Inches to Millimeters Converter
One Inch equals 25.4 Millimeters (1 in = 25.4 mm). Multiply an Inch value by 25.4 to get the equivalent in Millimeters. The calculator is above; the formula and worked examples are below.
Formula
mm = in × 25.4
Multiply the Inch value by 25.4 to get the equivalent value in Millimeters.
Example Conversions
1 Inch (in)
25.4 Millimeter (mm)
5 Inch (in)
127 Millimeter (mm)
10 Inch (in)
254 Millimeter (mm)
25 Inch (in)
635 Millimeter (mm)
100 Inch (in)
2540 Millimeter (mm)
Conversion Table
| Inch (in) | Millimeter (mm) |
|---|---|
| 0.1 | 2.54 |
| 0.5 | 12.7 |
| 1 | 25.4 |
| 2 | 50.8 |
| 5 | 127 |
| 10 | 254 |
| 25 | 635 |
| 50 | 1270 |
| 100 | 2540 |
| 250 | 6350 |
| 500 | 12700 |
| 1000 | 25400 |
How to Convert Manually
Use these steps to convert Inch values to Millimeter without the calculator.
- Write down the value in Inch (in) that you want to convert.
- Multiply that value by 25.4.
- For example, 10 Inches × 25.4 = 254 Millimeters.
- Label the result as Millimeter (mm).
Relative Size Comparison
This chart shows the visual difference in size between exactly one Inch and one Millimeter.
About Inch and Millimeter
The inch is defined as exactly 25.4 millimeters, one twelfth of a foot. Screen sizes, pipe diameters, and US manufacturing drawings are specified in inches. The millimeter is one thousandth of a meter. Engineering and manufacturing drawings in metric countries are dimensioned almost exclusively in millimeters to avoid decimal points. One Inch equals 25.4 Millimeters (1 in = 25.4 mm). Inch is used in screen and display sizes and in US manufacturing and plumbing specifications. Millimeter is used in engineering and CAD drawings.
Precision and Rounding:Length factors between metric and imperial units are exact by definition (1 ft = 0.3048 m since 1959), so any rounding error comes from display precision, not the factor. Two or three decimal places suit carpentry and DIY work; machining and surveying often require four or more.
Conversion factors per NIST Special Publication 811 and the BIPM SI Brochure (9th edition) (reference).
Worked Example
15Inches equals381Millimeters.
15 in × 25.4 = 381 mm
FAQ
How do I convert Inch to Millimeter?
Multiply the Inch value by 25.4 to get the equivalent value in Millimeters.
What is the formula for Inch to Millimeter?
The formula is mm = in × 25.4.
How many Millimeters are in 1 Inch?
1 Inch equals 25.4 Millimeters.
Can I convert Millimeter back to Inch?
Yes. 1 Millimeter equals 0.03937007874 Inches, and the related converter can reverse the calculation.
What are common names for Inch and Millimeter?
Inch is shown as in and may also be searched as inches, ". Millimeter is shown as mm and may also be searched as millimetre, millimeters, millimetres.
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