Inch to MM Converter
Convert between inches and millimeters using the exact 25.4 mm definition. Works both directions, entirely in your browser.
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Show the math
One inch is defined as exactly 25.4 millimeters, so the conversion is a simple factor:
mm = inch × 25.4
inch = mm ÷ 25.4
Because the inch is defined from the millimeter, this conversion is exact, not an approximation.
What this does
This tool converts between inches and millimeters in both directions, using the exact definition that one inch equals 25.4 millimeters.
How to use it
- Enter a value in inches or in millimeters.
- Read the converted value in the other unit.
- Use it for drawings, tooling, and stock sizes.
How it works
The inch is defined from the millimeter, so multiplying inches by 25.4, or dividing millimeters by 25.4, is an exact conversion, not a rounded estimate. That precision matters on tight tolerances where errors stack up.
mm = inches × 25.4
Example
1 inch is exactly 25.4 mm, and 0.5 inch is 12.7 mm. Going the other way, 100 mm is about 3.937 inches.
Sources & methodology
Last updated .
Frequently asked questions
How many millimeters are in an inch?
Exactly 25.4 millimeters. The inch is defined from the millimeter, so multiplying inches by 25.4 (or dividing mm by 25.4) is an exact conversion, not a rounded estimate.
Why does machining mix inches and millimeters?
Drawings, tooling, and stock come from both imperial and metric standards, so shops convert constantly. Keeping the exact 25.4 factor avoids stack-up errors on tight tolerances.
Is my data stored?
No. Everything is calculated locally in your browser.
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