If you have an o-ring that has failed and you require a replacement, while shopping you may be surprised to find out just how expansive the o-ring market is. If you are new to shopping for fluid sealing products you may feel you are reading a different language with different combinations of letters and numbers. Luckily, it is not as complicated as it may seem!

AS568 and the Global O-Ring and Seal Size Reference Chart

The good news is that there is a Global O-Ring and Seal Size Reference Chart and Guide that was published by the Society of Automotive Engineers that covers the AS568 o-ring sizes, including the extended sizes in the 400 series. This chart helps with sizing because it specifies diameter, cross-section, tolerances, and size identification codes for o-rings.

This o-ring size chart was published in 1971 by the A-6c2 Seals Committee at SAE as Aerospace Standard AS568. In 1974, these standards were republished as AS568A and then again in 2001 as AS568B, in 2008 as AS568C, and in 2014 as AS568D. Today any of these terms can be used interchangeably when referring to o-ring dash size.

O-Ring Sizing Formulas

With this bit of history in mind, you may still be curious how you calculate o-ring formulas. 

• To calculate the outside diameter or OD, you use this simple formula:  2(CS) + ID = OD I
• If you have the OD and you need to find the ID you can use this formula:  OD-2(CS) = ID ACtu
  

BS 1806 Metric Sizing