1/64 = 0.0156, 1/32 = 0.0313, 1/16 = 0.0625, 1/8 = 0.125, 3/16 = 0.1875, 1/4 = 0.25, 5/16 = 0.3125, 3/8 = 0.375, 1/2 = 0.5, 5/8 = 0.625, 3/4 = 0.75, 7/8 = 0.875 inch. One inch is exactly 25.4 mm.
Machinists work in thousandths, drawings and drill sets in fractions, and imported machines in millimeters. This is the conversion table for all three.
Tools that go with this chart
Fractions of an inch
| Fraction | Decimal (in) | mm |
|---|---|---|
| 1/64" | 0.01563 | 0.397 |
| 1/32" | 0.03125 | 0.794 |
| 3/64" | 0.04688 | 1.191 |
| 1/16" | 0.06250 | 1.587 |
| 5/64" | 0.07813 | 1.984 |
| 3/32" | 0.09375 | 2.381 |
| 7/64" | 0.10938 | 2.778 |
| 1/8" | 0.12500 | 3.175 |
| 9/64" | 0.14063 | 3.572 |
| 5/32" | 0.15625 | 3.969 |
| 11/64" | 0.17188 | 4.366 |
| 3/16" | 0.18750 | 4.762 |
| 13/64" | 0.20313 | 5.159 |
| 7/32" | 0.21875 | 5.556 |
| 15/64" | 0.23438 | 5.953 |
| 1/4" | 0.25000 | 6.350 |
| 17/64" | 0.26563 | 6.747 |
| 9/32" | 0.28125 | 7.144 |
| 19/64" | 0.29688 | 7.541 |
| 5/16" | 0.31250 | 7.938 |
| 21/64" | 0.32813 | 8.334 |
| 11/32" | 0.34375 | 8.731 |
| 23/64" | 0.35938 | 9.128 |
| 3/8" | 0.37500 | 9.525 |
| 25/64" | 0.39063 | 9.922 |
| 13/32" | 0.40625 | 10.319 |
| 27/64" | 0.42188 | 10.716 |
| 7/16" | 0.43750 | 11.112 |
| 29/64" | 0.45313 | 11.509 |
| 15/32" | 0.46875 | 11.906 |
| 31/64" | 0.48438 | 12.303 |
| 1/2" | 0.50000 | 12.700 |
| 33/64" | 0.51563 | 13.097 |
| 17/32" | 0.53125 | 13.494 |
| 35/64" | 0.54688 | 13.891 |
| 9/16" | 0.56250 | 14.287 |
| 37/64" | 0.57813 | 14.684 |
| 19/32" | 0.59375 | 15.081 |
| 39/64" | 0.60938 | 15.478 |
| 5/8" | 0.62500 | 15.875 |
| 41/64" | 0.64063 | 16.272 |
| 21/32" | 0.65625 | 16.669 |
| 43/64" | 0.67188 | 17.066 |
| 11/16" | 0.68750 | 17.462 |
| 45/64" | 0.70313 | 17.859 |
| 23/32" | 0.71875 | 18.256 |
| 47/64" | 0.73438 | 18.653 |
| 3/4" | 0.75000 | 19.050 |
| 49/64" | 0.76563 | 19.447 |
| 25/32" | 0.78125 | 19.844 |
| 51/64" | 0.79688 | 20.241 |
| 13/16" | 0.81250 | 20.637 |
| 53/64" | 0.82813 | 21.034 |
| 27/32" | 0.84375 | 21.431 |
| 55/64" | 0.85938 | 21.828 |
| 7/8" | 0.87500 | 22.225 |
| 57/64" | 0.89063 | 22.622 |
| 29/32" | 0.90625 | 23.019 |
| 59/64" | 0.92188 | 23.416 |
| 15/16" | 0.93750 | 23.813 |
| 61/64" | 0.95313 | 24.209 |
| 31/32" | 0.96875 | 24.606 |
| 63/64" | 0.98438 | 25.003 |
| 1" | 1.00000 | 25.400 |
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Frequently asked questions
How do I convert inches to millimeters?
Multiply by 25.4, which is exact by definition. 0.250 inch is 6.35 mm and 1.000 inch is 25.4 mm. To go the other way divide by 25.4, or multiply by 0.03937. Most digital calipers and micrometers switch units with a button, which avoids the arithmetic entirely.
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Figures are from published standards and handbook values, reproduced for convenience. Check critical dimensions against the standard or your tooling manufacturer.