Inches to Millimeters

About Inches and Millimeters

Inches (in): From the Latin uncia meaning one-twelfth; originally a thumb-width unit, defined as exactly 2.54 centimeters since the 1959 International Yard and Pound Agreement. Common uses: US construction, screens and devices, clothing sizes (waist, chest), paper sizes, and a near-universal global unit for TV and monitor diagonals.

Millimeters (mm): One thousandth of a meter, where the meter was originally defined in 1799 as one ten-millionth of the equator-to-pole distance, and is now defined by the speed of light. Common uses: The default precision unit in engineering drawings, machining tolerances, manufacturing specifications, and most metric-country construction blueprints.

How the conversion works

Length conversions use the SI definition: 1 inch is exactly 0.0254 meters and 1 mile is exactly 1,609.344 meters. The factor above is the exact ratio between inch and millimeter.

The exact relationship is millimeters = inches × 25.4, which the calculator at the top of this page applies in both directions. Type into either field and the other updates immediately.

When this conversion matters

Converting between inches and millimeters comes up wherever length measurements move between systems — from one country's conventions to another's, from a scientific reference to a practical specification, or from one industry's working unit to another's. The calculator and reference table above cover the everyday range; for unusual values you can type any number into either field.