MM to Fraction Inch Converter
Enter a millimeter value to see the nearest inch fraction at four precision levels side by side.
Why show four precisions at once
A single millimeter value rounds to a different fraction depending on how fine a denominator you allow. 10 mm rounds to 3/8" at 1/8" precision, but to the more exact 25/64" at 1/64" precision. Comparing all four levels side by side shows how much rounding error each precision level introduces for your specific measurement, instead of committing to one denominator before you have seen the trade-off.
The conversion steps
- Divide the millimeter value by 25.4 to get decimal inches.
- Multiply the decimal part by your chosen precision's denominator (8, 16, 32, or 64).
- Round to the nearest whole number to get the fraction's numerator.
- Simplify the fraction using the Greatest Common Divisor (GCD).
Common metric-to-fraction values
| Millimeters | Decimal inches | Nearest 1/16" |
|---|---|---|
| 3 mm | 0.1181" | 3/16" |
| 5 mm | 0.1969" | 3/16" |
| 6 mm | 0.2362" | 1/4" |
| 8 mm | 0.3150" | 5/16" |
| 10 mm | 0.3937" | 3/8" |
| 12 mm | 0.4724" | 1/2" |
| 15 mm | 0.5906" | 9/16" |
| 20 mm | 0.7874" | 13/16" |
When to use this instead of a single-precision chart
If you already know the precision your tool measures at, the MM to Inches Calculator gives one fraction at a time with a precision selector. Use this page instead when you want to compare rounding error across precision levels before choosing which fraction to cut or drill to. To go the other direction, from an inch fraction to millimeters, use the Inch Fraction to MM Converter.
Frequently Asked Questions
Related Tools
Back to the main decimal to inches calculator.