Reference guide
How to Measure an O-Ring
For identification, the most useful measurements are inside diameter (ID), outside diameter (OD), and cross-section (CS).
1. Measure ID
Measure across the inside opening without stretching the ring. Use light caliper contact so the elastomer is not visibly deformed.
2. Measure CS
Measure the round cross-section at a point where the ring is relaxed. Avoid squeezing it with the caliper jaws.
3. Verify with OD
As a consistency check, the nominal relationship is OD = ID + 2 × CS.
Important: dimensional matching does not prove interchangeability. Application conditions, elastomer, hardness, temperature, fluid compatibility, pressure, groove geometry and tolerances also matter.
AS568 vs metric
AS568 is an inch-based size designation. A millimetre number shown on an AS568 page is a unit conversion of the inch nominal dimensions; it should not automatically be treated as an identical metric-standard O-ring.