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
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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Last Updated 2026 | This converter was reviewed for accuracy