Standard O-Rings are available in 369 sizes as set up by the Aerospace Standard 568 published by the Society of Automotive Engineers. These sizes are designated by dash numbers shown on the 568 DASH Chart and are standardized by imperial dimensions. Standard O-Rings are specified by their inside diameter and cross section width. AS568 is the most commonly used standard in the U.S. for aerospace, automotive and general industrial applications.For more information, please see: Comparison Properties Chart, PTFE Encapsulated O-rings, Material Shelf Life, and O-ring Groove Design Chart.