How to Measure an O-Ring
Two dimensions, one common mistake, and when to measure the groove instead.
You only need two numbers to identify an O-Ring: the inside diameter (ID) and the cross-section (CS): the thickness of the cord.
The measurement
Lay the O-Ring flat and measure across the inside for the ID. Then measure the cord
thickness for the CS. With those two figures you can look up the AS568 dash number or
specify a metric ID x CS size directly.
The common mistake
Measuring a used O-Ring. Seals swell in service and take a compression set, so an old ring can read noticeably larger or flatter than the part you actually need. Whenever you can, measure the groove instead and pick the size from the hardware.
For the full standard-size breakdown, see the Sizing page.
