CM to MM Converter
Convert centimeters to millimeters — ten to the centimeter. Results update as you type; exact factors shown.
One centimeter is 10 millimeters — see the exact factor, not a rounded rule of thumb.
How the cm to mm converter works
Every length is converted through the meter, 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 US uses customary units (US gallon, short ton, °F) in daily life; NIST SP 811 gives the exact SI factors.
Common length conversions
| From | To | Units |
|---|---|---|
| 0.1 cm | 1 mm | centimeters to millimeters |
| 0.5 cm | 5 mm | centimeters to millimeters |
| 1 cm | 10 mm | centimeter to millimeters |
| 2 cm | 20 mm | centimeters to millimeters |
| 2.5 cm | 25 mm | centimeters to millimeters |
| 5 cm | 50 mm | centimeters to millimeters |
| 10 cm | 100 mm | centimeters to millimeters |
| 12 cm | 120 mm | centimeters to millimeters |
| 15 cm | 150 mm | centimeters to millimeters |
| 20 cm | 200 mm | centimeters to millimeters |
| 50 cm | 500 mm | centimeters to millimeters |
| 100 cm | 1,000 mm | centimeters to millimeters |
Units in this converter and their exact factors
| Unit | Symbol | One unit in meters |
|---|---|---|
| millimeter | mm | 0.001 meter |
| centimeter | cm | 0.01 meter |
| meter | m | 1 meter |
| kilometer | km | 1,000 meter |
| inch | in | 0.0254 meter |
| foot | ft | 0.3048 meter |
| yard | yd | 0.9144 meter |
| mile | mi | 1,609.344 meter |
| nautical mile | nmi | 1,852 meter |
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 millimeters, avoiding decimals entirely.
How many mm in an inch?
25.4.
What is the smallest mark on a ruler?
Usually one millimeter, 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 Institute of Standards and Technology (NIST) — United States's national measurement authority.
Last checked: September 2026. Conversion factors are fixed by definition and do not change with tax years or regulations.