Complete AS568 / ISO 3601 O-ring size chart with ID, cross-section and OD in inches and mm.
O-rings are circular sealing elements used in almost every mechanical system that contains fluid or gas under pressure — from hydraulic cylinders and pneumatic systems to engines, pumps, valves, and household appliances. An O-ring sits in a groove and is compressed between two surfaces to create a leak-proof seal.
Getting the right O-ring size is critical — too small and it won't seat properly, too large and it won't fit in the groove. O-ring sizes are standardised internationally under several standards: AS568 (American standard, most widely used globally), ISO 3601 (international metric standard), BS1806 (British standard), and JIS B2401 (Japanese standard).
CalcGlobe's O-ring size chart covers the complete AS568 standard from size 001 to 475, and the ISO 3601 metric series. Enter the O-ring code or inner diameter to instantly find the inner diameter (ID), cross section diameter (CS), and outer diameter (OD) in both millimetres and inches.
OD = ID + 2 \times CS
Inputs: AS568 O-ring code: 214
AS568 is the American standard published by the Society of Automotive Engineers (SAE) and uses imperial-based sizing with dash numbers (e.g. -214). It is the most widely used standard globally, especially in the USA, UK, and Australia. ISO 3601 is the international metric standard used primarily in Europe and Asia. The two standards are not directly interchangeable — the sizes do not always match exactly, so always confirm which standard your equipment uses before ordering replacement O-rings.
Material selection depends on the fluid being sealed and the operating temperature. NBR (Nitrile) is the most common and works with mineral oils and fuels at −40 °C to +120 °C. FKM (Viton) is used for aggressive chemicals, fuels, and high temperatures up to +200 °C. EPDM is best for water, steam, and brake fluids. Silicone is used for extreme temperature ranges (−60 °C to +230 °C) but has poor resistance to oils. PTFE is chemically inert but is not an elastomer — used as a backup ring.
Place the O-ring on a flat surface. Measure the inner diameter (ID) across the inside of the ring. Then measure the cross section (CS) — the thickness of the rubber cord. Use these two measurements to find the matching standard size in the chart. A digital calliper gives the most accurate measurements. Note that used O-rings may be slightly stretched or compressed — measure carefully or compare to the groove dimensions.
Durometer measures the hardness of the rubber on the Shore A scale. Standard O-rings are 70 Shore A — suitable for most applications. Softer O-rings (60 Shore A) provide better sealing at low pressures and on rough surfaces. Harder O-rings (80–90 Shore A) are used in high-pressure applications and extrusion-prone environments. The durometer is not part of the size standard but is specified separately when ordering.
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