Multiply by 10, and it is exact
A centimetre is ten millimetres by definition, so this conversion has no rounding error. Move the decimal one place right to go from cm to mm, and one place left to go back.
mm = cm × 10, cm = mm ÷ 10
Quick reference
| Centimetres | Millimetres |
|---|---|
| 0.5 cm | 5 mm |
| 1 cm | 10 mm |
| 2.5 cm | 25 mm |
| 5 cm | 50 mm |
| 10 cm | 100 mm |
| 15 cm | 150 mm |
| 30 cm | 300 mm |
| 100 cm | 1,000 mm |
Decimals carry over directly: 4.73 cm is 47.3 mm, and a 42 mm watch case is 4.2 cm.
When to use which
- Centimetres suit everyday sizes where a millimetre does not matter, such as height, clothing and room dimensions.
- Millimetres are the default in engineering, medicine and jewellery, where drawings, sutures and ring sizes are all specified to the millimetre.
- Smaller still, a micrometre (µm) is a thousandth of a millimetre, so 1 cm is 10,000 µm. Below a millimetre you are usually into microscopy.
Common questions
How many millimetres are in a centimetre?
Exactly 10. This is fixed by definition, not a rounded value, because centi- means one hundredth of a metre and milli- means one thousandth, a difference of ten.
How do I convert 47 mm to cm?
Divide by 10, which gives 4.7 cm. Going the other way, multiply cm by 10, so 8.5 cm is 85 mm.
Is 1 cm equal to 10 mm everywhere?
Yes. The metric system is defined the same way in every country, so 1 cm is 10 mm whether you are in Japan, Brazil or Germany. There are no regional variations.
