# How do I convert mm to cm?

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

- Page: https://www.acalculator.org/convert/mm-to-cm
- JSON spec: https://www.acalculator.org/convert/mm-to-cm.json
- Version: 961e59e34716

## Default answer

Example with the default inputs (Millimeters (mm) 1): 1 mm = 0.1 cm.

## Inputs

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

## Outputs

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

## Method

cm = mm × 0.1

## Assumptions

- 1 millimeter = 0.1 centimeters, 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 = 0.1. Source: Exact unit definitions in NIST SP 811 (2008), Appendix B.8.
2. value = 25.4 gives result = 2.54. Source: Exact unit definitions in NIST SP 811 (2008), Appendix B.8.
3. result = 2.54 gives value = 25.4. Source: Exact unit definitions in NIST SP 811 (2008), Appendix B.8.

## FAQ

### How many centimeters are in a millimeter?

1 millimeter is 0.1 centimeters, from the exact unit definitions (NIST SP 811).

### How do I convert mm to cm?

Multiply by 0.1: cm = mm × 0.1. For example, 1 mm × 0.1 = 0.1 cm.

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

Multiply by 10, or type the centimeters into the second box and the converter works it out. 1 centimeter is 10 millimeters.

### What is 25.4 mm in cm?

2.54 cm, exactly 1 inch: 25.4 ÷ 10. Milli means one thousandth and centi one hundredth (NIST SP 811).

## 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
- NIST SP 811 (2008), Guide for the Use of the International System of Units, Appendix B.8: Factors for Units Listed Alphabetically (bold factors are exact), retrieved 2026-10-18. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8