The exact equivalent
An inch is exactly 25.4 millimetres, so a millimetre is 0.0393701 inches. Like all imperial lengths, it is a defined value rather than a measured one.
1 mm = 0.03937008 in
1 in = 25.4 mm
How to convert it by hand
Just multiply by the conversion factor. To go from millimetres to inches, multiply by 0.03937; to come back, divide by that same number or multiply by 25.4.
For example: 25 mm × 0.03937 = 0.9843 in.
A tip that saves mistakes: check the result goes the way you expect. If inches are a smaller unit than millimetres, the number has to come out bigger. If it comes out smaller, you divided when you should have multiplied.
Conversion table
| mm | in |
|---|---|
| 1 | 0.0394 |
| 2 | 0.0787 |
| 5 | 0.1969 |
| 10 | 0.3937 |
| 20 | 0.7874 |
| 50 | 1.9685 |
| 100 | 3.937 |
Reference points to picture it
| Reference | Equals |
|---|---|
| A 10 mm spanner | 0.394 in |
| The thickness of a credit card | 0.76 mm = 0.030 in |
| A 25.4 mm drill bit | exactly 1 in |
| A phone 8 mm thick | 0.31 in |
The trick for doing it in your head
Divide by 25 and take off about 1.6%. For 100 mm: 100 ÷ 25 = 4, minus a little, gives 3.94 inches.
It is fine for getting by without a phone, but not for anything that depends on precision: in a recipe, on a building site or in a dose, use the exact conversion.