Conversion tool
Convert inches to millimeters instantly
Enter a value, see the result, copy it, and save a PDF snapshot.
Input
Type a value, then press Enter to calculate.
Result
0.000 mm
Rounded for readability. Use the arrows to increase or decrease the number of shown digits.
Estimation mode
Enter your estimate in mm, then reveal to compare.
Reveal summary
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- Within 10%
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- Within 25%
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Trend
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Use this space for project notes before saving as PDF.
Disclaimer: Use calculations at your own risk. For critical applications, verify results against your governing standards/specifications.
How it works
We use mm = in x 25.4.
Exact relationship: 1 in = 25.4 mm.
Example: 0.5 in = 12.700 mm.
Notes: Results are rounded in the default view.
Examples
- 0.5 in = 12.700 mm
- 1 in = 25.400 mm
- 12 in = 304.800 mm
FAQ
What physical quantity do inches and millimeters express?
An inch measures length and is commonly used for component size, stock dimensions, fastener specs, and layout work. A millimeter expresses length, meaning one-dimensional distance such as thickness, part size, spacing, or travel.
What is the difference between inches and millimeters?
Inches belong to the imperial and U.S. customary family, while millimeters belong to the metric SI system. These pages help bridge that system crossover in engineering, construction, manufacturing, and technical communication.
What is the history of the inch?
The inch has roots in older English systems and is now fixed internationally as exactly 25.4 millimeters.
What is the history of the millimeter?
The millimeter comes from the metric system developed in France in the late eighteenth century and is defined as one-thousandth of a meter.
Were the inch and millimeter discovered by a specific person?
The inch evolved from customary practice over time rather than being discovered or invented by one person. The millimeter was not discovered by one person; it emerged from metric-system standardization work by scientists and committees.
Where are inches and millimeters used in science and engineering?
Inches remain common in U.S. tooling, machine parts, legacy prints, construction products, and industrial catalogs. Millimeters are widely used in CAD, machining, inspection, international manufacturing drawings, and precision metrology.
Why show units with every result?
Units remove ambiguity and help prevent copy-and-paste mistakes when dimensions move between drawings, purchasing notes, setup sheets, calculations, and inspection records.
Can I trust this for production-critical design?
Use this for convenience and verify against your governing standard, print, or specification for critical applications. The conversion math is simple, but process control still matters more than a quick lookup tool.
References
- Exact constant used: 1 in = 25.4 mm.
- Unit definitions are aligned with modern customary and SI relationships.