About this tool
Type inches to get millimeters and centimeters, or type in either metric box to get inches. The conversion is exact: 1 inch = 25.4 mm = 2.54 cm, fixed by the 1959 international agreement. It runs in your browser and nothing is uploaded.
How to use it
Enter a value in any of the three boxes and the other two update. For fractions such as 3/8 inch, divide first (3 divided by 8 is 0.375) and enter the decimal. If you only need centimeters to inches, the cm to inches converter is written for that direction.
How to convert inches to mm by hand
Multiply inches by 25.4. For a quick mental figure, multiply by 25 and add a little: 4 inches times 25 is 100, and the exact answer is 101.6 mm. For centimeters, multiply by 2.54, or take mm and divide by 10.
Inches to millimeters and centimeters
| Inches | Millimeters / centimeters |
|---|---|
| 1/8 in | 3.175 mm / 0.3175 cm |
| 1/4 in | 6.35 mm / 0.635 cm |
| 1/2 in | 12.7 mm / 1.27 cm |
| 3/4 in | 19.05 mm / 1.905 cm |
| 1 in | 25.4 mm / 2.54 cm |
| 2 in | 50.8 mm / 5.08 cm |
| 3 in | 76.2 mm / 7.62 cm |
| 6 in | 152.4 mm / 15.24 cm |
| 12 in (1 ft) | 304.8 mm / 30.48 cm |
| 24 in | 609.6 mm / 60.96 cm |
| 36 in (1 yd) | 914.4 mm / 91.44 cm |
Where people need inches to mm
- Hardware and plumbing: bolt sizes, drill bits and pipe diameters are labeled in inches in the US and in millimeters elsewhere.
- 3D printing and CAD: models are often sized in millimeters while parts are bought in inches.
- Screens and electronics: display sizes are in inches, but cases and cutouts are specified in millimeters.
- Sewing and crafts: patterns from one country use the other country's unit.
Things to watch out for
Check which way the number should go. Inches are bigger than millimeters, so the mm number is always larger: 1 inch is 25.4 mm, not 0.04 mm. Tolerances matter for machined parts, so do not round to the nearest millimeter if a fit has to be exact. Also be careful with sizes called "1/2 inch" in plumbing, because the label is a nominal size and the real pipe diameter is different.
Frequently asked questions
1 inch is exactly 25.4 millimeters, which is also exactly 2.54 centimeters.
Divide 3 by 8 to get 0.375 inches, then multiply by 25.4. The answer is 9.525 mm.
Yes. Since 1959 the international inch has been defined as exactly 25.4 mm in the US, UK, Canada and other countries.