# Millimeters to Inches (mm to in)

> Convert millimeters to inches: 1 mm = 0.0393701 in. Exact conversion factor, formula, 22-row table and worked examples, with JSON API and MCP access.

- Page id: `mm-to-inches` · Quantity: Length (`length`, base unit m) · Tool: `convert_units`
- Canonical page: https://tttkmbb.com/conversion/mm-to-inches · This document: https://tttkmbb.com/conversion/mm-to-inches.md · JSON: https://tttkmbb.com/conversion/mm-to-inches.json (add `?value=` for a result)

## Formula

- `1 mm = 0.0393701 in`
- `inches = millimeters × 0.03937007874`
- Reverse: `millimeters = inches × 25.4`

The factor follows from the SI definitions of both units (NIST SP 811); values below are rounded to 6 significant figures, the API returns up to 12.

## Worked examples

- 5 mm = 0.19685 in
- 12.5 mm = 0.492126 in
- 100 mm = 3.93701 in

## Conversion table

| Millimeters (mm) | Inches (in) |
| --- | --- |
| 1 | 0.0393701 |
| 2 | 0.0787402 |
| 3 | 0.11811 |
| 4 | 0.15748 |
| 5 | 0.19685 |
| 6 | 0.23622 |
| 7 | 0.275591 |
| 8 | 0.314961 |
| 9 | 0.354331 |
| 10 | 0.393701 |
| 15 | 0.590551 |
| 20 | 0.787402 |
| 25 | 0.984252 |
| 30 | 1.1811 |
| 40 | 1.5748 |
| 50 | 1.9685 |
| 75 | 2.95276 |
| 100 | 3.93701 |
| 200 | 7.87402 |
| 250 | 9.84252 |
| 500 | 19.685 |
| 1000 | 39.3701 |

## API

- `GET https://tttkmbb.com/api/v1/convert?value=5&from_unit=mm&to_unit=in` → unified JSON envelope with `result.values.result`, `formula`, `factor`, `sources`, `freshness`
- MCP: server `https://tttkmbb.com/mcp`, tool `convert_units` with `{"value":5,"from_unit":"mm","to_unit":"in"}`
- All length units: https://tttkmbb.com/api/v1/units?quantity=length · full converter: [Length Converter](https://tttkmbb.com/conversion/length.md)
- Authentication: none. Freshness: `static` (fixed conversion factors).

## FAQ

**How many inches are in one millimeter?**

1 mm = 0.0393701 in. The factor comes from the SI definitions of both units (NIST SP 811 / BIPM), so it is exact or correct to at least 10 significant figures.

**What is the formula to convert millimeters to inches?**

Multiply the value in millimeters by 0.03937007874: inches = millimeters × 0.03937007874. For example, 5 mm = 0.19685 in.

**How do I convert inches back to millimeters?**

Multiply by the inverse factor 25.4 (millimeters = inches × 25.4), or use the inches-to-millimeters page.

## Related conversions

- [Inches to millimeters](https://tttkmbb.com/conversion/inches-to-mm.md) (reverse)
- [Kilometers to Miles](https://tttkmbb.com/conversion/km-to-miles.md)
- [Miles to Kilometers](https://tttkmbb.com/conversion/miles-to-km.md)
- [Meters to Feet](https://tttkmbb.com/conversion/meters-to-feet.md)
- [Feet to Meters](https://tttkmbb.com/conversion/feet-to-meters.md)
- [Centimeters to Inches](https://tttkmbb.com/conversion/cm-to-inches.md)
- [Inches to Centimeters](https://tttkmbb.com/conversion/inches-to-cm.md)
- [Meters to Yards](https://tttkmbb.com/conversion/meters-to-yards.md)
- [Yards to Meters](https://tttkmbb.com/conversion/yards-to-meters.md)

## Data Sources

- NIST Special Publication 811 – Guide for the Use of the International System of Units (SI), Appendix B conversion factors — https://www.nist.gov/pml/special-publication-811 (standard, retrieved 2026-09-24)
- BIPM – The International System of Units (SI Brochure, 9th edition) — https://www.bipm.org/en/publications/si-brochure (standard, retrieved 2026-09-24)
- Wikipedia – Conversion of units — https://en.wikipedia.org/wiki/Conversion_of_units (reference, retrieved 2026-09-24)