CM to MM Converter
Convert centimetres to millimetres — ten to the centimetre. Results update as you type; exact factors shown.
One centimetre is 10 millimetres — see the exact factor, not a rounded rule of thumb.
How the cm to mm converter works
Every length is converted through the metre, the SI base unit of length. The inch has been defined as exactly 25.4 mm since the 1959 international yard and pound agreement, so a foot is exactly 0.3048 m, a yard 0.9144 m and a statute mile 1,609.344 m. The nautical mile is 1,852 m by international definition.
The UK is metric for most trade and science but keeps miles, pints, stone and mpg (with the imperial gallon) in daily life.
Common length conversions
| From | To | Units |
|---|---|---|
| 0.1 cm | 1 mm | centimetres to millimetres |
| 0.5 cm | 5 mm | centimetres to millimetres |
| 1 cm | 10 mm | centimetre to millimetres |
| 2 cm | 20 mm | centimetres to millimetres |
| 2.5 cm | 25 mm | centimetres to millimetres |
| 5 cm | 50 mm | centimetres to millimetres |
| 10 cm | 100 mm | centimetres to millimetres |
| 12 cm | 120 mm | centimetres to millimetres |
| 15 cm | 150 mm | centimetres to millimetres |
| 20 cm | 200 mm | centimetres to millimetres |
| 50 cm | 500 mm | centimetres to millimetres |
| 100 cm | 1,000 mm | centimetres to millimetres |
Units in this converter and their exact factors
| Unit | Symbol | One unit in metres |
|---|---|---|
| millimetre | mm | 0.001 metre |
| centimetre | cm | 0.01 metre |
| metre | m | 1 metre |
| kilometre | km | 1,000 metre |
| inch | in | 0.0254 metre |
| foot | ft | 0.3048 metre |
| yard | yd | 0.9144 metre |
| mile | mi | 1,609.344 metre |
| nautical mile | nmi | 1,852 metre |
FAQFrequently asked questions about length conversion
How many mm in a cm?
Exactly ten.
What is 2.5 cm in mm?
25 mm.
Why do drawings use mm?
Because most part sizes come out as whole millimetres, avoiding decimals entirely.
How many mm in an inch?
25.4.
What is the smallest mark on a ruler?
Usually one millimetre, which is about the thickness of a credit card.
Where these figures come from
- BIPM — The International System of Units (SI Brochure, 9th edition) — definitions of the base and derived units.
- NIST Special Publication 811 — Guide for the Use of the International System of Units — the exact conversion factors used here.
- National Physical Laboratory (NPL) — United Kingdom's national measurement authority.
Last checked: September 2026. Conversion factors are fixed by definition and do not change with tax years or regulations.