Identify an o-ring from its measurements.
Measure the inside diameter and the cross section. Get the AS568 dash number. 361 standard sizes, free, no signup, no quote form.
Why measuring a used o-ring misleads you
An o-ring that has been compressed in a groove takes a permanent set. Pull one out after a few years of service and it will measure undersized, sometimes noticeably so, and it will no longer be round. Match those numbers to a chart literally and you order a size too small, which then fails the same way.
That is what the checkbox above is for. Tick it and the search weights results towards sizes slightly larger than what you measured, which is almost always the size that came out of the box.
Measuring properly
Lay the ring flat on a hard surface without stretching it and measure the inside distance across the centre — that is the ID. Then measure the thickness of the rubber itself with calipers, gently, without squeezing: that is the cross section. The outside diameter is simply ID plus twice the cross section, so you never need to measure it. Full measuring guide →
The AS568 series
Dash numbers are grouped into series by cross section. Knowing the series narrows the search enormously, because every ring in it shares one thickness.
| Series | Cross section | Dash range | Typical use |
|---|---|---|---|
| 000 | 1/16" | 001–050 | Pneumatics, instrumentation, small static seals |
| 100 | 3/32" | 102–178 | Hydraulics, pumps, valves |
| 200 | 1/8" | 201–284 | General purpose, cylinders |
| 300 | 3/16" | 309–387 | Heavy hydraulics, higher pressure |
| 400 | 1/4" | 425–475 | Industrial flanges and piping |
| 900 | mixed | 901–932 | SAE straight-thread boss gaskets |