Millimetres to inches
One millimetre is 0.03937007874 inches.
1 millimetre = 0.03937007874 inches · 1 inch = 25.4 millimetres
This factor is exact, not rounded. It comes from the definition of the units rather than from a measurement, so the only limit on precision is how many digits are worth showing.
Millimetres to inches table
| Millimetres (mm) | Inches (in) |
|---|---|
| 1 | 0.03937007874 |
| 2 | 0.07874015748 |
| 3 | 0.1181102362 |
| 5 | 0.1968503937 |
| 10 | 0.3937007874 |
| 20 | 0.7874015748 |
| 25 | 0.9842519685 |
| 50 | 1.968503937 |
| 100 | 3.937007874 |
| 500 | 19.68503937 |
| 1,000 | 39.37007874 |
How the conversion works
Multiply by 0.03937007874 to go from millimetres to inches, or by 25.4 to go back the other way. Both figures are exact rather than rounded: they follow from the definitions of the units, not from anybody measuring something.
The exactness is more interesting than it sounds. Since the International Yard and Pound Agreement of 1959, the inch has been defined as exactly 25.4 millimetres and the pound as exactly 0.45359237 kilograms. Every other imperial length and weight follows from those two by whole-number ratios, which means the imperial system is now defined in terms of the metric one rather than alongside it.