Millimeters to Inches
Convert millimeters (mm) to inches (in). Both measure distance and dimension, and the conversion comes up in construction, navigation, sports work.
The Formula
``
inches = millimeters × 0.0393700787402
millimeters = inches × 25.4
`
One millimeter is 0.03937 inches, and one inch is 25.4 millimeters.
Conversion Table
| Millimeters | Inches |
|---|---|
| 1 mm | 0.03937 in |
| 2 mm | 0.07874 in |
| 5 mm | 0.19685 in |
| 10 mm | 0.393701 in |
| 25 mm | 0.984252 in |
| 50 mm | 1.9685 in |
| 100 mm | 3.937 in |
| 500 mm | 19.685 in |
| 1,000 mm | 39.3701 in |
The Other Direction
| Inches | Millimeters |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 25 in | 635 mm |
| 50 in | 1,270 mm |
| 100 in | 2,540 mm |
| 500 in | 12,700 mm |
| 1,000 in | 25,400 mm |
What Each Unit Is
Millimeters (mm) — one thousandth of a metre. The metric unit engineering drawings default to, because it removes decimal points from most dimensions.
Inches (in) — exactly 2.54 centimetres — an exact figure, not a rounded one. From the Latin uncia, one twelfth (of a foot); the exact metric value was agreed in 1959.
A Sense of Scale
A credit card is 0.76 mm thick- A paperclip wire is about 1 mm
- Rainfall is measured in mm
- A thumb is about an inch wide
- Screens are measured diagonally in inches
- US rainfall is in inches
Precision
The factor 0.0393700787402 is rounded. For engineering work carry more digits than the table above, and round only at the end — rounding at each step compounds.
Watch Out For
- Because the inch is exactly 2.54 cm, inch↔cm conversions are exact and any rounding you see comes from your own input precision, not the factor.
In Code
`javascript
const millimeterToInch = (value) => value * 0.0393700787402;
``
Where This Conversion Comes Up
- Construction
- Navigation
- Sports
- Sewing
- Mapping
How Many Decimals Do You Need?
Rounding the factor costs accuracy, and how much depends on the factor. Converting 137 mm with the factor truncated at each decimal place:
| Decimals kept | Factor | 137 mm becomes | Error |
|---|---|---|---|
| 0 | 0 | 0 | 100.0000% |
| 1 | 0 | 0 | 100.0000% |
| 2 | 0.04 | 5.48 | 1.6000% |
| 3 | 0.039 | 5.343 | 0.9400% |
| 4 | 0.0394 | 5.3978 | 0.0760% |
The right number of significant figures is set by your input, not by the converter. If you measured millimeters to three significant figures, the answer in inches is only good to three — writing more digits claims precision the measurement never had.