Inches to mm Converter
Convert inches to millimeters for engineering, hardware, and precision measurement.
How It Works
Formula: in to mm = value × 25.4
Example: 1 in = 25.4 mm
Note: 1 inch equals exactly 25.4 millimeters by the international definition adopted in 1959.
Common Uses
Quick Conversion Table
| in | mm |
|---|---|
| 1 | 25.4 |
| 5 | 127 |
| 10 | 254 |
| 50 | 1270 |
| 100 | 2540 |
| 500 | 12700 |
| 1000 | 25400 |
Rounding Rule
Rounded to 4 decimal places
Error Margin
±0.0001 mm
Precision Guidance
For engineering, keep 2-4 decimals. For hardware, round to 1 decimal.
Common Mistakes
The inch-to-mm factor is exact (25.4), not an approximation. Use 25.4, not 25, for precision work.
Frequently Asked Questions
What is 1 inch in mm?
1 inch equals exactly 25.4 millimeters by international definition.
How do I convert inches to mm?
Multiply the inch value by 25.4. For example, 0.5 in × 25.4 = 12.7 mm.
Why is 25.4 the factor?
Since 1959, the inch has been defined as exactly 25.4 millimeters worldwide.
Is this used for hardware?
Yes, bolt diameters and tool sizes often need conversion between inches and millimeters.
Deep Dive Guide
Inches to mm: Precision Conversion for US Makers
Convert inches to mm with the exact 25.4 factor. See how US hardware, screens, and crafting specs translate into metric precision.
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Exact Constants & Authoritative Sources
Every conversion on this page uses the fixed, internationally agreed constants below — drawn from NIST and the SI Brochure. No values are estimated or rounded as if exact. See the full list on our Sources & Standards page.
- EXACT1 inch (in) = 25.4 millimetres (exact)
- EXACT1 foot (ft) = 0.3048 metres (exact, 12 in)
- EXACT1 yard (yd) = 0.9144 metres (exact, 3 ft)
- EXACT1 mile (mi) = 1609.344 metres (exact, 5280 ft)
- EXACT1 centimetre (cm) = 0.01 metre (definition)
Common Pitfalls & How to Avoid Them
US survey foot is deprecated
The old US survey foot (0.304800609601 m) was retired in 2023 in favor of the international foot (0.3048 m). Modern conversions use the international foot; mixing them drifts over long distances.
Inches vs. centimetres for screens
Display and wheel sizes are often rounded to the nearest convenient number; a "27-inch" monitor is nominal, not a precise 68.58 cm panel.
A unit-conversion reference site has no manufacturer-specific or device calibration parameters, so "layered tuning" does not apply here. The only real error-handling is input validation: the converter rejects non-numeric, empty, or out-of-range entries and shows a clear message, then falls back to exact constants above. No estimated or rounded figures are ever presented as exact.
Authoritative sources for this page
- NIST (U.S. National Institute of Standards and Technology) — NIST SP 330 — The International System of Units (SI), 2019 edition
- BIPM (International Bureau of Weights and Measures) — SI Brochure — The International System of Units (9th edition)
Last Updated 2026 | This converter was reviewed for accuracy