The short answer
1 millimeter = 0.0393700787 inches. Divide millimeters by 25.4.
inches = millimeters ÷ 25.4
millimeters = inches × 25.425 mm ÷ 25.4 = 0.9843 in — just under an inch, which is why 25 mm sockets do not fit 1 in fasteners.
Exact by definition
Since 1959 the international inch is defined as exactly 25.4 mm. Before that, the US inch and the imperial inch differed by about two parts per million — enough to matter in precision metrology, which is why old gauge blocks are marked with their standard.
Fractional inch chart
| Fraction | Decimal in | Millimeters |
|---|---|---|
| 1/64 | 0.015625 | 0.397 mm |
| 1/32 | 0.03125 | 0.794 mm |
| 1/16 | 0.0625 | 1.588 mm |
| 1/8 | 0.125 | 3.175 mm |
| 3/16 | 0.1875 | 4.763 mm |
| 1/4 | 0.25 | 6.35 mm |
| 3/8 | 0.375 | 9.525 mm |
| 1/2 | 0.5 | 12.7 mm |
| 5/8 | 0.625 | 15.875 mm |
| 3/4 | 0.75 | 19.05 mm |
| 1 | 1.0 | 25.4 mm |
Near-miss sizes that cause damage
| Metric | Imperial | Difference |
|---|---|---|
| 10 mm | 3/8 in (9.525 mm) | 0.475 mm |
| 13 mm | 1/2 in (12.7 mm) | 0.30 mm |
| 19 mm | 3/4 in (19.05 mm) | 0.05 mm |
| 22 mm | 7/8 in (22.225 mm) | 0.225 mm |
19 mm and 3/4 in are close enough to interchange in a pinch; 13 mm on a 1/2 in bolt will round the corners. Use the metric socket on metric fasteners.
How much rounding can you afford?
Rounding to three decimal places on an inch value is a 0.0254 mm step — fine for woodworking, marginal for machining. Typical tolerance bands:
- Carpentry: ±1 mm (±0.04 in) — round to 1/16 in
- Sheet metal: ±0.5 mm — two decimals
- General machining: ±0.05 mm — three decimals
- Precision/bearing fits: ±0.005 mm — four decimals, never a fraction
Drill sizes
Drill bits come in metric, fractional, number and letter series, and they interleave:
| Needed | Nearest metric | Nearest fractional |
|---|---|---|
| 3.0 mm | 3.0 mm | 1/8 in (3.175 mm) |
| 5.0 mm | 5.0 mm | 13/64 in (5.159 mm) |
| 6.0 mm | 6.0 mm | 15/64 in (5.953 mm) |
| 8.0 mm | 8.0 mm | 5/16 in (7.938 mm) |
For a clearance hole, oversize is fine. For a tapped hole, use the correct tap-drill size — a fractional substitute can leave too little or too much thread engagement.
Key takeaways
- mm ÷ 25.4 = inches, exact by international definition
- Round to 1/16 in for wood, decimals for metal
- 13 mm ≠ 1/2 in — the 0.3 mm gap rounds fastener heads
- Match tap drills exactly; do not substitute the nearest fraction