FractionReference

Metric → fractional inches

Type a metric measurement, get the marks a rule actually has: the nearest fraction on each grid, each with its exact difference from what you asked for.

Try:

Presets are conveniences, not standards: they pick which denominators count as “usable.” School stops at twelfths; machining runs to 128ths.

Common questions

What is 10 mm in fractional inches?
10 mm is 0.3937008″ — a hair under 25/64″. The nearest 64th is 25/64″ at 9.921875 mm, 0.078 mm undersize; the nearest 16th is 3/8″ at 9.525 mm, 0.475 mm undersize.
What is 6.7 mm in fractional inches?
6.7 mm is 0.2637795″. The nearest 64th is 17/64″ at 6.746875 mm, 0.047 mm oversize — close enough for most work, and the table shows the neighbours in case it is not.
Why show the error instead of just the answer?
Because “nearest” without “how near” is how holes end up the wrong size. Every candidate carries its exact signed difference, so whether it is close enough stays your call, made with real numbers.
Where this comes from. Exact rational arithmetic on the values shown — fractions are pairs of integers until display, and inch–metric conversion uses 25.4 mm to the inch, exact by definition since 1959. See the full methodology.