Inches to Millimeters Converter
Convert inches to millimeters instantly. Exact factor: 1 in = 25.4 mm.
Free toolNo signupFormula shown
Convert inches to millimeters
The exact conversion factor is 1 in = 25.4 mm. Type any value and the result updates instantly; the math is shown beneath the board.
Worked by hand
Multiply the inches value by 25.4 to get millimeters.
Worked example
- Take 1 in
- multiply by 25.4
- the result is 25.4 mm
How to use it
- Enter your inches
- Read the millimeters result
- Check the factor line to verify by hand
Frequently asked questions
How many millimeters are in one inches?
Exactly 25.4 mm, by the SI/NIST definition.
How do I convert millimeters back to inches?
Divide by 25.4 — or use the full converter where both directions are one click.
Is this rounding or exact?
The factor is exact; only the displayed decimals are rounded.