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Convert in to mm
1 in = 25.4 mm
Type a value in inches and the answer in millimetres appears as you go. The conversion is exactly 25.4 mm to the inch, 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 in is exactly 25.4 mm — 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.
Inch and Millimetre
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 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.
inches to millimetres in practice
70 in is 1778 mm
— a height of five foot ten.
15.6 in is 396.2 mm
— a common laptop screen, measured corner to corner.
inches to millimetres at a glance
| in | mm |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 5 | 127 |
| 10 | 254 |
| 20 | 508 |
| 50 | 1270 |
| 100 | 2540 |
Every figure here is computed from the same definition the calculator uses, so the table cannot drift away from the answer above it.
Convert in to mm: common questions
How many millimetres are in a inch?
Exactly 25.4. One inch is 25.4 millimetres, 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.