1 inches to millimeters
25.40 mm
1 in = 25.40 mm.
millimeters
25.40mm
1 in = 25.40 mm
In short
How many millimeters is 1 inches?
1 in = 25.40 mm. 1 inch = 25.400000 millimeters, so the working is 1 × 25.400000 = 25.40.
The working
The factor below is an exact definition rather than a measurement, so nothing is approximated in the arithmetic. The only rounding on this page happens where a number is displayed.
1 inch = 25.400000 millimeters 1 × 25.400000 = 25.40
How much precision do you need?
Each row is the same answer cut at a different number of decimals, with what that choice costs you. Pick the shortest row whose error is smaller than whatever you are measuring with.
| Rounded to | Answer (mm) | Off by |
|---|---|---|
| Whole number | 25 | −0.40000 |
| 1 decimal | 25.4 | exact |
| 2 decimals | 25.40 | exact |
Round trip: take 25.40 mm back the other way and you land on 1.0000 in — a drift of 0.0000 in. That gap is the rounding, not the conversion.
What 1 in looks like
1 in
One inch is exactly 25.4 mm, a definition agreed internationally in 1959 and unchanged since.
Frequently asked questions
What is 1 inches in millimeters?
1 inches is 25.40 millimeters. 1 inch = 25.400000 millimeters, so the working is 1 × 25.400000 = 25.40.
How accurate is 25.40 mm?
Rounding to 25.40 mm and converting straight back gives 1.0000 in, a drift of 0.0000 in from the 1 you started with. The factor itself is exact; every bit of that difference comes from the rounding, not the conversion.
Nearby values
Any value at all
Inches to Mm Converter
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