Inches to Millimeters
Convert inches (in) to millimeters (mm). Both measure distance and dimension, and the conversion comes up in construction, navigation, sports work.
The Formula
``
millimeters = inches × 25.4
inches = millimeters × 0.0393700787402
`
One inch is 25.4 millimeters, and one millimeter is 0.03937 inches.
Conversion Table
| 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 |
The Other Direction
| 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 |
What Each Unit Is
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.
Millimeters (mm) — one thousandth of a metre. The metric unit engineering drawings default to, because it removes decimal points from most dimensions.
A Sense of Scale
A thumb is about an inch wide- Screens are measured diagonally in inches
- US rainfall is in inches
- A credit card is 0.76 mm thick
- A paperclip wire is about 1 mm
- Rainfall is measured in mm
Doing It in Your Head
Multiply by 25. That is off by 1.57% — close enough to judge a
distance, a weight or a price, and not close enough to sign off on a drawing.
Exact Mental shortcut 254 mm 250 mm 2,540 mm 2,500 mm
Precision
The factor 25.4 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 inchToMillimeter = (value) => value * 25.4;
``
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 in with the factor truncated at each decimal place:
| Decimals kept | Factor | 137 in becomes | Error |
|---|---|---|---|
| 0 | 25 | 3,425 | 1.5748% |
| 1 | 25.4 | 3,479.8 | — |
| 2 | 25.4 | 3,479.8 | — |
| 3 | 25.4 | 3,479.8 | — |
| 4 | 25.4 | 3,479.8 | — |
The right number of significant figures is set by your input, not by the converter. If you measured inches to three significant figures, the answer in millimeters is only good to three — writing more digits claims precision the measurement never had.