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.