Inches to Millimeters (in to mm) Converter
1 inch equals exactly 25.4 millimeters — multiply the number of inches by 25.4 to convert. The inch is defined internationally as 2.54 cm, which is exactly 25.4 mm, so this factor has no rounding.
The formula
To convert inches to millimeters, multiply by 25.4:
millimeters = inches × 25.4
For example, 10 in × 25.4 = 254 mm. To reverse it, divide millimeters by 25.4 (or multiply by 0.03937008).
Conversion table
| Inches (in) | Millimeters (mm) |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 5 | 127 |
| 10 | 254 |
| 12 | 304.8 |
| 25 | 635 |
| 50 | 1,270 |
| 100 | 2,540 |
| 500 | 12,700 |
| 1000 | 25,400 |
Worked example
Say a fitting is 3.5 inches and you need millimeters for a metric spec. Multiply: 3.5 × 25.4 = 88.9 mm. Reversing, 88.9 ÷ 25.4 = 3.5 inches. A 1/4-inch bolt is 0.25 × 25.4 = 6.35 mm, and a 1/2-inch pipe is 0.5 × 25.4 = 12.7 mm — figures machinists reach for daily. Since the factor is exactly 25.4, each added inch contributes another 25.4 millimeters, so 10 inches is 254 mm and 100 inches is 2,540 mm. The converter is invaluable in CAD and 3D printing, where imperial part sizes must be entered as the millimeters used on metric drawings, and rounding by hand risks costly fit errors. Reverse conversions simply divide the millimeter value by 25.4.
How to convert online
Let the tool apply the exact factor for you:
- Type the number of inches into the first box.
- Read the equivalent in millimeters instantly in the second box.
- Enter a millimeter value to convert back to inches.
Use the Inch to Millimeter Converter — free and instant in your browser.
Where this conversion is used
Inches-to-millimeters conversions are essential in machining, CAD, 3D printing, plumbing and hardware, where imperial part sizes must be expressed in the millimeters used on metric drawings. A 1/2-inch pipe, for example, is 12.7 mm.
FAQ
How many millimeters are in an inch?
One inch equals exactly 25.4 millimeters. Ten inches equal 254 mm.
What is the formula to convert inches to millimeters?
Multiply the inch value by 25.4: mm = in × 25.4.
How do I convert millimeters back to inches?
Divide the millimeter value by 25.4, or multiply by 0.03937008. For example, 127 mm = 5 inches.
Is the conversion exact?
Yes. The inch is defined as exactly 25.4 millimeters, so there is no rounding in the factor.
Related converters
- Millimeter to Inch Converter
- Inch to Centimeter Converter
- Inch to Meter Converter
- Inch to Foot Converter
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