Millimeters to Inches Converter
Convert millimeters to inches instantly. Enter mm to get inches by dividing by 25.4, with the exact factor and the reverse inches to mm direction shown.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
How the Millimeters to Inches Converter Works
This tool converts millimeters (mm) to inches (in) and back. Millimeters are a metric length unit common in engineering and manufacturing, while inches are the imperial unit used in the US. The inch is defined as exactly 25.4 mm.
The Formula
Inches = millimeters ÷ 25.4 Millimeters = inches × 25.4
Because the inch has an exact definition of 25.4 mm, the conversion is precise.
Worked Example
Convert 100 mm to inches:
- Inches = 100 ÷ 25.4 = 3.94 in
A small measurement, 10 mm:
- 10 ÷ 25.4 = 0.394 in
Going the other way, 2.5 inches in millimeters:
- mm = 2.5 × 25.4 = 63.5 mm
Millimeters to Inches Quick Reference
| Millimeters | Inches |
|---|---|
| 10 | 0.394 |
| 25.4 | 1 |
| 50 | 1.969 |
| 100 | 3.937 |
| 250 | 9.843 |
Where You Use This Conversion
This conversion shows up constantly in DIY, woodworking, 3D printing and machining, where part drawings may be in millimeters but tools and rulers are marked in inches. It is also handy for reading screen and device dimensions, picture frame sizes and hardware specs that mix units. Because the factor is exact, you can rely on the result for precision work.
Frequently asked questions
How many millimeters are in an inch?+
There are exactly 25.4 millimeters in one inch. So 100 mm is about 3.94 inches. To convert mm to inches, divide by 25.4.
How do I convert millimeters to inches?+
Divide the millimeter value by 25.4. For example, 100 mm ÷ 25.4 = 3.94 inches. To reverse it, multiply inches by 25.4 to get millimeters.
What is 10 mm in inches?+
10 millimeters equals about 0.394 inches, a little under four tenths of an inch. Dividing 10 by 25.4 gives 0.3937.
Is the mm to inch conversion exact?+
Yes. Since 1959 the inch has been defined as exactly 25.4 mm, so the conversion involves no approximation. Any rounding only happens when you limit decimal places.
How many mm is a quarter inch?+
A quarter inch equals 6.35 mm (0.25 × 25.4). A half inch is 12.7 mm and three quarters of an inch is 19.05 mm.