The exact equivalent
Multiply by 2.54. Since 1959 the inch has been defined as exactly 2.54 centimetres, so this conversion is exact rather than rounded.
1 in = 2.54 cm
1 cm = 0.39370079 in
How to convert it by hand
Just multiply by the conversion factor. To go from inches to centimetres, multiply by 2.54; to come back, divide by that same number or multiply by 0.393701.
For example: 25 in × 2.54 = 63.5 cm.
A tip that saves mistakes: check the result goes the way you expect. If centimetres are a smaller unit than inches, the number has to come out bigger. If it comes out smaller, you divided when you should have multiplied.
Conversion table
| in | cm |
|---|---|
| 1 | 2.54 |
| 2 | 5.08 |
| 5 | 12.7 |
| 10 | 25.4 |
| 20 | 50.8 |
| 50 | 127 |
| 100 | 254 |
Reference points to picture it
| Reference | Equals |
|---|---|
| 1 inch | 2.54 cm |
| 1 foot (12 inches) | 30.48 cm |
| A 27-inch monitor | 68.6 cm diagonal |
| A 32-inch waist | 81.3 cm |
The trick for doing it in your head
Multiply by 2.5 and add 1.6%. For 12 inches: 12 × 2.5 = 30, plus a little, gives 30.5 cm.
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.