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Convert mm to in
1 mm = 0.0393700787402 in
Type a value in millimetres and the answer in inches appears as you go. The conversion is exactly 0.0393700787402 in to the millimetre, and it happens on your own machine — this page asks nothing of a server once it has loaded.
Where it runs
In your browser. The number you type is never part of a request.
Exact by definition
1 mm is exactly 0.0393700787402 in — a definition, not a rounded factor.
Answers as you type
No button, no wait. The worked answer is already on the page before any script runs.
Millimetre and Inch
Millimetres are what engineering drawings and datasheets use, because a whole number of them is precise enough for almost anything made by machine and avoids a decimal point that can be lost in a photocopy.
An inch has been exactly 25.4 mm since 1959, when six countries agreed to stop using slightly different ones. Before that a US inch and a UK inch differed in the seventh decimal place.
millimetres to inches in practice
210 mm is 8.268 in
— the short side of a sheet of A4 paper.
1800 mm is 70.87 in
— a standard interior doorway.
millimetres to inches at a glance
| mm | in |
|---|---|
| 1 | 0.0393700787402 |
| 2 | 0.0787401574803 |
| 3 | 0.11811023622 |
| 5 | 0.196850393701 |
| 10 | 0.393700787402 |
| 20 | 0.787401574803 |
| 50 | 1.96850393701 |
| 100 | 3.93700787402 |
Every figure here is computed from the same definition the calculator uses, so the table cannot drift away from the answer above it.
Convert mm to in: common questions
How many inches are in a millimetre?
Exactly 0.0393700787402. One millimetre is 0.0393700787402 inches, and the box above will do it for any figure you type.
Is this conversion exact, or rounded?
The ratio is exact. It is not a rounded constant typed into a table: it is derived from the definitions of the two units, so it agrees with any other correct converter to as many digits as you care to compare.
Why does another converter show a slightly different number?
Almost always rounding rather than disagreement. Many converters cut the answer to two or four decimals, and some carry a factor that was rounded before the multiplication rather than after. The figures here are computed from the exact definitions and only rounded on the way to the screen.
Where these figures come from
The claims this page makes about length units are checkable, and these are the documents that settle them.
How this page works
The factor is a constant in the page and the arithmetic is four operations, so nothing is sent anywhere and nothing needs to be. The number you type never leaves the browser — there is no request for it to travel in.