How to convert in to mm
mm = in × 25.4. The scale factor is the source-unit factor divided by the destination-unit factor.
Worked example
For a measured distance or component dimension of 5 in converts to 127 mm.
- Start with 5 in.
- Multiply 5 by 25.4.
- Report 127 mm, rounded to the precision of your original measurement.
Check the reverse conversion
1 mm = 0.03937007874 in. Use the millimeter to inch converter to calculate in the opposite direction. This has its own formula and reference table.
Definitions that matter
An international inch is exactly 2.54 cm and a foot is exactly 0.3048 m. These tools do not use historical US survey feet. For square measurements, use the area category instead.
The base unit for this category is m. One in corresponds to 0.0254 m; one mm corresponds to 0.001 m. These definitions explain the relationship rather than relying on a rounded display value.
Sources and precision
Definitions: NIST Guide to the SI. Calculations use double precision; the tool defaults to a readable rounded result, while tables show up to ten significant digits. Displaying more digits does not increase the accuracy of the original measurement.
Read our calculation methodology and content standards, or continue with length definitions and common conversions.