How to Measure an O-Ring

Two measurements identify any AS568 size. Neither of them is the outside diameter.

Inside diameter

Lay the ring flat on a hard surface. Do not stretch it, do not hold it up, do not thread it over anything. Measure the inside distance across the centre with calipers, or lay it over a printed scale. Take two measurements at right angles and use the larger — a ring that has been in service will have gone slightly oval, and the larger figure is closer to what it started as.

Cross section

This is the thickness of the rubber itself. Close the calipers gently until they just touch. Rubber compresses under caliper pressure, and squeezing even slightly will read low, which is enough to put you in the wrong series.

Cross section matters more than diameter for identification, because it only takes a handful of values. If your measurement is close to 1.78, 2.62, 3.53, 5.33 or 6.99 mm, it is almost certainly that value exactly, and you have narrowed 361 sizes to a few dozen.

Outside diameter

Don't. It is the least reliable of the three: a flexible ring distorts under the caliper jaws and the error is doubled because it spans two walls. Calculate it instead — OD = ID + 2 × CS — and use it only as a check.

If the ring has been in service

Everything above assumes an undamaged ring. A used one will read undersized because compression set is permanent, and no measuring technique recovers the original dimension. The finder has a checkbox for this that biases results towards slightly larger sizes. Why this happens →

Without calipers

Lay the ring on a millimetre rule and read the inside edges. You will get the ID within half a millimetre, which combined with a cross section estimated against known drill bits or a stack of feeler gauges is usually enough to get within one or two candidate sizes. Take those candidates to the finder and compare against a new ring before ordering a box.