 5/8″ |
| 3/8″ | 7/16″, 1/2″ | 5/8″ | 5/8″ | 3/4″ | 3/4″ |
| 1/2″ | 5/8″ | — | 3/4″ | 7/8″ | 1″ |
| 9/16″ | — | — | 7/8″ | 1″, 1-1/8″ | — |
| 5/8″ | 3/4″ | — | 1″ | 1-1/4″, 1-3/8″ | 1-1/4″ |
| 3/4″ | 7/8″, 1″ | — | — | 1-1/2″ | — |
| 7/8″ | 1-1/8″, 1-1/4″ | — | — | — | 1-1/2″ |
| 1″ | 1-3/8″, 1-1/2″ | — | — | — | 1-3/4″ |
| 1-1/4″ | 1-3/4″ | — | — | — | 2″ |
| 1-1/2″ | 2″ | — | — | — | — |
| Key size | Socket head cap (DIN 912) | Low head (DIN 7984) | Flat head (DIN 7991) | Button head (ISO 7380) | Set screw |
|---|---|---|---|---|---|
| 0.7 mm | — | — | — | — | M1.6 |
| 0.9 mm | — | — | — | — | M2 |
| 1.3 mm | — | — | — | — | M2.5 |
| 1.5 mm | M2 | — | — | — | M3 |
| 2 mm | M2.5 | M3 | M3 | M3 | M4 |
| 2.5 mm | M3 | M4 | M4 | M4 | M5 |
| 3 mm | M4 | M5 | M5 | M5 | M6 |
| 4 mm | M5 | M6 | M6 | M6 | M8 |
| 5 mm | M6 | M8 | M8 | M8 | M10 |
| 6 mm | M8 | — | — | M10 | M12 |
| 7 mm | — | M10 | — | — | — |
| 8 mm | M10 | M12 | M12 | M12 | M16 |
| 10 mm | M12 | M14 | M14, M16 | M16 | M20 |
| 12 mm | M14 | M16 | M20 | — | M24 |
| 14 mm | M16 | M20 | M24 | — | — |
| 17 mm | M20 | — | — | — | — |
| 19 mm | M24 | — | — | — | — |
| 22 mm | M30 | — | — | — | — |
| 27 mm | M36 | — | — | — | — |
| 32 mm | M42 | — | — | — | — |
| 36 mm | M48 | — | — | — | — |
Good to know
Inch and metric keys don’t mix. A 5/32″ key is .1547″–.1562″ across the flats; a 4 mm key is 3.95–4.00 mm. Close is not the same: the wrong key rocks in the socket and rounds it out. Use the key the screw was made for.
Use the short arm in the screw for the most turning force; the long arm gives reach and speed. A ball-end key turns a screw at an angle but carries less force — break it loose and do the final tightening with a straight hex end. Ball-end sizes and torque ratings aren’t on our chart — we don’t have two sources that agree on them.
Quick answers
- What size Allen wrench fits a 1/4-20 socket head cap screw?
- A 3/16″ hex key (ASME B18.3).
- What size hex key fits an M6 socket head cap screw?
- A 5 mm hex key (DIN 912 / ISO 4762).
- What hex key fits each inch socket head cap screw?
- #0 screw – .050″ key, #1 screw – 1/16″ key, #2 screw – 5/64″ key, #3 screw – 5/64″ key, #4 screw – 3/32″ key, #5 screw – 3/32″ key, #6 screw – 7/64″ key, #8 screw – 9/64″ key, #10 screw – 5/32″ key, 1/4″ screw – 3/16″ key, 5/16″ screw – 1/4″ key, 3/8″ screw – 5/16″ key, 7/16″ screw – 3/8″ key, 1/2″ screw – 3/8″ key, 5/8″ screw – 1/2″ key, 3/4″ screw – 5/8″ key, 7/8″ screw – 3/4″ key, 1″ screw – 3/4″ key, 1-1/8″ screw – 7/8″ key, 1-1/4″ screw – 7/8″ key, 1-3/8″ screw – 1″ key, 1-1/2″ screw – 1″ key, 1-3/4″ screw – 1-1/4″ key, 2″ screw – 1-1/2″ key.
- How long is a 1/4 inch hex key?
- Short arm 1.031″–1.219″. Long arm 3.156″–3.344″ (short series) or 5.156″–5.344″ (long series), min–max, ASME B18.3.
- Is a 5/32 Allen wrench the same as a 4 mm?
- No. A 5/32″ key is .1547″–.1562″ across the flats; a 4 mm key is 3.95–4.00 mm. Use the key the screw was made for.