All O-Rings

O Rings are measured by their outer diameter, inner diameter, and cross-section diameter, so for example, an AS568 O Ring could have the measurement of 3.69mm (inner diameter) x 7.25mm (outer diameter) x 1.78mm (cross-section diameter). 

In most instances, O Rings are measured by metric measurements to identify their dimensions and tolerance. The sizes vary from different specifications of O Rings such as JASO (Japanese Automobile Standard Organization), which has its own dimensions and tolerances to various environmental influences and chemicals. Therefore, it is imperative that when choosing O-Rings, its size, material, and resistance are accounted for as each classification of O Rings is unique in its specifications.