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O-Ring Sizes...how to understand them

O-Rings come in sizes established by the Aerospace Standard 568 published by the Society of Automotive Engineers (SAE). These sizes are designated by "dash" numbers. In the charts linked below, you'll find O-rings identified and grouped by numbers that look like this: -010 or -076. Likewise, Superior Seals and Service's O-ring part numbers also contain dash numbers. A typical part number for us is ob-70-013. All letters and numbers have meaning in the part number, but the last four characters, -013, are the dash number.

So, what does all that mean? Well, if you consult the chart for the dash number -013, you will find that O-ring has an inside diameter of .426 (in inches). The cross section is .070 (in inches), and the tolerance for that ring's cross section is .003 (in inches). Our charts also provide corresponding dimensions in millimeters, and we provide a calculator on each screen of this portion of our website in case you want to make your own calculations.

Here is an explanation of our O-ring part numbering system. We will use the example given above of ob-70-013.

  • ob...The "o" means it's an O-ring. The "b" signifies that the ring is made from BUNA-N.
  • 70...This is the Durometer of the O-ring's material, in this instance, 70 Shore D. Not all of our O-ring part numbers have a listed Durometer.
  • 013...That's the dash number that's already been explained.

With the exception of the very smallest sizes (-001, -002 and -003), O-Rings share just a few cross section dimensions.

  • -004 thru -050 have a cross section of .070"
  • -102 thru -178 have a cross section of .103"
  • -201 thru -284 have a cross section of .139"
  • -309 thru -395 have a cross section of .210
  • -425 thru -475 have a cross section of .275"

To help the charts download quickly, we've broken up the possible sizes into managable groups. If you know the dash number you need, then click on the range that includes it.