Inches and 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.
The millimeter is one thousandth of a meter. Engineering drawings, bolt and drill sizes, rainfall, camera lenses and jewelry use millimeters.
Worked example
Convert 0.5 in: 0.5 × 25.4 = 12.7, so 0.5 inches is 12.7 millimeters. Likewise 0.1 in = 2.54 mm.
Estimating in your head
Multiply by 25 for a quick figure; it is off by about 1.6%. For 0.5 in that gives 12.5 mm, against 12.7 exactly.
Going the other way
To convert millimeters to inches, divide by 25.4 (or multiply by 0.0393700787402). The swap button between the boxes opens that page.
How this works
mm = in × 25.4
1 in = 0.0254 m and 1 mm = 0.001 m, both by definition, so the factor is their ratio: 25.4 exactly. 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 millimeters are in a inch?
1 inch = 25.4 millimeters, exactly. The value follows from the unit definitions, so it never changes.
How do I convert inches to millimeters?
Multiply by 25.4. For example, 0.1 in × 25.4 = 2.54 mm.
What is 0.2 in in millimeters?
0.2 in = 5.08 mm. 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/inches-to-mm/