mm to Inches Converter
1 inch equals exactly 25.4 mm, so 100 mm is about 3.94 inches. This converter switches between millimeters, centimeters, meters, and inches in either direction. Enter a value and pick the units to size a bolt, read a drawing, or convert a metric spec to imperial.
Quick answer
1 inch is defined as exactly 25.4 millimeters.
Result
3.937008 in
What this tells you
- •1 inch is defined as exactly 25.4 millimeters.
- •To convert mm to inches, divide by 25.4.
- •To convert inches to mm, multiply by 25.4.
- •Switch the from and to units to convert in either direction.
How to Use
- 1Enter the measurement you want to convert.
- 2Choose the unit you are converting from.
- 3Choose the unit you are converting to.
- 4Read the converted measurement below.
How It Works
Formula
inches = mm / 25.4The inch has been defined as exactly 25.4 mm since 1959, so the conversion is exact rather than approximate. Divide millimeters by 25.4 to get inches, or multiply inches by 25.4 to get millimeters. Centimeters are 10 mm and meters are 1000 mm, so they convert through the same base.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Convert 100 mm to inches
Divide 100 by 25.4 to get about 3.94 inches.
Convert 2.5 inches to mm
Multiply 2.5 by 25.4 to get 63.5 mm.
Common mm to Inches Conversions
Frequent sizes, rounded to 4 decimal places.
| Millimeters | Inches | Close fraction |
|---|---|---|
| 1 mm | 0.0394 in | 3/64 in |
| 5 mm | 0.1969 in | 13/64 in |
| 10 mm | 0.3937 in | 25/64 in |
| 20 mm | 0.7874 in | 25/32 in |
| 25.4 mm | 1 in | 1 in |
| 50 mm | 1.9685 in | 1 31/32 in |
| 100 mm | 3.937 in | 3 15/16 in |
Fractions are the nearest 64th of an inch, which is how tape measures and drill bits are usually marked.
Common mistakes
- Dividing by 25 instead of 25.4. The rounding error grows fast on larger measurements.
- Mixing up mm and cm. 10 mm is 1 cm, so a 100 mm part is 10 cm, not 100 cm.
- Rounding too early when chaining conversions. Convert first, then round the final answer.