Multiply by the precision, then round
Split off the whole inches, then multiply what is left by the denominator you want and round to a whole number. 0.375″ at 1/16: 0.375 × 16 = 6 → 6/16 → 3/8″. Reduce the result by its greatest common divisor, or 6/16 never becomes 3/8.
Decimal to the nearest 1/16 and 1/32
| Decimal (in) | Nearest 1/16 | Nearest 1/32 |
|---|---|---|
| 0.0625 | 1/16 | 2/32 |
| 0.125 | 1/8 | 4/32 |
| 0.1875 | 3/16 | 6/32 |
| 0.25 | 1/4 | 8/32 |
| 0.3125 | 5/16 | 10/32 |
| 0.333 | 5/16 | 11/32 |
| 0.375 | 3/8 | 12/32 |
| 0.5 | 1/2 | 16/32 |
| 0.625 | 5/8 | 20/32 |
| 0.75 | 3/4 | 24/32 |
| 0.875 | 7/8 | 28/32 |
Rounding to 1/16 is off by at most 1/32 (about 0.8 mm). Go to 1/32 for machining (max error ~0.4 mm) or 1/64 (~0.2 mm).
Full sixteenths, with the decimal and millimetres
| Fraction | Decimal (in) | mm |
|---|---|---|
| 1/16 | 0.0625 | 1.588 |
| 1/8 | 0.1250 | 3.175 |
| 3/16 | 0.1875 | 4.763 |
| 1/4 | 0.2500 | 6.350 |
| 5/16 | 0.3125 | 7.938 |
| 3/8 | 0.3750 | 9.525 |
| 7/16 | 0.4375 | 11.113 |
| 1/2 | 0.5000 | 12.700 |
| 5/8 | 0.6250 | 15.875 |
| 3/4 | 0.7500 | 19.050 |
| 7/8 | 0.8750 | 22.225 |
| 1 | 1.0000 | 25.400 |
Where people go wrong
- Not reducing. 4/16 and 1/4 are the same measurement; a tape measure only labels the reduced form, so always divide out the common factor.
- Wrong precision for the job. 1/16 vs 1/32 is roughly 0.8 mm apart — fine for framing, too coarse for machining.
- Thirds and fifths never land exactly. Inch fractions are powers of two, so 1/3 or 1/5 of an inch always rounds to a near neighbour.
Common questions
How do I turn a decimal inch into a fraction?
Multiply the decimal part by the precision you want, then round. For 0.375 at 1/16 precision: 0.375 times 16 = 6, so 6/16, which reduces to 3/8. The whole-number part stays as it is.
What fraction is 0.333 inches?
There is no exact match, because inch fractions are halves of halves and 1/3 is not. The nearest sixteenth is 5/16 (0.3125); the nearest thirty-second is 11/32 (0.3438).
How do I reduce a fraction like 12/32?
Divide the top and bottom by their greatest common divisor. For 12 and 32 that is 4, so 12/32 becomes 3/8. Always reduce, or you get 4/16 where you meant 1/4.
