Convert inches to mm

One inch equals 25.4 millimeters. To convert any number of inches to millimeters, multiply by 25.4.

The converter above works in both directions and updates as you type. Below you will find the formula, a worked example and a reference table of common values.

Length conversions come up wherever a measurement crosses a border: a height given in feet and inches on a form that wants centimetres, a running distance recorded in kilometres and compared against miles, a component specified in millimetres and ordered from a supplier working in inches. The conversions themselves are exact — the inch, foot, yard and mile have all been defined in metric terms since 1959 — so any discrepancy comes from rounding, not from the units disagreeing.

inches to mm formula

To go the other way, use inch = millimeter ÷ 25.4.

About these units

The inch has been defined as exactly 25.4 millimetres since the 1959 international yard and pound agreement, which finally reconciled slightly different British and American definitions.

The millimetre is the working unit of engineering drawings and machining across most of the world, precise enough that dimensions rarely need decimals.

Inch is also written inches and ". Millimeter is also written millimetre, millimetres and millimeters.

Sources

1. National Bureau of Standards, "Refinement of Values for the Yard and the Pound", Federal Register, 1 July 1959 — the international yard as exactly 0.9144 m and the avoirdupois pound as exactly 0.45359237 kg.