1/16 inch is 1.5875 mm and 1/4 inch is 6.35 mm. Every fraction converts exactly, since 1 inch is 25.4 mm.
| Inches (fraction) | Inches (decimal) | Millimeters (mm) |
|---|---|---|
| 1/16 | 0.0625 | 1.59 |
| 1/8 | 0.1250 | 3.18 |
| 3/16 | 0.1875 | 4.76 |
| 1/4 | 0.2500 | 6.35 |
| 5/16 | 0.3125 | 7.94 |
| 3/8 | 0.3750 | 9.52 |
| 7/16 | 0.4375 | 11.11 |
| 1/2 | 0.5000 | 12.70 |
| 9/16 | 0.5625 | 14.29 |
| 5/8 | 0.6250 | 15.88 |
| 11/16 | 0.6875 | 17.46 |
| 3/4 | 0.7500 | 19.05 |
| 13/16 | 0.8125 | 20.64 |
| 7/8 | 0.8750 | 22.22 |
| 15/16 | 0.9375 | 23.81 |
| 1 | 1.0000 | 25.40 |
| 2 | 2.0000 | 50.80 |
| 3 | 3.0000 | 76.20 |
| 4 | 4.0000 | 101.60 |
| 5 | 5.0000 | 127.00 |
| 6 | 6.0000 | 152.40 |
| 7 | 7.0000 | 177.80 |
| 8 | 8.0000 | 203.20 |
| 9 | 9.0000 | 228.60 |
| 10 | 10.0000 | 254.00 |
| 11 | 11.0000 | 279.40 |
| 12 | 12.0000 | 304.80 |
How it is computed
Decimal inches = numerator ÷ denominator; millimeters = decimal inches × 25.4. Fractions are reduced (8/16 shows as 1/2), as on a tape measure.
FAQ
Why do tape measures use sixteenths?+
Imperial rulers halve the inch repeatedly: halves, quarters, eighths, sixteenths. Each step is twice as fine, and 1/16 in (1.5875 mm) is the usual working precision.
What metric drill bit replaces a 1/4 in bit?+
1/4 in is 6.35 mm, so a 6.5 mm bit gives a slightly loose fit and 6 mm a tight one.