Inch and metric size conventions
Seal drawings move between two measurement systems. North American AS568 sizes were originally defined in inches, while ISO 3601 and most modern engineering drawings use millimetres. The conversion is exact — 1 inch = 25.4 mm — but rounding at each step matters: a 0.070 in cross-section becomes 1.78 mm, and small rounding differences can push a dimension across a tolerance band. Convert from the original design units wherever possible rather than chaining approximations.
Inside, cross-section, outside and mean diameter
An O-ring is fully described by its inside diameter (ID) and cross-section (CS). From those two, the outside diameter is OD = ID + 2 × CS and the mean diameter is ID + CS. The mean diameter is the figure used for stretch calculations when fitting a ring onto a rod or piston, so having it computed alongside the OD makes it easy to check that a candidate size will seat without over-stretching.
Durometer (Shore A) hardness
Hardness is measured on the Shore A durometer scale for elastomeric seals. Soft compounds (30–50 Shore A) conform to rough surfaces and seal at low load but extrude easily; medium compounds (60–75 Shore A) are the general-purpose default, with 70 Shore A specified for the vast majority of standard O-rings; hard compounds (80–90 Shore A) resist extrusion in high-pressure or large-clearance glands but require more squeeze force and conform less to surface imperfections. Choosing hardness is a trade-off between sealing force, extrusion resistance and the surface finish of the mating parts.