Inches to Millimeters
( in → mm )
Instantly convert inches to millimeters. Enter any value below and get an accurate result with the full formula and reference table.
How to Convert Inches to Millimeters
To convert inches to millimeters, multiply the number of inches by 25.4. This factor comes from the definition: 1 inch = 2.54 centimeters, and 1 centimeter = 10 millimeters, so 1 in = 2.54 × 10 = 25.4 mm exactly.
Step-by-Step Example
Let’s convert 3 inches to millimeters:
1. Multiply by 25.4: 3 × 25.4 = 76.2 mm
2. So 3 inches equals 76.2 millimeters.
For a measurement of 0.5 inches: 0.5 × 25.4 = 12.7 mm.
Why 25.4?
In 1959, the International Yard and Pound Agreement standardized the inch as exactly 25.4 mm across the US, UK, Canada, Australia, New Zealand, and South Africa. Before that, slight regional differences caused small inconsistencies. This exact definition makes conversions precise and reliable.
When You’d Use Each Unit
Inches are commonly used in the US and UK for screen sizes, clothing, and construction. Millimeters are widely used in engineering, machining, and for small measurements where precision matters. Converting between them is essential for international manufacturing, DIY projects, and reading technical drawings.
Common inches values converted to millimeters — typical sizes highlighted.
| Inches (in) | Millimeters (mm) | Approximate |
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