How to Convert Millimetres to Inches (mm to in)
Length conversions come up when reading overseas furniture specs, cycling the Round Island Route in kilometres while a foreign app shows miles, or checking marine distances in the Singapore Strait in nautical miles. This guide shows how to convert millimetres into inches by hand, with examples and a quick-reference table.
The conversion
To convert millimetres to inches, multiply the value by 0.0393700787. To go the other way, divide by the same number (or multiply by 25.4). For a quick estimate, multiply by about 0.039.
Inches = millimetres × 0.0393700787
Worked examples
- 3 mm = 0.11811 in
- 12.5 mm = 0.492126 in
- 75 mm = 2.9528 in
Millimetres to Inches table
| Millimetres | Inches |
|---|---|
| 1 mm | 0.03937 in |
| 2 mm | 0.07874 in |
| 5 mm | 0.19685 in |
| 10 mm | 0.393701 in |
| 20 mm | 0.787402 in |
| 25 mm | 0.984252 in |
| 50 mm | 1.9685 in |
| 100 mm | 3.937 in |
| 250 mm | 9.8425 in |
| 500 mm | 19.685 in |
| 1,000 mm | 39.3701 in |
Common questions
Is mm to in exact?
The factors used here are the internationally defined values where they exist; results are rounded for display only.
How do I convert back?
Use the Inches to Millimetres converter.
Mistakes to avoid
Dropping the units
A bare number without a unit is easy to misread later. Always write the unit with the result.
Mixing similar-sounding units
US and imperial gallons, pints and tons are different sizes, and decimal kilobytes differ from binary kibibytes.
Multiplying when you should divide
Converting to a smaller unit gives a bigger number, and converting to a larger unit gives a smaller one. If your answer moves the wrong way, you have inverted the factor.
Background
For everyday use, three or four significant figures are usually enough. Engineering, science and trade documents may need more, and rounding should happen only at the end of a calculation.
A conversion table is handy for common values, but a calculator is faster and avoids interpolation errors for anything in between.
Singapore adopted the metric system in the 1970s, replacing imperial units and traditional Chinese measures in most official uses. Metric units are now standard in schools, trade and government, but older units survive in property, markets and imported goods.
For any value, use the Millimetres to Inches Converter or browse all length converters.
Run your own numbers: Millimetres to Inches Converter