How to convert millimeters to inches
in = x × 0.0393700787401575
One inch is exactly 25.4 mm. The result is decimal inches, useful for comparing metric dimensions with inch specifications.
Worked example
in = 25.4 × 0.0393700787401575
25.4 mm = 1 in.
Conversion table in both directions
Values are generated by the same engine as the interactive converter.
| Input | mm → in | in → mm |
|---|---|---|
| 1 | 0.03937 | 25.4 |
| 5 | 0.19685 | 127 |
| 10 | 0.393701 | 254 |
| 25 | 0.984252 | 635 |
| 50 | 1.968504 | 1270 |
| 100 | 3.937008 | 2540 |
Unit definitions and sources
Ordinary feet, yards and miles use international definitions. The separately labeled US survey foot is for legacy records only. A nautical mile is exactly 1,852 meters; it is not a constant minute of latitude on Earth.
| Unit and system | Use | Definition | Relationship |
|---|---|---|---|
| Millimeter (mm); si | Common units | Millimeter measures length. m = (value in mm) × 0.001. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
| Inch (in); us | Common units | Inch measures length. m = (value in in) × 0.0254. Common use: Dimensions and distances. | Exact definition; numerical display may be rounded |
Sources: BIPM: International System of Units; NIST: SP 811 Appendix B.8: conversion factors; IAU: Measuring the Universe: astronomical unit; NIST: US survey foot: definition and retirement; BIPM: SI prefixes
Precision of length conversions
These 2 length units convert through Meter (m). 2 have an exact defined relationship to the base unit and 0 are conventional or measured values. Results and the tables on this page come from the same engine, so a table value can be checked against the converter directly.
Standard display shows up to six decimal places, switching to scientific notation for very large or small values. Calculation uses JavaScript binary floating point independently of display rounding, so extra digits are not a claim about measurement accuracy. Each unit lists its own source; a citation records where the definition comes from, not an independent review. See Conversion methodology for the review and correction process.