To convert inches to millimeters (in to mm), multiply by 25.4. 1 inch equals 25.4 mm, about 30% of a credit card's long edge.
| inches | millimeters |
|---|---|
| 1 inch | 25.4 mm |
| 2 inches | 50.8 mm |
| 5 inches | 127 mm |
| 10 inches | 254 mm |
| 25 inches | 635 mm |
| 50 inches | 1,270 mm |
| 100 inches | 2,540 mm |
| 250 inches | 6,350 mm |
| 500 inches | 12,700 mm |
| 1000 inches | 25,400 mm |
Definition: 1/12 of a foot, or exactly 25.4 millimeters.
History: The Romans had the "uncia" - one-twelfth of their foot, the same root word that gives us "ounce" today. Centuries later, medieval England needed something more practical and checkable than "one twelfth of a foot," so a 1308 statute under King Edward I defined an inch as the length of three grains of barley, dry and round, laid end to end - a definition ordinary people could actually verify for themselves without a ruler.
Current use: Screen sizes, paper formats, pipe diameters, and everyday measurements across the US, UK, and other imperial-leaning countries.
Definition: 1/1000 of a meter - the same "milli" prefix as milliliter or milligram.
History: Like the centimeter, it arrived as part of the same 1790s French decimal system, built specifically to make precise, small-scale measurement simple - no more converting between competing local systems of inches, lines, and points.
Current use: Engineering tolerances, screen and camera specs, and rainfall measurements - which is why weather reports outside the US say "20mm of rain" rather than inches.
One inch is exactly 25.4 millimetres by international definition. There is no rounding in this conversion at all.
For a quick estimate, multiply by 25. That is off by about 1.6%, which is fine for a rough figure and not fine for anything you are paying for or building to. Use the exact factor of 25.4000 when it matters.
The reverse factor is 0.039, so one millimeter is that many inches. Two decimal places is plenty for most purposes, and three is more than a tape measure can justify.
To anchor the scale: 1 inch is about 25.4 mm.