Millimeters and centimeters
The millimeter is one thousandth of a meter. Engineering drawings, bolt and drill sizes, rainfall, camera lenses and jewelry use 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.
Worked example
Convert 45 mm: 45 × 0.1 = 4.5, so 45 millimeters is 4.5 centimeters. Likewise 20 mm = 2 cm.
Estimating in your head
The factor is a power of ten, so just move the decimal point 1 place to the left: 45 mm = 4.5 cm.
Going the other way
To convert centimeters to millimeters, divide by 0.1 (or multiply by 10). The swap button between the boxes opens that page.
How this works
cm = mm × 0.1
1 mm = 0.001 m and 1 cm = 0.01 m, both by definition, so the factor is their ratio: 0.1 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 centimeters are in a millimeter?
1 millimeter = 0.1 centimeters, exactly. The value follows from the unit definitions, so it never changes.
How do I convert millimeters to centimeters?
Multiply by 0.1. For example, 20 mm × 0.1 = 2 cm.
What is 100 mm in centimeters?
100 mm = 10 cm. 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/mm-to-cm/