MM to Fraction Inch Converter
Millimetres to convert
Enter millimetres to see the nearest inch fraction at four precisions at once.
Decimal inches
awaiting inputAwaiting input.
Enter millimetres to convert.
- Nearest 1/8"
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- Nearest 1/16"
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- Nearest 1/32"
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- Nearest 1/64"
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This calculator rounds a millimetre value to the nearest inch fraction at four precisions simultaneously, so you can compare them at a glance. 10 mm rounds to 3/8" at both eighth- and sixteenth-inch precision, but to 25/64" at sixty-fourth precision.
One MM Value, Four Answers
"What's this in inches?" doesn't have one answer once fractions are involved — it depends entirely on how fine a fraction you're allowed to use.
A tape measure only shows sixteenths, so rounding a millimetre spec to the nearest 1/16" tells you what a carpenter would actually mark. A machinist's rule shows sixty-fourths, so the same millimetre value rounds to a different, finer fraction for that context. Seeing all four side by side — eighths, sixteenths, thirty-seconds and sixty-fourths — shows exactly how much precision you gain (or lose) at each step.
The MM to Fraction Formula
The same rounding rule runs four times, once per denominator.
i is the nearest numerator at a chosen denominator (8, 16, 32 or 64), after first converting the millimetre value to decimal inches.
- MMThe input length in millimetres, such as 10.
- DenThe denominator being tested: 8, 16, 32 or 64.
- iThe rounded numerator at that denominator.
Worked calculation — 10 mm at 1/64"
- 10 ÷ 25.4 = 0.3937Convert to decimal inches
- 0.3937 × 64 = 25.197Multiply by the denominator
- round(25.197) = 25Round to the nearest whole numerator
10 mm ≈ 25/64" at sixty-fourth-inch precision
Millimetres Across All Four Precisions
| MM | 1/8" | 1/16" | 1/32" | 1/64" |
|---|---|---|---|---|
| 5 mm | 1/4" | 3/16" | 3/16" | 13/64" |
| 10 mm | 3/8" | 3/8" | 13/32" | 25/64" |
| 15 mm | 5/8" | 9/16" | 19/32" | 19/32" |
| 20 mm | 3/4" | 13/16" | 25/32" | 25/32" |
| 25 mm | 1/1" | 1/1" | 31/32" | 63/64" |
Rounding Without a Calculator
- Convert mm to decimal inches. 15 mm ÷ 25.4 = 0.5906".
- Multiply by your chosen denominator and round. At 1/16": 0.5906 × 16 = 9.45, which rounds to 9.
- Place the rounded number over the denominator and simplify. 9/16" — already in lowest terms.
Frequently Asked Questions
What is 10mm as a fraction of an inch?
10 mm is 0.3937" as a decimal. That rounds to 3/8" at both eighth- and sixteenth-inch precision, but to 13/32" at thirty-second precision and 25/64" at sixty-fourth precision — the best-fitting fraction depends on how fine a measurement you need.
Why does the fraction change depending on precision?
Rounding to a coarser denominator, like eighths, and a finer one, like sixty-fourths, means picking the closest mark on two different rulers. A value that sits almost exactly between two eighth-inch marks can land on a noticeably different — and more accurate — mark once you allow finer sixty-fourths.
Which precision should I trust for a real measurement?
Match the precision to the tool that will actually use the result: eighths or sixteenths for a tape measure and general woodworking, thirty-seconds or sixty-fourths when a machinist's rule or caliper is involved and the extra precision matters.
What is 5mm to the nearest 64th?
5 mm rounds to 13/64" (0.203125") at sixty-fourth-inch precision, compared with its exact decimal value of 0.19685".
Can two different mm values round to the same fraction?
Yes. Many mm values close to each other fall into the same rounding "bucket." For example, both 9.9 mm and 10.1 mm round to 3/8" at eighth-inch precision, even though their exact decimal-inch values (0.3898" and 0.3976") are different.
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