About this tool
This converter uses the exact international standard: 1 inch equals exactly 25.4 millimeters. Type a value into either field and the other updates instantly — nothing is sent to a server.
Quick reference table
| Millimeters | Inches |
|---|---|
| 1 mm | 0.039 in |
| 5 mm | 0.197 in |
| 10 mm | 0.394 in |
| 25 mm | 0.984 in |
| 50 mm | 1.969 in |
| 100 mm | 3.937 in |
Common use cases
- Converting an engineering drawing or parts spec listed in millimeters
- Checking screw, bolt, or hardware sizes between metric and imperial systems
- Converting a small precise measurement, like a phone or watch dimension
Things to watch out for
The 25.4 mm-per-inch figure is an exact international definition, not a rounded approximation — small differences between calculators usually come from how many decimal places the result is rounded to. Because millimeters are a small unit, precision matters more here than in the cm-to-inches conversion — use enough decimal places for parts or fittings where a small error could cause a fit problem.
Frequently asked questions
1 millimeter equals 0.03937 inches. Since 1 inch is exactly 25.4 mm, 1 mm is exactly 1 ÷ 25.4 inches.
Same underlying math, different unit of length — a millimeter is one-tenth of a centimeter, so this tool is useful when your starting measurement is already in mm, like on an engineering drawing or a small parts spec.
Millimeters give finer precision than inches without needing fractions — instead of writing 1/64 inch, a measurement can be written as a clean decimal like 0.4 mm, which is why many international manufacturing and engineering specs default to millimeters.