Convert inches to millimetres

One inch is 25.4 millimetres. To convert any amount, multiply by 25.4. Going the other way, one millimetre is 0.0393700787 inches.

Multiply the inch figure by 25.4. So mm = in × 25.4. For a quick mental estimate, multiplying by about 25.4 gets you within a percent or so, which is close enough for most everyday purposes.

One inch is roughly the width of an adult thumb. The inch has been exactly 25.4 millimetres since 1959. Screen sizes, pipe diameters and tyre rims are still specified in inches almost universally.

One millimetre is roughly the thickness of a credit card, which is 0.76 mm. The millimetre is the working unit of engineering drawings and machining tolerances almost everywhere metric is used, because it avoids decimals for most part dimensions.

Yes. Since the 1959 international yard and pound agreement, the imperial and metric systems are related by exact definitions rather than measurements — the inch is exactly 25.4 mm and the pound exactly 0.45359237 kg. Everything derived from those, including this conversion, is therefore exact. Round at the end rather than partway through, since rounding in before converting introduces an error the conversion then multiplies.