Centimeters and millimeters
The centimeter is one hundredth of a meter (10 mm). Body height, clothing measurements, screen and paper sizes are commonly given in centimeters outside the US.
The millimeter is one thousandth of a meter. Engineering drawings, bolt and drill sizes, rainfall, camera lenses and jewelry use millimeters.
Worked example
Convert 2.5 cm: 2.5 × 10 = 25, so 2.5 centimeters is 25 millimeters. Likewise 2 cm = 20 mm.
Estimating in your head
The factor is a power of ten, so just move the decimal point 1 place to the right: 2.5 cm = 25 mm.
Going the other way
To convert millimeters to centimeters, divide by 10 (or multiply by 0.1). The swap button between the boxes opens that page.
How this works
mm = cm × 10
1 cm = 0.01 m and 1 mm = 0.001 m, both by definition, so the factor is their ratio: 10 exactly. Results drop binary floating-point noise beyond 15 significant digits and display at the precision you choose (6 significant figures by default). Copy result copies the full value.
Frequently asked
How many millimeters are in a centimeter?
1 centimeter = 10 millimeters, exactly. The value follows from the unit definitions, so it never changes.
How do I convert centimeters to millimeters?
Multiply by 10. For example, 2 cm × 10 = 20 mm.
What is 10 cm in millimeters?
10 cm = 100 mm. The table next to the converter lists more common values.
Is the result rounded?
Only on screen. The page computes with the exact definitions and removes floating-point noise past 15 significant digits; pick 3 to 12 significant figures, or use Copy result for the full value.
Sources
- NIST SP 811 Appendix B.8 — Factors for units listed alphabetically
- Federal Register 59-5442 (1959) — Refinement of values for the yard and the pound
- Wikipedia — Conversion of units of length
Reviewed Oct 17, 2026 · test vectors fixtures/units/length/cm-to-mm/