The exact figure
Millimetres to inches is the same 2.54 relationship shifted one decimal place, and it is the version that dominates practical work because millimetres are the working unit of engineering drawings almost everywhere. A metric drawing gives whole millimetres; an imperial drawing gives fractions.
Useful anchors: 25.4 mm is exactly one inch, 12.7 mm is exactly half an inch, and 6.35 mm is exactly a quarter. Those three cover a surprising amount of workshop conversion without a calculator.
The fractions that matter
Imperial tools work in halves of halves, so a decimal answer needs rounding to something a rule is actually marked with. The common sixteenths are 1/16 at 1.5875 mm, 1/8 at 3.175 mm, 3/16 at 4.7625 mm, 1/4 at 6.35 mm, 3/8 at 9.525 mm, 1/2 at 12.7 mm, 5/8 at 15.875 mm, 3/4 at 19.05 mm and 7/8 at 22.225 mm.
Reading that list the other way is the more useful skill. A 10 mm hole is between 3/8 (9.525) and 13/32 (10.319), closer to 13/32. A 20 mm dimension sits just above 25/32. Nothing in metric lands neatly on an imperial fraction, because the systems divide by different numbers — ten against two.
Spanners, sockets and the near misses
Metric and imperial fastener sizes overlap closely enough to be genuinely dangerous to threads. A 13 mm spanner is 0.512 inches while a 1/2 inch spanner is 12.7 mm — a 0.3 mm gap that will often engage and will round the corners of a bolt head under load.
The notorious pairs are 10 mm against 3/8 inch (9.525 mm), 19 mm against 3/4 inch (19.05 mm, close enough to be nearly interchangeable), and 22 mm against 7/8 inch (22.225 mm). The 19 mm and 3/4 pairing works in practice; the 13 mm and 1/2 inch pairing does not, and knowing which is which saves fasteners.
Where mm is standard regardless of country
Millimetres dominate several fields worldwide even in the United States. Firearm calibres, camera lens focal lengths, screen bezels, PCB layouts, bearing dimensions and most automotive fasteners are all specified in mm globally. Photographic film sizes — 35 mm, 120 — and ammunition designations like 9 mm are so embedded that they function as names rather than measurements.
The practical implication is that a US workshop needs both sets of tools and both mental scales, which is precisely why a converter that also gives the nearest fraction is more useful than one that stops at four decimal places.
Frequently asked questions
How many mm in an inch?
Exactly 25.4 mm, by definition since 1959. Half an inch is exactly 12.7 mm and a quarter is exactly 6.35 mm.
What is 10 mm in inches as a fraction?
0.3937 inches, which falls between 3/8 (9.525 mm) and 13/32 (10.319 mm), closer to 13/32. No round metric figure lands neatly on an imperial fraction.
Can I use a 13 mm spanner on a 1/2 inch bolt?
It will often engage but should be avoided — a 1/2 inch spanner is 12.7 mm, so a 13 mm leaves 0.3 mm of play and tends to round the bolt head under load. The 19 mm and 3/4 inch pairing is close enough to work; 13 mm and 1/2 inch is not.