Get the dimensions right.

Two measurements. One clear size.

The inside diameter (ID) is measured across the opening. The cross-section (CS) is the thickness of the ring. Express the size as ID × CS and always include units.

Measure an unstretched sample

Place the O-ring on a flat surface without stretching it. Use appropriate measuring equipment and avoid compressing the rubber. A used seal may have swollen, shrunk or taken a permanent set; use the original drawing when possible.

Search standard O-ring dimensions ↗

Match the reference, not the nearest size

A metric size and an inch-series size can look similar without being interchangeable. Record the exact size designation, units and drawing tolerances. Check the applicable ISO 3601 or AS568 reference before ordering.

Geometry does not establish seal performance

Outside diameter = inside diameter + twice the cross-section. This calculation describes the free ring only. Gland design, squeeze, stretch and clearance still need engineering review.

What should accompany the drawing?

• Compound and hardness requirements
• Media, minimum and maximum temperatures, working pressure
• Static or dynamic duty and any cleaning cycles
• Quantity, required documentation and inspection criteria

General dimensional context: Trelleborg O-ring standards overview. This guide is not a substitute for an approved gland design or current standard.