Compress an O-ring 20–30% of its cross-section for static seals, only 10–20% for dynamic.
Too little squeeze leaks; too much accelerates compression set and, in dynamic service, friction and wear.
Not on a rotating shaft, where 20% squeeze generates enough friction heat to harden the elastomer in service. And a hard-durometer ring in a wide gland can hit the right squeeze and still extrude into the clearance gap under pressure — the squeeze was never the only thing being checked.
Estimate with the rule, then check it against the calculator that models it properly.
Open O-Ring Calculator →