This free length converter changes millimetres (mm) into inches (in) and back again, updating live as you type. It is useful for hardware sizes, screen dimensions, screw lengths, and any drawing or spec that mixes metric and imperial.
How it works
One inch is defined as exactly 25.4 mm, so the conversion uses that fixed factor:
- in = mm ÷ 25.4 (i.e. mm × 0.03937007874)
- mm = in × 25.4
The relationship is exact and linear, so the converter is bidirectional — edit either field and the other updates instantly.
Example
A bolt is 8 mm in diameter and you need the inch equivalent.
8 ÷ 25.4 = 0.315 in, which is close to a 5/16-inch (0.3125 in) size.
| Millimetres (mm) | Inches (in) |
|---|---|
| 1 mm | 0.03937 in |
| 25.4 mm | 1 in |
| 50 mm | 1.9685 in |
| 100 mm | 3.937 in |
| 152.4 mm | 6 in |
All calculations run entirely in your browser — no data is sent to any server.