# How do I convert cm to mm?

Converts centimeters (cm) to millimeters (mm) and back, with the exact unit definitions.

- Page: https://www.acalculator.org/convert/cm-to-mm
- JSON spec: https://www.acalculator.org/convert/cm-to-mm.json
- Version: 51074c423504

## Default answer

Example with the default inputs (Centimeters (cm) 1): 1 cm = 10 mm.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| value | Centimeters (cm) | The amount in centimeters to convert. |
| result | Millimeters (mm) | The same amount in millimeters. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| value | Centimeters (cm) | The amount in centimeters to convert. |
| result | Millimeters (mm) | The amount in millimeters. |

## Method

mm = cm × 10 (the same as cm ÷ 0.1)

## Assumptions

- 1 centimeter = 10 millimeters, from the exact definitions in NIST SP 811.
- Exact on the typed decimals: at most 6 decimal places (6 significant digits below 0.0001), rounded half up; an answer of 1 billion or more shows at most 15 significant digits.

## Worked examples

1. value = 1 gives result = 10. Source: Exact unit definitions in NIST SP 811 (2008), Appendix B.8.
2. value = 2.5 gives result = 25. Source: Exact unit definitions in NIST SP 811 (2008), Appendix B.8.
3. result = 25 gives value = 2.5. Source: Exact unit definitions in NIST SP 811 (2008), Appendix B.8.

## FAQ

### How many millimeters are in a centimeter?

1 centimeter is 10 millimeters, from the exact unit definitions (NIST SP 811).

### How do I convert cm to mm?

Multiply by 10: mm = cm × 10 (the same as cm ÷ 0.1). For example, 1 cm × 10 = 10 mm.

### How do I convert mm back to cm?

Multiply by 0.1, or type the millimeters into the second box and the converter works it out. 1 millimeter is 0.1 centimeters.

### What is 2.5 cm in mm?

25 mm: 2.5 × 10. Centi means one hundredth and milli one thousandth (NIST SP 811), so 1 cm is 10 mm.

## Sources

- NIST SP 811 (2008), Chapter 4, Table 5: SI prefixes (kilo 10³, centi 10⁻², milli 10⁻³), retrieved 2026-10-18. https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-4-two-classes-si-units-and-si-prefixes