How to convert inches to millimeters
1 in = 25.4 mm
1 in = 25.4 mm exactly (NIST SP 811). Multiply inches by 25.4; common fractions such as 1/8 in convert to exact millimeter values.
Examples
- 0.25 in = 6.35 mm: A quarter-inch drill bit
- 0.125 in = 3.175 mm: An eighth-inch sheet thickness
- 0.75 in = 19.05 mm: A three-quarter-inch mark on a ruler
Inches to mm chart
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.125 in | 3.175 mm |
| 0.25 in | 6.35 mm |
| 0.75 in | 19.05 mm |
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 15 in | 381 mm |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 50 in | 1,270 mm |
| 75 in | 1,905 mm |
| 100 in | 2,540 mm |
Common mistakes
- A mil (thou) is 0.001 inch, which is 0.0254 mm, not a millimeter.
- Rounding 25.4 to 25 makes a 4-inch length 1.6 mm short.