O-Ring Measurement Guide — ID, CS & OD

How to Measure an O-Ring

An O-Ring is defined by two dimensions: Inside Diameter (ID) and Cross-Section (CS, also called cord diameter or wire diameter). Outside Diameter (OD) is calculated: OD = ID + 2 × CS.

1. Place the unstretched O-Ring flat on a clean surface at room temperature.
2. Measure the Inside Diameter (ID) with calipers across the inside of the ring.
3. Measure the Cross-Section (CS) by closing the calipers on the rubber cord itself at any point.
4. Calculate OD = ID + 2×CS, or measure directly across the outer edges as a cross-check.
5. If the O-Ring has deformed or swelled in service, measure the groove (gland) dimensions instead and size from there.

Important: O-Rings stretch when fitted, so always measure an unstretched ring. A used O-Ring that has compression-set will give an inaccurate CS — compare against the groove depth instead.