# How do I convert mm to inches?

Converts millimeters (mm) to inches (in) and back, with the exact unit definitions.

- Page: https://www.acalculator.org/convert/mm-to-inches
- JSON spec: https://www.acalculator.org/convert/mm-to-inches.json
- Version: b2375577e790

## Default answer

Example with the default inputs (Millimeters (mm) 1): 1 mm = 0.03937 in.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| value | Millimeters (mm) | The amount in millimeters to convert. |
| result | Inches (in) | The same amount in inches. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| value | Millimeters (mm) | The amount in millimeters to convert. |
| result | Inches (in) | The amount in inches. |
| fraction | Nearest 1/64 inch | The length in inches rounded to the nearest 1/64 inch, as on a tape measure. |

## Method

in = mm × 0.0393700787402 (the same as mm ÷ 25.4)

## Assumptions

- 1 millimeter = 0.0393700787402 inches, from the exact definitions in NIST SP 811.
- The answer shows at most 6 decimal places; the calculation keeps full precision.

## Worked examples

1. value = 1 gives result = 0.03937. Source: exact unit definitions in NIST SP 811 (2008), Appendix B.8.
2. value = 25 gives result = 0.984252. Source: exact unit definitions in NIST SP 811 (2008), Appendix B.8.
3. result = 0.984252 gives value = 25. Source: exact unit definitions in NIST SP 811 (2008), Appendix B.8.

## FAQ

### How many inches are in a millimeter?

1 millimeter is 0.03937 inches, from the exact unit definitions (NIST SP 811).

### How do I convert mm to inches?

Multiply by 0.03937: in = mm × 0.0393700787402 (the same as mm ÷ 25.4). For example, 1 mm × 0.03937 = 0.03937 in.

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

Multiply by 25.4, or type the inches into the second box and the converter works it out. 1 inch is 25.4 millimeters.

## Sources

- NIST SP 811 (2008), Guide for the Use of the International System of Units, Appendix B.8. https://www.nist.gov/pml/special-publication-811