Millimeters to Inches Converter (mm to in)
Convert millimeters to inches or inches to millimeters using the exact international definition of 25.4 millimeters per inch.
How it works
Millimeters ÷ 25.4 = inches. Inches × 25.4 = millimeters.
Formula / calculation method
inches = millimeters ÷ 25.4millimeters = inches × 25.4
Worked examples
- 25.4 mm equals exactly 1 inch.
- 10 mm is approximately 0.39370079 inches.
When this tool helps
Useful for hardware, machining, technical drawings, screen sizes, product specifications, fasteners, and metric–imperial dimension checks.
Related calculations: centimeters to inches, meters to feet, and feet to inches.
Browse this topic: Length & Distance Converters · Length & Distance Standards Guide.
Quick reference
| Input | Result |
|---|---|
| 1 mm | 0.03937 in |
| 10 mm | 0.39370 in |
| 25.4 mm | 1 in |
Accuracy and definitions
The inch is defined as exactly 25.4 millimeters, so the conversion factor itself is exact. Displayed results may be rounded for readability.
Reference standards
Where a published measurement definition applies, Solveberry links to primary standards rather than relying only on secondary converter sites.
- NIST Guide to the SI — Appendix B.8 conversion factors
- BIPM SI Brochure — International System of Units
Methodology & verification
- Method
- Millimeters ÷ 25.4 = inches. Inches × 25.4 = millimeters.
- Verification
- The displayed formula is cross-checked against Solveberry regression cases before release. Calculator inputs and results stay in the browser.
- Last reviewed
- 2026-08-19
Results are calculated locally in your browser. Optional analytics do not include calculator input values or results.
Common questions
Is 25.4 mm exactly one inch?
Yes. The international inch is defined as exactly 25.4 millimeters.
Why do some converted values have many decimals?
The exact conversion can produce a repeating or long decimal. Solveberry keeps calculation precision internally and rounds only the displayed value.
Can I convert in the opposite direction on this page?
Yes. Use the “Convert from” selector or Swap units button; Solveberry keeps both directions on one canonical page.
Why can my result differ slightly from another converter?
Differences usually come from rounding or from selecting a different regional measurement standard. Solveberry shows the factor or standard used on the page.