To convert millimeters to inches (mm to in), multiply by 0.03937. 1 millimeter equals 0.03937 in, about 17% of a grain of rice.
| millimeters | inches |
|---|---|
| 1 millimeter | 0.03937 in |
| 2 millimeters | 0.07874 in |
| 5 millimeters | 0.19685 in |
| 10 millimeters | 0.3937 in |
| 25 millimeters | 0.98425 in |
| 50 millimeters | 1.9685039 in |
| 100 millimeters | 3.9370079 in |
| 250 millimeters | 9.8425197 in |
| 500 millimeters | 19.685039 in |
| 1000 millimeters | 39.370079 in |
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.
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.
Because an inch is defined as exactly 25.4 millimetres, this conversion is exact rather than approximate.
No simple round multiplier is close enough to be useful, so this is one to look up rather than estimate. The exact factor is 0.0393701.
Going the other way, one inch is 25.4 mm, which is usually the easier direction to work in. Because the forward factor is small, watch the decimal place rather than the digits: a factor-of-ten slip is the common error, not a rounding one.
To anchor the scale: 50 millimeters is about 1.97 in.