O-ring size charts typically list the O-ring part number, the actual inner diameters and cross sections in inches and millimeters, as well as nominal references for the inner and outer diameters and the cross sections. The AS568 standard from the Society of Automotive Engineers is the common sizing chart for O-rings in the United States.
The standard AS568 sizing chart typically includes measurements in both inches and millimeters so that users can find the right size to match the desired equipment pieces. When incorrect sizes are used, the O-rings can extrude from internal pressure, causing the equipment to wear down and break away the O-ring. The sizing charts feature the intended size as well as the expected variances.
When placing the O-ring on a flat surface, the outer diameter is the measurement from the top to the bottom of the outside of the O-ring. The inner diameter is the interior measurement from the top of the O-ring to the bottom. The cross section measurement refers to the O-ring's thickness. The nominal reference for the inner and outer diameters, as well as the cross section, is listed in fractions, whole numbers or whole numbers with fractions. This number refers to the theoretical or expected size of the O-ring, whereas the actual size may vary slightly.