This converter turns inches into millimeters instantly, using the exact factor 1 inch = 25.4 millimeters. Type any number and the result updates as you go — nothing is sent to a server, so the arithmetic happens entirely in your browser.
Exactly 25.4 millimetres since the 1959 international yard agreement. One thousandth of a metre — the finest gradation on a standard ruler.
Because the relationship is a simple proportion, one multiplication does the whole job. The table further down gives pre-computed values for the amounts people look up most often, and the result panel also reports the reverse conversion so you can check the figure both ways.
The formula
inches- The amount you are converting, measured in inches
25.4- The exact number of millimeters in one inch
millimeters- The converted result
How it works, step by step
- Enter the number of inches you want to convert.
- The value is multiplied by 25.4, the exact conversion factor.
- The result is displayed in millimeters, rounded for readability.
- The unrounded value and the reverse conversion appear underneath so you can verify it.
Worked examples
Converting 5 inches
Multiply 5 by 25.4. That gives 127 millimeters. A quick sanity check: one inch is larger than one millimeter, so the number should get bigger — and it does.
Converting 72 inches
72 × 25.4 = 1,828.8 millimeters. Working backwards, 1,828.8 ÷ 25.4 returns 72 inches, which confirms the factor is applied the right way round.
How to read your score
Frequently asked questions
How many millimeters are in one inch?
Exactly 25.4. Every conversion on this page is that single number multiplied by your input, so the result carries no rounding error beyond what is shown.
What is the formula to convert inches to millimeters?
Millimeters = inches × 25.4. To reverse it, divide instead: inches = millimeters ÷ 25.4, which is the same as multiplying by 0.03937007874.
Is the inch bigger than the millimeter?
Yes. One inch equals 25.4 millimeters, so it is the larger unit.
Why does my answer differ slightly from other inches to mm converters?
Most differences come from rounding the factor too early. This tool keeps full double precision through the calculation and only rounds at the final display step, and it also shows the unrounded value so you can see exactly where any difference appears.
What is a inch?
Exactly 25.4 millimetres since the 1959 international yard agreement.
What is a millimeter?
One thousandth of a metre — the finest gradation on a standard ruler.
Inches to millimeters reference table
| Inches | Millimeters |
|---|---|
| 0.5 in | 12.7 mm |
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 15 in | 381 mm |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 50 in | 1,270 mm |
| 75 in | 1,905 mm |
| 100 in | 2,540 mm |
| 250 in | 6,350 mm |
| 500 in | 12,700 mm |
| 1000 in | 25,400 mm |
Exact factor: 1 inch = 25.4 millimeters. Values rounded for display.