Millimeters to Inches
Convert millimeters to inches in your browser, with the fractional inch that workshops actually use.
in = mm ÷ 25.4
NIST SP 811 (2008), Appendix B.8 — inch (in) to meter (m) = 2.54 E−02
This is the workshop conversion. Drill bits, bolt threads, sheet thicknesses, filament diameters and bearing sizes are catalogued in millimeters in one hemisphere and in inches in the other, and the two rarely line up on a round number. An inch is exactly 25.4 millimeters, so the arithmetic at least is clean.
How it is calculated
in = mm ÷ 25.4
Workshops usually want the answer as a fraction rather than a decimal, because that is how drills and spanners are labelled. A 10 millimeter bolt is 0.3937 inches, which sits between 25/64 and 13/32 — near enough to reach for the wrong spanner if you round carelessly.
Source: NIST SP 811 (2008), Appendix B.8 — inch (in) to meter (m) = 2.54 E−02
Common values
| millimeters | inches |
|---|---|
| 1 | 0.039370079 |
| 3 | 0.11811024 |
| 5 | 0.19685039 |
| 6 | 0.23622047 |
| 8 | 0.31496063 |
| 10 | 0.39370079 |
| 12 | 0.47244094 |
| 20 | 0.78740157 |
| 25.4 | 1 |
| 50 | 1.9685039 |
| 100 | 3.9370079 |
Questions people ask
- Is a 10 mm bolt the same as a 3/8 inch bolt?
- No, and this catches people out. Ten millimeters is 0.3937 inches while 3/8 is 0.375 — a difference of nearly half a millimeter. Close enough to start the thread, far enough to strip it.
- How do I get a fraction instead of a decimal?
- Multiply the decimal part by 64 and round to the nearest whole number, then reduce. 0.3937 inches gives 25.2 sixty-fourths, so 25/64. Workshops rarely go finer than 64ths for hand tools.
- Why is 25.4 exact?
- It was agreed rather than measured. In 1959 the countries still using the inch settled on 25.4 millimeters exactly, replacing several slightly different national inches with one definition tied to the metre.
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