Millimeters and inches
The millimeter is one thousandth of a meter. Engineering drawings, bolt and drill sizes, rainfall, camera lenses and jewelry use millimeters.
The inch is exactly 25.4 millimeters, one twelfth of a foot. Screen diagonals, wheel and tire sizes, pipe threads, and most US measurements of small objects use inches.
Worked example
Convert 25 mm: 25 × 0.03937007874 = 0.9842519685, so 25 millimeters is 0.9842519685 inches. Likewise 20 mm = 0.7874015748 in.
Estimating in your head
Divide by 25 for a quick figure; it is off by about 1.6%. For 25 mm that gives 1 in, against 0.984252 exactly.
Going the other way
To convert inches to millimeters, divide by 0.03937007874 (or multiply by 25.4). The swap button between the boxes opens that page.
How this works
in = mm × 0.03937007874
1 mm = 0.001 m and 1 in = 0.0254 m, both by definition, so the factor is their ratio: 0.03937007874 (rounded here; the page uses the full value). Results drop binary floating-point noise beyond 15 significant digits and display at the precision you choose (6 significant figures by default). Copy result copies the full value.
Frequently asked
How many inches are in a millimeter?
1 millimeter = 0.03937007874 inches. The factor is exact by definition but has endless decimals, so it is shown rounded; the page uses the full value.
How do I convert millimeters to inches?
Multiply by 0.03937007874. For example, 20 mm × 0.03937007874 = 0.7874015748 in.
What is 100 mm in inches?
100 mm = 3.937007874 in. The table next to the converter lists more common values.
Is the result rounded?
Only on screen. The page computes with the exact definitions and removes floating-point noise past 15 significant digits; pick 3 to 12 significant figures, or use Copy result for the full value.
Sources
- NIST SP 811 Appendix B.8 — Factors for units listed alphabetically
- Federal Register 59-5442 (1959) — Refinement of values for the yard and the pound
- Wikipedia — Conversion of units of length
Reviewed Oct 17, 2026 · test vectors fixtures/units/length/mm-to-inches/