Mm to Inches: Millimeters to Inches Converter
Down to sixty-fourths, with the rounding error shown, for machining and woodworking rather than rough measurement.
Enter a number.
The formula
mm = in × 25.4 (exact)
Exact by definition in both directions, following from the inch being defined as exactly 2.54 centimeters.
Conversion table
| Millimeters | Inches (decimal) | Nearest 1/64 in | Nearest 1/16 in |
|---|---|---|---|
| 1 mm | 0.0394 | 3/64 | 1/16 |
| 2 mm | 0.0787 | 5/64 | 1/16 |
| 3 mm | 0.1181 | 1/8 | 1/8 |
| 5 mm | 0.1969 | 13/64 | 3/16 |
| 6 mm | 0.2362 | 15/64 | 1/4 |
| 8 mm | 0.3150 | 5/16 | 5/16 |
| 10 mm | 0.3937 | 25/64 | 3/8 |
| 12 mm | 0.4724 | 15/32 | 1/2 |
| 15 mm | 0.5906 | 19/32 | 9/16 |
| 20 mm | 0.7874 | 25/32 | 13/16 |
| 25 mm | 0.9843 | 63/64 | 1 |
| 50 mm | 1.9685 | 1 31/32 | 1 15/16 |
| 100 mm | 3.9370 | 3 15/16 | 3 15/16 |
Why the rounding error matters here
A metric fastener or drill rarely has an exact imperial equivalent. 6 mm is 0.2362 inches, and the nearest common imperial drill at 15/64 is 0.2344, roughly two thousandths under. For a clearance hole that is fine. For an interference fit it isn't.
The tool reports the error rather than rounding silently, so you can decide whether the nearest fraction is close enough for the job. For the general case see the decimal to fraction converter.
Common questions
How many inches is 10 mm?
10 millimeters is 0.3937 inches, near enough 25/64 on a machinist's rule.
Is 25.4 mm exactly 1 inch?
Yes. The inch is defined as exactly 25.4 millimeters, so the relationship is exact in both directions.
What drill size is closest to 6 mm?
6 mm is 0.2362 inches. The nearest common imperial drill is 15/64, at 0.2344 inches, about 0.002 inches under.
Why does this show 1/64 rather than 1/16?
Because millimeter values are fine enough that sixteenths lose real precision. At 1/64 the worst-case rounding error is about 0.2 mm; at 1/16 it is four times that.
Related converters and guides
Sources
- NIST Guide to the SI, Appendix B: Conversion Factors. National Institute of Standards and Technology. Source for every conversion factor on this page, and for which factors are exact rather than rounded.