# mm to Inches Converter

URL: https://speedycalculator.com/calculators/conversion/mm-to-inches/
Last updated: 2026-10-25

> Convert millimetres (mm) to inches (in): 1 millimetre = 0.039370079 in, so multiply by 0.039370079 or divide by 25.4. Both fields are editable, so you can also type in inches and get millimetres back.

## Inputs

- Millimetres (mm)

## Results

- Inches

## Formula

```
1 mm = 0.039370079 in
1 in = 25.4 mm
in = mm × 0.039370079
```

## Explanation

Divide millimetres by 25.4 to get inches. Drill bits, wrench sizes, rainfall totals and jewellery are often given in millimetres.

### How to use the mm to Inches Converter

- Type the number of millimetres into the **Millimetres** field. Decimals such as 12.7 are fine.
- Read the answer in **Inches** – it updates as you type, with no button to press.
- To convert the other way, type into **Inches** and the millimetres are recalculated.
- Press **Copy results** to copy both values with the page title, or **Copy link** to share this exact conversion.

This page converts only mm ⇄ in. Need a different unit? Use the general length converter, where you can choose any “from” and “to” unit.

### How it works

Both units measure length, so they are always in the same ratio: one millimetre equals 0.039370079 inches. The calculator multiplies the number of millimetres you type by 0.039370079 to give inches, and when you type into the inches field it divides by the same factor to give millimetres. Because the ratio never changes, doubling the millimetres always doubles the inches. Results are calculated at full precision and rounded only for display (up to eight decimal places, with very large or small values shown in scientific notation).

### How to convert millimetres to inches

- Write down the value in millimetres, for example **25.4 mm**.
- Multiply by the conversion factor 0.039370079 (because 1 millimetre = 0.039370079 in).
- 25.4 × 0.039370079 = **1 in**.
- To go back from inches to millimetres, divide by 0.039370079 – or multiply by 25.4.

### mm to in conversion table

MillimetresInchesInchesMillimetres1 mm0.039370079 in1 in25.4 mm2 mm0.078740157 in2 in50.8 mm5 mm0.19685039 in5 in127 mm6 mm0.23622047 in10 in254 mm8 mm0.31496063 in20 in508 mm10 mm0.39370079 in50 in1,270 mm15 mm0.59055118 in100 in2,540 mm20 mm0.78740157 in1,000 in25,400 mm25 mm0.98425197 in25.4 mm1 in50 mm1.9685039 in100 mm3.9370079 in

### What is a millimetre?

A millimetre is one thousandth of a metre. Engineering drawings, construction plans, rainfall, screws, bolts and jewellery are all dimensioned in millimetres.

### What is an inch?

The inch has been defined as exactly 25.4 mm since 1959. It is used in the U.S. for almost everything and worldwide for screen sizes, wheel and tyre sizes, pipe threads and many tools.

### Quick mental conversion

For a fast estimate, multiply millimetres by about **0.039** to get inches; the error is about 0.9%. Use the calculator above when you need the exact figure.

### Common mistakes to avoid

Mixing the international foot (0.3048 m) with the old u.s. survey foot, confusing nautical and statute miles, and writing decimal feet such as 5.5 ft as “5 ft 5 in” (it is 5 ft 6 in).

## Worked example

Inputs — Millimetres: 25.4 mm.

Results — **Inches = 1 in**.

## FAQ

**How many inches is 10 mm?**
About 0.394 in.

**How many mm in an inch?**
Exactly 25.4.

**How do I convert millimetres to inches?**
Multiply the number of millimetres by 0.039370079. For example, 25.4 mm × 0.039370079 = 1 in.

**How do I convert inches back to millimetres?**
Divide by 0.039370079 (or multiply by 25.4), or simply type the value into the inches field above.

**What is 10 mm in inches?**
10 mm = 0.39370079 in.

**What is 100 mm in inches?**
100 mm = 3.9370079 in.

**Is this conversion exact?**
Yes. The factor comes from the official definitions of both units (NIST and the SI), so the only difference you may see is rounding in the last displayed digit.

**Can I convert to other units on this page?**
This page converts only between millimetres and inches. To pick any length unit, use the general length converter linked under Related calculators.

**What formula does the mm to Inches Converter use?**
1 mm = 0.039370079 in.


## Sources

- [NIST Special Publication 811 – Guide for the Use of the International System of Units](https://www.nist.gov/pml/special-publication-811)
- [NIST Handbook 44 Appendix C – General tables of units of measurement](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition)