Fractional Inches to MM Converter (and mm to Nearest Fraction)
Convert fractional inches such as 3/8, 1 1/4 or 5/16 to millimeters, or find the nearest 1/8, 1/16, 1/32 or 1/64 inch for any mm size — for wrenches, drill bits, bolts and pipe.
How it works
decimal inches = whole + numerator ÷ denominator · mm = decimal inches × 25.4 · nearest fraction = round(mm ÷ 25.4 × denominator) ÷ denominator
Tools, fasteners and pipe in the US are sized in fractions of an inch, while metric parts use millimeters. Because 1 inch is exactly 25.4 mm, every fraction converts exactly: 1/16 in = 1.5875 mm. Going the other way, a metric size rarely lands on a fraction, so the converter rounds to the nearest 1/8, 1/16, 1/32 or 1/64 inch, reduces it (8/16 → 1/2) and shows how far off the closest fraction is.
Worked example
0 + 3/8 in = 0.375 in, and 0.375 × 25.4 = 9.525 mm (0.9525 cm). Going the other way, 10 mm = 0.3937 in, whose nearest 1/16 is 6/16 = 3/8 in (9.525 mm, 0.475 mm smaller).
Sources: NIST — Unit conversion (SI and US customary) · NIST SP 811, Appendix B — Conversion factors
Frequently asked questions
How many mm is 3/8 inch?
3/8 in = 9.525 mm. Close metric neighbors are 9.5 mm and 10 mm, so a 3/8″ wrench fits a 9.5 mm nut but is loose on 10 mm.
What fraction of an inch is 1 mm?
1 mm = 0.03937 in, about 1/25 of an inch; the nearest 1/32 is 1/32 in (0.794 mm) and the nearest 1/64 is 3/64 in (1.191 mm).
How do I convert a mixed number like 1 1/4 inch to mm?
Turn it into a decimal (1.25 in) and multiply by 25.4: 1.25 × 25.4 = 31.75 mm.
Is a 1/2 inch pipe really 1/2 inch?
No. Pipe sizes are nominal: 1/2″ pipe has an outside diameter of about 21.3 mm (0.840 in). Use this tool for true measured sizes such as bolts, drill bits and sockets.
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This calculator is for educational purposes. Double-check important results before acting on them.