Inches to Millimetres

How to use Inches to MM Converter

1. 1
   Enter the inches. Decimals work; fractions are shown as a nearest sixteenth.
2. 2
   Read the millimetres. Centimetres are shown too, for drawings marked in cm.

About converting inches to millimetres

Inches to millimetres is unusual among unit conversions in being exact. Since the 1959 international yard and pound agreement, an inch is defined as precisely 25.4 millimetres, so the conversion introduces no error at all — the only rounding is whatever you choose to do to the result. Most conversions between systems are not like this. A pound is also exactly defined, but a nautical mile, a horsepower and a calorie all have several definitions differing in the third or fourth digit. The agreement was reached because the national inches had drifted apart: the American and British inches differed by a fraction of a part per million, enough to matter for interchangeable manufactured parts across the Atlantic. Standardising on 25.4 was chosen partly because it made the conversion clean in metric terms. Millimetres are the practical unit for anything engineered. Technical drawings, machining tolerances and fastener sizes are specified in millimetres in most of the world, and one millimetre is a fine enough division that decimals are rarely needed — which is exactly why the trade uses it rather than centimetres. Working from an imperial drawing, the fractions matter more than the decimals: an eighth is 3.175 mm, a quarter 6.35, a half 12.7, and the sixteenths in between are what most dimensioned drawings are actually built from.

Frequently asked questions