The exact equivalent
An inch is exactly 2.54 centimetres. It is not an approximation: it has been defined that way by international agreement since 1959.
1 cm = 0.39370079 in
1 in = 2.54 cm
How to convert it by hand
Just multiply by the conversion factor. To go from centimetres to inches, multiply by 0.393701; to come back, divide by that same number or multiply by 2.54.
For example: 25 cm × 0.393701 = 9.8425 in.
A tip that saves mistakes: check the result goes the way you expect. If inches are a smaller unit than centimetres, the number has to come out bigger. If it comes out smaller, you divided when you should have multiplied.
Conversion table
| cm | in |
|---|---|
| 1 | 0.3937 |
| 2 | 0.7874 |
| 5 | 1.9685 |
| 10 | 3.937 |
| 20 | 7.874 |
| 50 | 19.685 |
| 100 | 39.3701 |
Reference points to picture it
| Reference | Equals |
|---|---|
| A 15.6-inch laptop screen | 39.6 cm diagonal |
| A4 paper (21 cm wide) | 8.27 inches |
| A 29-inch bike wheel | 73.7 cm |
| A height of 1.80 m | 70.9 inches |
The trick for doing it in your head
Divide by 2.5 and take about 1.6% off the result. For 30 cm: 30 ÷ 2.5 = 12, minus a little, gives 11.8 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.