📏 Mm to Inches Converter
Convert millimetres to inches and back. The exact factor, a table of the values people look up most, and what each unit is actually used for.
Converting millimetres to inches
One millimetre is 0.03937007874 inches, and one inch is 25.4 millimetres.
inches = millimetres × 0.03937007874
Getting the precision right
One inch is exactly 25.4 mm. A thousandth of an inch, called a thou or a mil, is 0.0254 mm, and that unit is still standard in American machining. Note that a British mil and an American mil historically differed, and that mil also means millimetre in casual European speech, which has caused real manufacturing errors.
Where each unit is used
Millimetres are the working unit of engineering drawings, machining and construction detail almost everywhere, because they avoid decimals for most dimensions. American machine shops often work in thousandths of an inch instead, so drawings crossing between the two conventions need care.
Values worth knowing
| mm | in |
|---|---|
| 1 | 0.0393701 |
| 5 | 0.19685 |
| 10 | 0.393701 |
| 12.7 | 0.5 |
| 25.4 | 1 |
| 50 | 1.9685 |
| 100 | 3.93701 |
| 150 | 5.90551 |
| 300 | 11.811 |
| 1000 | 39.3701 |
A credit card is 0.76 mm thick, about 30 thou. A 12.7 mm drill bit is exactly half an inch. Standard plasterboard at 12.5 mm is 0.49 inches, which is why it is sold as half-inch board.
Where the two units came from
Engineering adopted the millimetre because it removes decimals from most dimensions, and decimals are where errors hide. American machining went the other way and subdivided the inch instead, first into binary fractions and then into thousandths, which is why a US machinist talks about a part being two thou oversize while a European one says fifty microns. Both describe the same thing; the trouble starts when a drawing crosses between them without the unit stated.
Round the result to the precision your own work needs. These converters carry the full factor, which is useful for checking but rarely what you want to write down.
Most people arrive here on the way to something else, so it is worth knowing the cm to inches, which handles the next part, and the feet to cm, which covers the case just beyond.
Frequently Asked Questions (FAQs)
How many millimetres are in an inch?
Exactly 25.4. Unlike most conversions this is a definition rather than a measurement, so there is no uncertainty in it.
What is a thou or a mil?
One thousandth of an inch, 0.0254 mm, used in American machining and for material thickness. Be careful in Europe, where mil is sometimes used informally to mean millimetre, which is forty times larger.
Why do engineers prefer millimetres to centimetres?
Because most engineering dimensions then come out as whole numbers, which removes a whole class of decimal point errors. Technical drawings often omit the unit entirely on the understanding that everything is in millimetres.