O-Ring Size Chart

AS568 O-ring sizes

AS568 dash numbers are grouped by cross-section. Once you know the series, the ID rises in regular steps through the group. That makes identification quick: measure the cross-section first, which gives the series, then match the ID.

Metric O-ring sizes

Metric O-rings drop the dash-number system and state the two dimensions directly as ID x CS in millimetres. A 20 x 2.5 has a 20mm inside diameter and a 2.5mm cord.

The common metric cross-sections are 1.0, 1.5, 1.78, 2.0, 2.5, 2.62, 3.0, 3.53, 4.0, 5.0, 5.33, 6.0 and 7.0mm. The 1.78, 2.62, 3.53 and 5.33 figures are the metric conversions of the AS568 sections, which is why they look odd.

Measuring an O-ring

Lay the ring on a flat surface without stretching it and measure across the inside with a caliper. Then measure the cord thickness at a point the ring has not been squashed flat.

A used O-ring has taken a set and will read undersize on cross-section and oversize on ID, sometimes by 10 percent or more. Measure the groove instead where accuracy matters: groove ID plus twice the groove depth gives you what to order.

O-ring compression and groove sizing

The seal comes from squeeze, and there is a right amount. Too little and it leaks, too much and the material takes a permanent set and then leaks anyway.

Target 15 to 30 percent compression on a static face seal, 10 to 20 percent on a dynamic reciprocating seal, and no more than 10 percent on a rotary application. Groove fill should sit between 70 and 90 percent, leaving room for the elastomer to expand with heat and fluid absorption.