# Inches to millimeters converter

> Convert inches to millimeters (inches to mm) with the exact factor, a conversion chart and worked steps.

Interactive version: https://www.calcopenly.com/conversion/inches-to-mm
Subject: Unit converters

Inch dimensions on a product listing may include fractions such as a half or a quarter. Enter a decimal or fraction for the full inch measurement before converting. Do not read 5 feet 6 inches as 5.6 inches, or confuse a screen's diagonal with its width.

One inch is exactly 25.4 millimeters, so 1 in is 25.4 mm. To convert inches to mm, multiply by 25.4; to convert back, multiply by 0.0393700787.

The international inch is defined as exactly 25.4 mm (1959 international yard and pound agreement). Because both units are defined exactly, the factor above is exact and the results here carry no conversion error.

## Inputs

- **Inches (in)**

## Results

- Millimeters (mm) — main result
- Conversion
- 1 mm in inches (in)

## Formula

$$
\text{millimeters} = \text{inches} \times 25.4
$$

## Worked examples

### 1 in to mm

- Inches (in): 1 in
- **Millimeters: 25.4 mm**
- Checked against: Exact-fraction computation from the legal unit definitions (scripts/gen-pairs.py, Python fractions)

### 2 in to mm

- Inches (in): 2 in
- **Millimeters: 50.8 mm**
- Checked against: Exact-fraction computation from the legal unit definitions (scripts/gen-pairs.py, Python fractions)

### 48 in to mm

- Inches (in): 48 in
- **Millimeters: 1,219.2 mm**
- Checked against: Exact-fraction computation from the legal unit definitions (scripts/gen-pairs.py, Python fractions)

## Questions

### When would I convert inches to mm?

Inch dimensions on a product listing may include fractions such as a half or a quarter. Enter a decimal or fraction for the full inch measurement before converting. Do not read 5 feet 6 inches as 5.6 inches, or confuse a screen's diagonal with its width.

### How many millimeters are in a inch?

One inch is exactly 25.4 millimeters. The international inch is defined as exactly 25.4 mm (1959 international yard and pound agreement).

### How do you convert inches to mm?

Multiply the number of inches by 25.4. For example, 1 in × 25.4 = 25.4 mm. To go the other way, divide by 25.4.

### How can I estimate inches to mm without a calculator?

Multiply by 25. That is 1.6% low of the exact factor 25.4, so 1 in comes out as 25 mm instead of 25.4.

### How accurate is the inches to millimeters converter?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 3 worked examples whose answers come from independent sources; for example, “1 in to mm” is checked against Exact-fraction computation from the legal unit definitions (scripts/gen-pairs.py, Python fractions).

### Where does the method come from?

NIST SP 811 (2008), Guide for the Use of the International System of Units, Appendix B; NIST Handbook 44, Appendix C: General tables of units of measurement; BIPM, The International System of Units (SI Brochure), 9th edition.

## Sources

- [NIST SP 811 (2008), Guide for the Use of the International System of Units, Appendix B](https://www.nist.gov/pml/special-publication-811)
- [NIST Handbook 44, Appendix C: General tables of units of measurement](https://www.nist.gov/pml/weights-and-measures/publications/nist-handbooks/handbook-44)
- [BIPM, The International System of Units (SI Brochure), 9th edition](https://www.bipm.org/en/publications/si-brochure)