One inch is exactly 2.54 cm — so you divide
The inch is defined as exactly 2.54 centimeters, so all three factors above are exact, not rounded. In your head, 1 cm is about 0.4 inch and 25 mm is about 1 inch, which is close enough for a quick sanity check. For anything precise, divide by the exact factor.
Quick reference
| Metric | Inches |
|---|---|
| 10 mm | 0.39 in |
| 25 mm | 0.98 in |
| 1 cm | 0.39 in |
| 10 cm | 3.94 in |
| 30 cm | 11.81 in |
| 50 cm | 19.69 in |
| 100 cm | 39.37 in |
| 1 m | 39.37 in (3 ft 3.4 in) |
25.4 mm is exactly 1 inch, and 254 mm is exactly 10 inches — the only clean whole-number crossings between the two systems.
The mistakes to watch for
- mm and cm are ten apart. 1 cm is 10 mm, so divide millimeters by 25.4 but centimeters by 2.54. Mixing the two throws the answer off by a factor of ten.
- Feet come after inches. To get feet and inches, convert to inches first, then divide by 12: 180 cm is 70.87 in, which is 5 ft 10.9 in, not 71 "feet."
- Round at the end. Keep the full decimals through the working and round only the final figure, or small errors stack up on precision jobs.
Common questions
How many inches is 1 cm?
1 centimeter is 0.3937 inches, a little under four tenths of an inch. Ten centimeters comes to 3.94 inches.
Is the 2.54 factor exact?
Yes. Since 1959 the inch has been defined as exactly 2.54 centimeters, so 25.4 mm and 0.0254 m are exact too. Every other length conversion follows from that.
How do I convert millimeters to inches?
Divide the millimeters by 25.4. For example, 50 mm divided by 25.4 is 1.97 inches. For a rough check, 25 mm is almost exactly 1 inch.
