#1 What are metric O-ring dimensions?
Most O-ring dimensions are taken from the standardized dimension lists. The most important O-ring standard today is ISO 3601.
It is internationally recognized and contains both the most important metric and imperial dimensions.
#2 What is the difference between metric and imperial dimensions?
Metric dimensions originate from the international metric system of units and are given in millimeters.
Inch dimensions originate from the much older Anglo-American measurement system and are given in inches.
Note: In accordance with the international standard, the O-ring dimensions are now specified in millimeters.
Converting inches into millimeters (mm) produces the odd numbers you are surely familiar with.
Example: 20.35 x 1.78 mm (0.801 x 0.07 inch)