Inches to MM formula
1 inch = 25.4 millimetres, and 1 millimetre = 0.03937007874 inches. Exactly 2.54 cm, fixed by the 1959 international yard and pound agreement.
Inches to MM chart
| Inches (in) | Millimetres (mm) |
|---|---|
| 0.0625 | 1.5875 |
| 0.125 | 3.175 |
| 0.1875 | 4.7625 |
| 0.25 | 6.35 |
| 0.3125 | 7.9375 |
| 0.375 | 9.525 |
| 0.5 | 12.7 |
| 0.625 | 15.875 |
| 0.75 | 19.05 |
| 0.875 | 22.225 |
| 1 | 25.4 |
| 1.25 | 31.75 |
| 1.5 | 38.1 |
| 2 | 50.8 |
| 2.5 | 63.5 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 6 | 152.4 |
| 8 | 203.2 |
| 10 | 254 |
| 12 | 304.8 |
Values are rounded to 4 decimal places. The converter above shows more precision.
Worked examples
A 3/8-inch bolt
0.375 in × 25.4 = 9.525 mm
0.375 inches = 9.525 millimetres. About 9.5 mm across the thread. The nearest metric bolt, M10, is slightly bigger, so the two are not interchangeable.
A 2-inch strap or webbing
2 in × 25.4 = 50.8 mm
2 inches = 50.8 millimetres. Just under 51 mm. Sellers outside the US often list the same strap as 50 mm, which is close enough for buckles and bags.
Converting inches to millimetres in real life
This is the conversion for workshops and hardware. US plans, plumbing fittings, woodworking tools, guitar parts and bike components are often sized in fractions of an inch, while metric tools, 3D printers and CNC software expect millimetres. Converting carefully saves a ruined part.
To use a fraction, divide the top number by the bottom one first (3/8 = 0.375), then multiply by 25.4 (9.525 mm). Some sizes are names rather than measurements: a "1/2-inch" water pipe is not 12.7 mm across, because pipe sizes are trade names. Check the maker's table for pipes and threads. For bigger sizes, try inches to cm.