AS568 O-Ring Size Chart

The Global O-Ring and Seal Size Reference Chart covers the AS568 O-Ring Sizes published by the Society of Automotive Engineers (S.A.E.), and includes the Extended Sizes in the 400 Series. The purpose of this chart is to specify the inside diameters, cross-sections, tolerances, and size identification codes (dash numbers) for O-rings used in sealing applications and for straight thread tube fitting boss gaskets. Click here to download a digital copy of our AS568 O-Ring Reference Guide.

Series C/S Inches C/S Frac. C/S mm ID Range
Zero .070 1/16 1.78 .029 – 5.239″
100 .103 3/32 2.62 .049 – 9.737″
200 .139 1/8 3.53 .171 – 17.955″
300 .210 3/16 5.33 .412 – 25.940″
400 .275 1/4 6.99 1.350 – 25.940″
900 varies varies 1.42 – 3.0 .185 – 2.337″

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AS568 O-Ring Size Reference Chart

Dash # CS Nominal ID Nominal OD Nominal CS Inch ID Inch Tol Inch CS mm ID mm Tol mm


This size chart was first published in 1971 by the A-6c2 Seals Committee at SAE (Society of Automotive Engineers) as Aerospace Standard AS568. These standards were later republished in 1974 as AS568A, in 2001 as AS568B, in 2008 as AS568C, and most recently in 2014 as AS568D. Any of these terms (AS568, AS568A, AS568B, AS568C, AS568D) may be used interchangeably when referring to an o-ring dash size.

Actual vs. Nominal

To assist our customers in selecting the correct size o-ring we have included 3 measurements for cross reference purposes; the “Nominal”, “Actual Inch”, and “Actual Metric” sizes. We are often asked, “What is the difference between Actual and Nominal sizes for o-rings?”. The Nominal (fractional) dimensions are in name only (often used when verbally describing the size). For more accurate measurements, please refer to the actual inch or millimeter sizes, as these are the true dimensions of the o-ring.

O-Ring Formulas:

To calculate an o-ring’s outside diameter (OD) you may use this formula: 2(CS) + ID = OD
Conversely, if you have the OD and need to find the ID use this formula: OD – 2(CS) = ID