Calculate inches from millimeters instantly by dividing by 25.4, with decimal and fraction results plus a quick mm to inches conversion chart.
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MM to Inches Formula
To convert a length in millimeters to inches, divide the millimeter value by 25.4. To go the other way, multiply the inch value by 25.4.
in = mm / 25.4
mm = in * 25.4
- in is the length in inches.
- mm is the length in millimeters.
- 25.4 is the fixed number of millimeters in one inch.
The factor 25.4 is exact. Since 1959 an inch has been defined as exactly 25.4 millimeters, so the conversion is not a rounded approximation. The calculator divides by 25.4 when you convert mm to inches and multiplies by 25.4 when you convert inches to mm. The optional rounding setting rounds the inch result to the nearest fraction (such as 1/16 or 1/64) or to a chosen number of decimal places, which is useful when you need a value you can read off a tape measure.
MM to Inches Conversion Chart
Common millimeter values and their inch equivalents, shown as decimals and as the nearest 1/16 inch fraction.
| Millimeters (mm) | Inches (decimal) | Inches (nearest 1/16) |
|---|---|---|
| 1 | 0.0394 | 1/16 |
| 5 | 0.1969 | 3/16 |
| 10 | 0.3937 | 6/16 (3/8) |
| 20 | 0.7874 | 13/16 |
| 25 | 0.9843 | 1 |
| 25.4 | 1.0000 | 1 |
| 50 | 1.9685 | 1 15/16 |
| 100 | 3.9370 | 3 15/16 |
| 250 | 9.8425 | 9 13/16 |
| 500 | 19.6850 | 19 11/16 |
| 1000 | 39.3701 | 39 3/8 |
Example Problems
Example 1. You measure a bolt at 30 mm and want the length in inches.
- Start with the millimeter value: 30 mm.
- Divide by 25.4: 30 / 25.4 = 1.1811 inches.
- Rounded to the nearest 1/16 inch, that is about 1 3/16 inches.
Example 2. A drawing calls for a 2 inch gap and you need it in millimeters.
- Start with the inch value: 2 in.
- Multiply by 25.4: 2 * 25.4 = 50.8 mm.
FAQ
How many millimeters are in one inch?
There are exactly 25.4 millimeters in one inch. This value is defined and does not change, so any mm to inch conversion is just division by 25.4.
How do I convert mm to inches as a fraction?
First convert to a decimal by dividing the millimeters by 25.4, then round that decimal to the nearest common fraction such as 1/16, 1/32, or 1/64 of an inch. The calculator does this for you when you turn on the rounding option and pick a fraction size.
Why does my result have so many decimal places?
Most mm values do not land on a clean inch value, so the exact answer is a long decimal. Use the decimal places setting to shorten the result, or round to the nearest fraction if you are working from a ruler or tape measure.
