How it works
- 01
Type a length
Plain numbers, decimals, or tape-measure fractions like 3 1/2. The answer updates as you type.
- 02
Pick the units
Millimetres through miles, in either direction. Swap reverses the conversion and carries the answer across.
- 03
Choose the precision
Up to ten decimal places. The factor is exact, so rounding only happens where you ask for it.
Conversion table
| Inches | Centimetres | Inches | Centimetres |
|---|---|---|---|
| 1/8 | 0.32 | 10 | 25.40 |
| 1/4 | 0.64 | 12 | 30.48 |
| 1/2 | 1.27 | 18 | 45.72 |
| 1 | 2.54 | 24 | 60.96 |
| 2 | 5.08 | 36 | 91.44 |
| 6 | 15.24 | 60 | 152.40 |
Where the number comes from
The inch has been exactly 25.4 millimetres since the 1959 international yard and pound agreement. So this conversion is exact rather than close: multiply by 2.54 to get centimetres, divide by 2.54 to go back. Rounding only enters when you choose how many decimals to display.
Two older definitions still turn up in technical documents. The US survey inch was used in land surveying until 2022. It was longer by about 2 parts per million, which is invisible on a desk and not invisible across a mile of survey line.
- Exact factor
- 1 in = 2.54 cm = 25.4 mm
- Reverse
- 1 cm = 0.393701 in
- Rounding
- Up to 10 decimal places, your choice.
- Other units
- The From/To selects cover mm, m, ft, yd and more.