Convert inches to millimeters instantly. Enter any inch value and get the exact millimeter equivalent, plus the formula and a full reference table.
The conversion factor is derived from SI base values:
1 inch is exactly 25.4 mm by international definition - a whole number with no rounding.
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.25 in | 6.35 mm |
| 0.5 in | 12.7 mm |
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 6 in | 152.4 mm |
| 8 in | 203.2 mm |
| 10 in | 254 mm |
| 12 in | 304.8 mm |
1 inch = exactly 25.4 mm.
2 × 25.4 = 50.8 mm.
The international inch was defined as exactly 25.4 mm in 1959 to align US and UK standards with the metric system.
100 ÷ 25.4 = 3.937 inches.
Millimeters are widely used in engineering and manufacturing because of their precision. A standard credit card is 0.76 mm thick; a human hair is roughly 0.07 mm in diameter.