What this calculator does
Change a length from millimeters to centimeters, or back again, in one step. Both are metric units based on the meter. A millimeter (mm) is a thousandth of a meter, and a centimeter (cm) is a hundredth.
Engineers, printers and product listings tend to use millimeters. Everyday measurements, such as body height, clothing and school rulers, use centimeters. So product specs often need changing from one to the other.
There are exactly 10 millimeters in a centimeter. To go from millimeters to centimeters, divide by 10. That moves the decimal point one place to the left. To go from centimeters to millimeters, multiply by 10, which moves it one place right. The calculator shows that step so you can check it.
Where it's used
Why this math matters outside the classroom:
- Rainfall. A rain gauge reading of 25 mm is 2.5 cm of rain.
- Jewelry beads. Jewelry makers sizing a necklace know a 6 mm bead is 0.6 cm across.
- Phone sizes. Shoppers choosing a phone case read a phone height of 147 mm as 14.7 cm.
- Plans and drawings. Drafters convert 450 mm on a plan to 45 cm when checking it with a centimeter tape.
- Classroom rulers. Students count the 10 small marks between centimeter lines to see why 1 cm is 10 mm.
Grade level and standards: Grades 4–5; Common Core State Standards for Mathematics 4.MD.A.1, 5.MD.A.1. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Enter the length in millimeters or centimeters.
- Choose mm → cm or cm → mm.
- Read the result and working, or look up nearby values in the table.
What the inputs mean
- Value to convert
- A length in millimeters (mm → cm) or centimeters (cm → mm).
- Direction
- The unit your number is in.
Formula and method
cm = mm ÷ 10 mm = cm × 10
Here cm is the length in centimeters and mm is the length in millimeters. The metric prefixes milli (one thousandth) and centi (one hundredth) differ by exactly 10 times. So this conversion is exact and never needs rounding.
Only the decimal point moves, so every decimal place you enter is kept: 4.5 mm is exactly 0.45 cm.
Worked example
A bolt is 25 mm long. How long is it in centimeters?
- There are exactly 10 mm in 1 cm, so divide: 25 ÷ 10.
- 25 ÷ 10 = 2.5.
Result: 25 mm is 2.5 cm.
Understanding the result
On a metric ruler, the long numbered marks are centimeters. The nine small marks between them are millimeters. Reading 7 numbered marks and 3 small ones gives 7.3 cm, or 73 mm.
Some handy reference values: A4 paper is 210 × 297 mm, which is 21.0 × 29.7 cm. A standard bank card is 85.60 × 53.98 mm, or 8.56 × 5.398 cm. A tire marked 205 has a stated width of 205 mm, which is 20.5 cm. Rain totals in millimeters convert the same way: 15 mm of rain is 1.5 cm.
Watch out: the factor changes for area and volume. One square centimeter is 100 mm², and one cubic centimeter (1 mL) is 1,000 mm³. Dividing an area in mm² by 10 is a common mistake. It gives an answer ten times too large.
Assumptions and limitations
- Both units are standard metric (SI) units: 1 cm = 10 mm exactly.
- Values are lengths, not areas or volumes.
- Square or cubic units need factors of 100 or 1,000. Use the area or volume converters for those.
- Very small lengths (micrometers) and large ones (meters, kilometers) are on the general length converter.
Frequently asked questions
How many millimeters are in a centimeter?
Exactly 10.
Is 1 mm bigger than 1 cm?
No. A millimeter is one tenth of a centimeter, about the thickness of a bank card.
Why do engineering drawings use millimeters for everything?
Sticking to one unit avoids mixing scales on a single drawing. Millimeters also let most measurements be written as whole numbers. A 1.2 m part is labeled 1200.
How this calculator is checked
Method: SI prefix definitions: 1 cm = 10⁻² m, 1 mm = 10⁻³ m, so 1 cm = 10 mm exactly. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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