How to convert inches to millimeters
- Take the value in inches.
- Use the formula millimeters = inches × 25.4.
- For example, 0.25 in is 6.35 mm.
About inches and millimeters
Multiply inches by 25.4 to get millimeters. The factor is exact by definition, so a quarter inch is precisely 6.35 mm and 4 inches is 101.6 mm.
You need this when a US drawing, drill bit, wrench or lumber size meets metric parts and tools. Fractions are the tricky part: turn the fraction into a decimal first (3/8 is 0.375), then multiply.
Examples
- 0.25 in (a quarter-inch drill bit) is 6.35 mm
- 1.5 in (the real thickness of a 2x4 board) is 38.1 mm
- 3.5 in (the real width of a 2x4 board) is 88.9 mm
- 11 in (the long side of a US Letter sheet) is 279.4 mm
Inches to millimeters chart
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.0625 | 1.59 |
| 0.125 | 3.17 |
| 0.188 | 4.76 |
| 0.25 | 6.35 |
| 0.313 | 7.94 |
| 0.375 | 9.52 |
| 0.5 | 12.7 |
| 0.625 | 15.88 |
| 0.75 | 19.05 |
| 0.875 | 22.22 |
| 1 | 25.4 |
| 1.25 | 31.75 |
| 1.5 | 38.1 |
| Inches (in) | Millimeters (mm) |
|---|---|
| 2 | 50.8 |
| 2.5 | 63.5 |
| 3 | 76.2 |
| 3.5 | 88.9 |
| 4 | 101.6 |
| 5 | 127 |
| 6 | 152.4 |
| 8 | 203.2 |
| 10 | 254 |
| 12 | 304.8 |
| 18 | 457.2 |
| 24 | 609.6 |
| 36 | 914.4 |