Inches to Millimeters Converter

Inches (in). An inch is exactly 2.54 centimeters. It remains the standard unit for screen sizes, pipe and hose diameters, and tool sizes in imperial-measure countries.

Millimeters (mm). A millimeter is one thousandth of a meter. It is the unit for precision work — engineering tolerances, rainfall, wire gauges and the thickness of paper or sheet metal.

Metric-to-metric conversions are exact. Imperial figures are exact by definition but are rounded for display, so a value converted back and forth can differ in the final digit.

Inside the metric units every length conversion is a shift of the decimal point, so millimeters to centimeters, meters to kilometers and the rest can be done by moving it. Which way, and how far, follows from the prefix alone.

Converting between metric and imperial is not a shift but a single exact multiplication, and the factor is the same one for that pair however large or small the value. The trap is mixing the two systems halfway, as when a distance is stated in meters and inches in the same sentence.

To check a length conversion, run it backwards, or convert through a familiar middle value such as a meter or a foot. The one thing to keep straight is pairs that differ by a thousand rather than by ten, since those are where a misplaced decimal point survives unnoticed.