Convert Inches to Millimeters
Inches to millimeters is a workhorse conversion in manufacturing and engineering, where millimeters are the standard unit on drawings. One inch is exactly 25.4 millimeters.
Inches to Millimeters conversion table
| Inches (in) | Millimeters (mm) |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 5 | 127 |
| 10 | 254 |
| 20 | 508 |
| 50 | 1,270 |
| 100 | 2,540 |
| 500 | 12,700 |
| 1,000 | 25,400 |
The formula
To convert inches to millimeters, multiply the length in inches by 25.4:
millimeters = inches × 25.4
A worked example
Convert a 5-inch bolt length to millimeters:
5 × 25.4 = 127 mm
At a bigger scale, 100 inches is 2,540 mm.
Why it's exact
The inch has been defined as exactly 2.54 centimeters — exactly 25.4 millimeters — since 1959. There's no rounding anywhere in this conversion.
When you'll use it
Converting part dimensions, tool and hardware sizes, or any US-inch measurement that needs to match a millimeter-based engineering drawing or manufacturing spec.
Frequently asked questions
How many millimeters are in an inch?
Exactly 25.4 millimeters. This is a fixed international definition, not a rounded figure.
How many millimeters is 10 inches?
10 × 25.4 = 254 mm.
Why do engineering drawings use millimeters instead of inches?
Most of the world uses the metric system for manufacturing, so millimeters give finer, whole-number precision than fractional inches on a drawing.