Convert inches to millimeters
Converting inches to millimeters is a common length task, whether you are checking a recipe, a spec sheet, a travel distance or a homework problem. Since 1 in equals 25.4 mm, the math is a single multiplication — and this page does it for you instantly, with the formula, a table of common values and answers to the questions people ask most.
Exact factor
How to convert inches to millimeters
To convert inches to millimeters, multiply the number of inches by 25.4. For example, 10 in × 25.4 = 254 mm.
mm = in × 25.4
- Write down the value in inches you want to convert.
- Multiply it by 25.4 (the exact in→mm factor).
- The result is your value in millimeters. Check it below with the converter or the table.
Conversion table
Common values of inches and their equivalents in millimeters:
| in | mm |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 10 | 254 |
| 20 | 508 |
| 25 | 635 |
| 50 | 1,270 |
| 100 | 2,540 |
| 250 | 6,350 |
| 500 | 12,700 |
About these units
Inch (in). An inch is a unit of the imperial and US customary systems, legally defined as exactly 25.4 millimeters since 1959. It is used for screen sizes, tools and construction in the United States.
Millimeter (mm). A millimeter is one thousandth of a meter, the smallest length unit in everyday metric use. It is the standard precision unit in engineering drawings, machining and rainfall measurement.
Frequently asked questions
How do I convert inches to millimeters?
To convert inches to millimeters, multiply the number of inches by 25.4. For example, 10 in × 25.4 = 254 mm. Or skip the math: type the value into the converter at the top of this page and the result updates instantly.
How many millimeters are in 1 inch?
1 in = 25.4 mm. That is the fixed relationship used by this converter and by international standards.
How many inches are in 1 millimeter?
1 mm = 0.0393701 in. If you are converting in the opposite direction, use our Millimeter to Inch converter for the dedicated table and FAQ.
Is the inches to millimeters conversion exact?
Yes. Both units are defined by international agreement, so the factor of 25.4 is exact. Displayed results are rounded to 6 significant digits only for readability.
What is inch used for?
An inch is a unit of the imperial and US customary systems, legally defined as exactly 25.4 millimeters since 1959. It is used for screen sizes, tools and construction in the United States.