O-ring Selection and Size Standards Guide
The O-ring is the most widely used sealing element in industry. Despite its simple appearance, correct selection requires careful matching of material, size and operating conditions. A wrongly chosen O-ring can cause leakage, pressure loss and equipment failure.
O-ring size standards
An O-ring is defined by two key dimensions: inner diameter (ID) and cross-section diameter (CS, i.e. the thickness of the rubber cord). Two standards dominate internationally:
- AS568 (American/inch system) — a standardized table with fixed cross-section diameters.
- Metric (ISO 3601) — inner diameter and cross-section given in millimetres, for example 20x3 mm.
When choosing a size, the dimensions of the groove that seats the O-ring must also be considered. Groove depth and width directly determine the compression percentage.
Why compression percentage matters
For reliable sealing, an O-ring must be compressed by a certain percentage when installed. In static applications, 15–30% is typically recommended, while dynamic applications call for 10–20%. Excessive compression wears the material quickly, while too little fails to seal.
Material selection
The operating environment determines material choice. EPDM is ideal for water, steam and air. NBR excels in oil and fuel environments. For high temperatures and aggressive chemicals, Viton (FKM) is recommended. Shore hardness should also be considered — 70 Shore A is generally regarded as the universal choice.
Practical recommendations
- Determine the working temperature precisely — about 130 °C for EPDM, above 200 °C for Viton.
- Check the chemical compatibility chart.
- Compare groove dimensions against the standard.
- In dynamic applications, pay attention to surface roughness.
As NUR KAUÇUK MMC, we manufacture O-rings of various sizes from EPDM, NBR and other materials. Thanks to our production capabilities, we also fulfil custom-size orders. For more on general seal selection, see our seal selection guide.
To choose the right O-ring for your project, browse our product range and contact us for technical support.