MM to Inches Converter
Convert millimeters to inches instantly — or switch the units to convert cm, meters, km, feet, yards and miles either way.
100 mm
Formula used
MM to inches: the formula
One inch is exactly 25.4 millimeters — an international standard since 1959. So:
- mm → inches: divide by 25.4. Example: 100 mm ÷ 25.4 = 3.937 inches.
- inches → mm: multiply by 25.4. Example: 5 in × 25.4 = 127 mm.
The converter above handles all eight units both ways — handy when a TV screen is listed in inches but your wall space is measured in cm, or a marathon is 42.195 km and you want miles.
Quick conversion table
| Millimeters | Inches | Common use |
|---|---|---|
| 10 mm | 0.394 in | 1 cm — small screws |
| 25.4 mm | 1 in | The exact definition |
| 100 mm | 3.937 in | Phone width |
| 304.8 mm | 12 in | 1 foot — ruler |
| 1,000 mm | 39.37 in | 1 meter |
| 1,609,344 mm | 63,360 in | 1 mile |
Where you'll need this in India
India officially uses the metric system, but inches sneak in everywhere: TV and laptop screen sizes, pipe diameters, tyre sizes, cricket bat lengths, and tools. Construction drawings may mix mm and feet-inches. Whenever the units don't match, convert first — a 25.4× error is an expensive mistake on a building site.
Frequently asked questions
How many millimeters are in an inch?
Exactly 25.4 millimeters = 1 inch. This is the international standard agreed in 1959, used by every country including India.
What is the formula to convert mm to inches?
Divide millimeters by 25.4: inches = mm ÷ 25.4. For example, 100 mm ÷ 25.4 = 3.937 inches.
How do I convert inches back to mm?
Multiply inches by 25.4: mm = inches × 25.4. For example, 5 inches × 25.4 = 127 mm. Switch the units above to convert either way.
How many inches is 1 km, and how many miles is 1 km?
1 km = 39,370.08 inches. For miles: 1 km = 0.621371 miles, so 10 km = 6.21371 miles. Select km and miles in the unit boxes above for instant conversion.
Why is 1 inch exactly 25.4 mm?
In 1959, the US, UK, Canada, Australia, New Zealand and South Africa signed the international yard agreement, defining 1 yard as exactly 0.9144 meters. Since 1 yard = 36 inches, 1 inch became exactly 25.4 mm by definition.