Fraction to Decimal to mm Converter
Type into whichever box matches what you have — a fraction off a tape measure, a decimal off a micrometer, or a millimetre figure off a drawing — and the others follow. The fraction is resolved to the nearest 1/64, and the page tells you when that rounding is doing real harm.
Why the fraction is approximate
A decimal inch rarely lands exactly on a fraction with a small denominator. The standard shop fractions step in 1/64, so 0.370 in is not any marked value — it is between 23/64 (0.3594) and 24/64 = 3/8 (0.375). This converter rounds to the nearest 1/64 and tells you how far off that rounding is. If the error is large enough to matter, use the decimal and machine to that instead.
Common conversions
| Fraction (in) | Decimal (in) | Millimetres |
|---|---|---|
| 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 |
Standard: exact arithmetic (1 in = 25.4 mm, defined by the international yard and pound agreement) · Data checked: 2026-10-14
Things worth knowing
- An inch is exactly 25.4 mm. This is a definition, not a measurement — it has been exact since the 1959 international yard and pound agreement, so there is no rounding error in the conversion itself.
- Fractions stop at 1/64 in most shops, 1/32 on a tape measure. Read your tape before trusting a fraction down to a 64th.
- Machining drawings use decimals, not fractions. In a machine shop, tolerances are given in thousandths of an inch (0.001 in = 25.4 µm) or in millimetres to two decimal places. The fraction is for the lumber yard, not the milling machine.
- Watch mixed numbers. "1 3/8" means one and three eighths (1.375), not 13/8 by coincidence — though here they are the same. "1-1/2" and "1 1/2" both mean 1.5.