FractionReference

The ruler

One inch, laid out the way a rule engraves it: longer marks for coarser fractions. Tap any mark — or tab to it — to read it in every notation. Choose how fine a rule to show.

0 1″ 5mm 10mm 15mm 20mm 25mm 1/4 1/2 3/4
Tap a mark to read it here.

Common questions

How do I read the marks on a ruler?
By halving. The longest mark between the inch lines is 1/2; the next longest are the quarters, then eighths, then sixteenths. A mark’s length tells you its family — count which halving you are in and that is the denominator.
Why do finer marks get shorter?
So the eye can find the coarse marks first. Length encodes the denominator, which is why you can read 3/8 at a glance without counting sixteen ticks.
Where is 10 mm on this ruler?
Just short of the 25/64 mark — 10 mm is 0.3937 inches, and 25/64″ is 0.390625″. The metric overlay draws the millimetre grid underneath so the two systems can be compared directly.
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.