Inches to mm
One in equals 25.4 mm. Type any number and the answer appears as you type — the maths happens in your browser.
Result
The same value in every unit
| Unit | Value |
|---|
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Common values
| inches | mm |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 15 in | 381 mm |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 50 in | 1270 mm |
| 100 in | 2540 mm |
| 500 in | 12700 mm |
| 1000 in | 25400 mm |
How to use
- Enter the number of inches in the field.
- The result in mm updates on every keystroke.
- Use the table below for the values people look up most often.
Formula and reverse conversion
The formula
To convert by hand: mm = in × 25.4. Going back the other way, one mm is 0.03937007874 in.
Where the odd number comes from
The factor is not a round number because the imperial units are defined through the metric ones by international agreement: an inch is exactly 25.4 mm and a pound is exactly 0.45359237 kg. Everything else follows from those definitions.
mm = in × 25.4
Frequently asked questions
How many mm are in one inches?
One in is 25.4 mm.
How do I convert inches to mm?
Multiply by the factor: mm = in × 25.4.
Is the result exact?
The factors are the exact legal definitions, and up to ten significant digits are kept in the answer. Rounding happens only in the display.
How do I convert mm back to inches?
One mm is 0.03937007874 in — or open the reverse page linked below.