Verified by Rachel Mayfield, Supply Chain Analyst - August 2026
One inch is exactly 25.4 mm - fixed by international agreement in 1959, so every value in these tables is exact arithmetic, not an approximation. The first table covers every 1/64" fraction up to 1 inch; the second continues in 1/16" steps to 3 inches. To convert any other value, multiply inches by 25.4.
Fractions of an inch to mm (1/64" to 1")
| Fraction | Decimal inches | mm |
|---|
| 1/64" | 0.0156 | 0.397 |
| 1/32" | 0.0312 | 0.794 |
| 3/64" | 0.0469 | 1.191 |
| 1/16" | 0.0625 | 1.587 |
| 5/64" | 0.0781 | 1.984 |
| 3/32" | 0.0938 | 2.381 |
| 7/64" | 0.1094 | 2.778 |
| 1/8" | 0.1250 | 3.175 |
| 9/64" | 0.1406 | 3.572 |
| 5/32" | 0.1562 | 3.969 |
| 11/64" | 0.1719 | 4.366 |
| 3/16" | 0.1875 | 4.762 |
| 13/64" | 0.2031 | 5.159 |
| 7/32" | 0.2188 | 5.556 |
| 15/64" | 0.2344 | 5.953 |
| 1/4" | 0.2500 | 6.350 |
| 17/64" | 0.2656 | 6.747 |
| 9/32" | 0.2812 | 7.144 |
| 19/64" | 0.2969 | 7.541 |
| 5/16" | 0.3125 | 7.938 |
| 21/64" | 0.3281 | 8.334 |
| 11/32" | 0.3438 | 8.731 |
| 23/64" | 0.3594 | 9.128 |
| 3/8" | 0.3750 | 9.525 |
| 25/64" | 0.3906 | 9.922 |
| 13/32" | 0.4062 | 10.319 |
| 27/64" | 0.4219 | 10.716 |
| 7/16" | 0.4375 | 11.112 |
| 29/64" | 0.4531 | 11.509 |
| 15/32" | 0.4688 | 11.906 |
| 31/64" | 0.4844 | 12.303 |
| 1/2" | 0.5000 | 12.700 |
| 33/64" | 0.5156 | 13.097 |
| 17/32" | 0.5312 | 13.494 |
| 35/64" | 0.5469 | 13.891 |
| 9/16" | 0.5625 | 14.287 |
| 37/64" | 0.5781 | 14.684 |
| 19/32" | 0.5938 | 15.081 |
| 39/64" | 0.6094 | 15.478 |
| 5/8" | 0.6250 | 15.875 |
| 41/64" | 0.6406 | 16.272 |
| 21/32" | 0.6562 | 16.669 |
| 43/64" | 0.6719 | 17.066 |
| 11/16" | 0.6875 | 17.462 |
| 45/64" | 0.7031 | 17.859 |
| 23/32" | 0.7188 | 18.256 |
| 47/64" | 0.7344 | 18.653 |
| 3/4" | 0.7500 | 19.050 |
| 49/64" | 0.7656 | 19.447 |
| 25/32" | 0.7812 | 19.844 |
| 51/64" | 0.7969 | 20.241 |
| 13/16" | 0.8125 | 20.637 |
| 53/64" | 0.8281 | 21.034 |
| 27/32" | 0.8438 | 21.431 |
| 55/64" | 0.8594 | 21.828 |
| 7/8" | 0.8750 | 22.225 |
| 57/64" | 0.8906 | 22.622 |
| 29/32" | 0.9062 | 23.019 |
| 59/64" | 0.9219 | 23.416 |
| 15/16" | 0.9375 | 23.812 |
| 61/64" | 0.9531 | 24.209 |
| 31/32" | 0.9688 | 24.606 |
| 63/64" | 0.9844 | 25.003 |
| 1" | 1.0000 | 25.400 |
1 inch to 3 inches in mm
| Inches | Decimal inches | mm |
|---|
| 1-1/16" | 1.0625 | 26.987 |
| 1-1/8" | 1.1250 | 28.575 |
| 1-3/16" | 1.1875 | 30.162 |
| 1-1/4" | 1.2500 | 31.750 |
| 1-5/16" | 1.3125 | 33.337 |
| 1-3/8" | 1.3750 | 34.925 |
| 1-7/16" | 1.4375 | 36.512 |
| 1-1/2" | 1.5000 | 38.100 |
| 1-9/16" | 1.5625 | 39.688 |
| 1-5/8" | 1.6250 | 41.275 |
| 1-11/16" | 1.6875 | 42.862 |
| 1-3/4" | 1.7500 | 44.450 |
| 1-13/16" | 1.8125 | 46.037 |
| 1-7/8" | 1.8750 | 47.625 |
| 1-15/16" | 1.9375 | 49.212 |
| 2" | 2.0000 | 50.800 |
| 2-1/16" | 2.0625 | 52.387 |
| 2-1/8" | 2.1250 | 53.975 |
| 2-3/16" | 2.1875 | 55.562 |
| 2-1/4" | 2.2500 | 57.150 |
| 2-5/16" | 2.3125 | 58.737 |
| 2-3/8" | 2.3750 | 60.325 |
| 2-7/16" | 2.4375 | 61.912 |
| 2-1/2" | 2.5000 | 63.500 |
| 2-9/16" | 2.5625 | 65.087 |
| 2-5/8" | 2.6250 | 66.675 |
| 2-11/16" | 2.6875 | 68.263 |
| 2-3/4" | 2.7500 | 69.850 |
| 2-13/16" | 2.8125 | 71.438 |
| 2-7/8" | 2.8750 | 73.025 |
| 2-15/16" | 2.9375 | 74.612 |
| 3" | 3.0000 | 76.200 |
Common workshop values worth memorising
1/8" is 3.175 mm, 1/4" is 6.35 mm, 3/8" is 9.525 mm, 1/2" is 12.7 mm, 5/8" is 15.875 mm, 3/4" is 19.05 mm and 1" is 25.4 mm. Note that metric drill and spanner sizes near these values are NOT interchangeable with imperial ones: a 13 mm spanner is loose on a 1/2" (12.7 mm) head, and a 6 mm drill is undersize for 1/4". Where fit matters, use the exact converted figure and the correct-system tooling.
Going the other way: mm to inches
Divide millimetres by 25.4. So 10 mm is 0.3937", 12 mm is 0.4724", 20 mm is 0.7874" and 25 mm is 0.9843". For fractional answers, multiply the decimal result by the denominator you want: 10 mm = 0.3937 x 64 = 25.2, so the nearest 1/64 fraction is 25/64".