Inch to mm Calculator
Convert inches to millimetres, or millimetres to inches, in an instant. Enter a whole number, decimal, or fraction of an inch to get an accurate, exact conversion.
Length Conversion
Enter a length and choose your conversion direction
Optional: add a fraction of an inch (e.g. 1/2, 3/8) on top of the whole number.
Your Conversion Result
Converted length and unit breakdown
Enter a length above and click Convert Length to instantly see your result in the other unit.
Common Inch to mm Conversions
A quick-glance table of commonly searched inch-to-millimetre conversions, including standard fractional sizes used in engineering, woodworking, and DIY.
| Inches | Millimetres | Millimetres | Inches |
|---|---|---|---|
| 1/16 in | 1.5875 mm | 1 mm | 0.0394 in |
| 1/8 in | 3.175 mm | 5 mm | 0.1969 in |
| 1/4 in | 6.35 mm | 10 mm | 0.3937 in |
| 1/2 in | 12.7 mm | 25 mm | 0.9843 in |
| 1 in | 25.4 mm | 50 mm | 1.9685 in |
| 12 in (1 ft) | 304.8 mm | 300 mm | 11.811 in |
Inch to mm FAQ
Everything you need to know about converting between inches and millimetres, including exact conversion factors and fraction handling.
To convert inches to millimetres, multiply the length in inches by 25.4. For example, 2 inches multiplied by 25.4 equals 50.8 millimetres.
To convert millimetres to inches, divide the length in millimetres by 25.4. For example, 100 millimetres divided by 25.4 equals approximately 3.937 inches.
Yes. Since 1959, the international inch has been defined as exactly 25.4 millimetres by international agreement, so this conversion factor is exact rather than rounded or approximate.
To convert a fraction of an inch to millimetres, first convert the fraction to a decimal by dividing the numerator by the denominator, then multiply the result by 25.4. For example, 3/8 inch equals 0.375 inches, which equals 9.525 millimetres.
Many industries, particularly in the United States, still specify components in inches due to legacy tooling and standards, while most of the rest of the world uses millimetres under the metric system. Technical drawings often show both units side by side to support manufacturing and assembly across regions.
A quick approximation is to multiply the number of inches by 25. This gives a close estimate, though it slightly underestimates the true value since the exact factor is 25.4 rather than 25, with the error growing for larger measurements.