CM to Inches: The Complete Conversion Guide
One inch is exactly 2.54 centimetres — a defined value, not a measurement. Here is how to use it, and the trick for doing it in your head.
12 January 2026 · 5 min read · Length
Centimetres to inches is the single most searched conversion on the internet, and it is one of the few that is exact. Since 1959, one international inch has been defined as 25.4 millimetres. There is no rounding, no measurement uncertainty, no regional variation.
The formula
inches = centimetres ÷ 2.54
To go the other way, multiply by 2.54. That is the whole thing.
The mental shortcut
Dividing by 2.54 in your head is awkward. Instead, multiply by 0.4 and add a bit. A centimetre is 0.3937 inches, so 0.4 overshoots by about 1.6%.
- 10 cm → 4 in (true: 3.94)
- 25 cm → 10 in (true: 9.84)
- 100 cm → 40 in (true: 39.37)
For anything where 2% matters — machining, framing, medical measurements — use the centimetres to inches converter instead of the shortcut.
Reference table
| Centimetres | Inches | Common context |
|---|---|---|
| 1 cm | 0.394 in | Width of a fingernail |
| 2.54 cm | 1 in | Exact by definition |
| 15 cm | 5.91 in | A pencil |
| 30 cm | 11.81 in | Almost exactly a foot ruler |
| 91.44 cm | 36 in | One yard, exactly |
| 152.4 cm | 60 in | 5 feet |
| 180 cm | 70.87 in | 5 ft 10.9 in |
Where people get it wrong
Mixing up feet-and-inches with decimal inches. A height of 5 ft 11 in is not 5.11 feet. Convert the inches separately: 11 in = 0.9167 ft, so 5 ft 11 in = 5.9167 ft = 180.34 cm.
Assuming screen sizes are diagonal centimetres. A "15-inch" laptop is 38.1 cm measured corner to corner, not across the width.
Rounding too early. If you convert 2.54 cm to "1 inch" and then multiply by 100, you have quietly thrown away accuracy. Convert once, at the end.
Related conversions
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy