About the MM to Inches Converter
This mm to inches converter changes millimeters into decimal inches and into the nearest fraction of an inch, the way tape measures, drill bits and wrenches are marked in the US. Choose the fraction precision — eighths, sixteenths, thirty-seconds or sixty-fourths — and the fraction is automatically reduced (8/16 shows as 1/2).
It is built for woodworkers, machinists, mechanics, 3D-printing hobbyists and anyone matching a metric part to an imperial tool or reading a spec sheet from another country.
Because a fraction is rounded to the nearest mark, the tool also shows how far that fraction is from the exact size in millimeters, so you can see at a glance that, for example, a 3/8" socket (9.525 mm) is too small for a 10 mm nut.
With the default inputs, the inches (decimal) is 0.98425 in. Change any value above to recalculate instantly.
How to use the mm to inches converter
- 1Enter the length in millimeters.
- 2Pick the fraction precision that matches your tape measure or tool set.
- 3Read the decimal inches and the nearest fraction.
- 4Check the difference in mm to see how close the fraction is.
Formula and method
The inch is defined as exactly 25.4 millimeters, so dividing by 25.4 gives exact decimal inches. To get a fraction, the decimal inches are multiplied by the chosen denominator d (8, 16, 32 or 64), rounded to the nearest whole number of those marks, and then the fraction is reduced by the greatest common divisor.
The rounding difference is converted back to millimeters so you can judge whether the nearest imperial size is close enough for your fit or tolerance.
- mm
- Length in millimeters
- in
- Length in inches
- d
- Fraction denominator (8, 16, 32 or 64)
Worked examples
25 mm to inches
25 ÷ 25.4 = 0.984 in. To the nearest 1/16 that is 16/16 = 1 inch, which is 0.4 mm larger than 25 mm.
10 mm socket vs 3/8"
10 mm = 0.3937 in. 0.3937 × 16 = 6.3, rounded to 6/16 = 3/8 in (9.525 mm), about 0.475 mm smaller than 10 mm — too small to fit over a 10 mm nut.
6 mm at 1/32" precision
6 ÷ 25.4 = 0.2362 in. × 32 = 7.56, rounded to 8/32, which reduces to 1/4 inch.
300 mm ruler
300 ÷ 25.4 = 11.811 in. × 16 = 188.98 → 189/16 = 11 13/16 inches, just under one foot.
Frequently asked questions
How many mm are in an inch?+
Exactly 25.4 millimeters. This has been the international definition of the inch since 1959.
What is 10 mm in inches?+
10 mm is 0.3937 inches — a little over 3/8" (0.375 in, 9.525 mm). That means a 3/8" socket is too small to fit a 10 mm nut; the nearest imperial size that fits is 13/32" (10.32 mm), which will be slightly loose.
How do I convert mm to a fraction of an inch?+
Divide mm by 25.4, multiply by 16 (or 32 or 64), round to a whole number and put it over 16, then simplify. For 12 mm: 0.472 × 16 = 7.56 → 8/16 = 1/2 in.
What is 1/4 inch in mm?+
1/4 inch is exactly 6.35 mm, 1/8 inch is 3.175 mm and 1/2 inch is 12.7 mm.
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