MM to Inches Converter
An inch is exactly 25.4 millimetres, so millimetres to inches is one division: inches = mm / 25.4. That makes 10 mm 0.393700787402 in, 25.4 mm exactly 1 in, and 100 mm 3.93700787402 in. This page opens the converter already set to millimetres and inches, shows the answer to twelve significant digits as you type, and lists the same length in centimetres, metres, feet, yards and miles underneath, so a drawing dimensioned in millimetres does not need a second lookup.
To
1 mm in every length unit
Common questions
- How many inches is 1 mm?
- 0.0393700787402 inches, which is 1 divided by 25.4. Ten millimetres is 0.393700787402 in, 100 mm is 3.93700787402 in and 1,000 mm is 39.3700787402 in: the digits are the same and only the point moves, because the whole conversion is a single division. Going the other way, 25.4 mm is exactly 1 inch and 12.7 mm is exactly half of one.
- What is the formula for mm to inches?
- Inches = millimetres / 25.4. The inch has been exactly 2.54 centimetres since the 1959 international agreement on the yard and the pound, so 25.4 mm to the inch is a definition rather than a measurement, and nothing about it drifts with time or country. This converter divides by that exact figure instead of multiplying by a rounded 0.03937, which is why 25.4 mm comes back as 1 and not as 0.99999.
- Is the decimal exact, or is it rounded?
- The definition is exact; the decimal is not. One millimetre is 5/127 of an inch, and 127 is a prime that is neither 2 nor 5, so the decimal repeats for ever: 0.039370078740157480... The page shows twelve significant digits and rounds there, which is well inside what the arithmetic carries. Lengths that land on a round number, like 25.4 mm or 12.7 mm, come back exact rather than as a trail of nines.
- How do I turn the decimal into a fraction of an inch, like sixteenths?
- Multiply the decimal by 16 and round to the nearest whole number: that is the count of sixteenths. Ten millimetres is 0.3937 in, times 16 is 6.3, so 6/16, which reduces to 3/8 of an inch. Check it by converting back: 3/8 in is 0.375 in, which is 9.525 mm, about half a millimetre short of the 10 you started with. This page prints decimals rather than fractions, because a fraction is always a rounding and the size of that rounding is worth seeing.
- Can I convert inches back to millimetres here?
- Yes. The swap button between the two boxes reverses the pair, so the page becomes inches to millimetres: 1 in gives 25.4 mm and 3.5 in gives 88.9 mm. The two unit menus stay open as well, so the same number will go from millimetres to feet, metres, yards or thou without leaving the page.
- Where are thou, mils and the other engineering units?
- Under the answer, the Uncommon units switch adds seven more: nanometre, micrometre, thou, furlong, astronomical unit, light year and parsec. Thou, also written mil, is the one a machine drawing usually wants. One thou is a thousandth of an inch, exactly 0.0254 mm, so 1 mm is 39.3700787402 thou and 0.5 mm is 19.6850393701 thou.
- Why does the page open on millimetres and inches when I last converted something else?
- Because a page that names a pair should show that pair. This one sets millimetres and inches on arrival, ahead of whatever the converter last remembered, and it leaves the main converter's own memory alone: go back to the full unit converter and it is still on the pair you left it on. The number you last typed does travel with you, since it is usually the one you are still working on.
Conversion factors are the exact legal definitions where one exists. An inch is exactly 2.54 cm and a pound exactly 0.45359237 kg, so those conversions are exact rather than approximate. Temperature uses offsets rather than factors, and that is shown in the working.