AS568 O-Ring Sizes

Let’s start super basic with standard AS568 o-ring sizes before we get into the details of static o-ring groove design.

In the US, the most common o-ring standard is called Aerospace Size Standard for O-Rings (AS-568 ).  Each o-ring size is officially written as AS568-XXX, where the -XXX denoted a particular size of o-ring.  In practice, this gets shortened to simply “-XXX” and is commonly referred to as a “dash number”.  So if you are specifying an o-ring size, you might ask for a -015 or -314 o-ring.

as568 o-ring inner diameter cross section cs

The first digit of this dash number specifies the o-ring’s cross-sectional diameter, or “CS”.:

-0XX means 0.070″ CS diameter
-1XX means 0.103″ CS diameter
-2XX means 0.139″ CS diameter
-3XX means 0.210″ CS diameter
-4XX means 0.275″ CS diameter


The second two digits simply specify the inner diameter of the o-ring in sequential size order.  You need to refer to tables to know the actual size of the o-rings.  Fortunately, the tables are easy to find:

-001 thru -114 O-Rings
-115 thru -178 O-Rings
-201 thru -264 O-Rings
-265 thru -349 O-Rings
-350 thru -439 O-Rings

Make sense?  Now that we understand o-ring sizes, here’s a tool to help you size and design grooves for your static o-ring seals.