how to determine diameter of ring
an O-ring, a good fit is critical. O-rings are measured in millimetres. Place the O-ring on a clean, flat, level surface. Determine the inside diameter. Use a ruler to measure from one inner edge to another. To measure the outside diameter, use a ruler to measure from one outer corner of the O-ring to the other outer corner. Measure cross section. Place the O-ring into a vernier calliper. Lightly clamp the jaws of the calliper onto the O-ring, but do not compress it. Record the thickness of the ring (the cross section). If you only have 2 of the O-ring diameters then you can determine the third diameter with the following equations: Measurements: Outside Diameter (OD) = Inside Diameter + (2 x Cross Section). Inside Diameter (ID) = OD - (2 x CS). Cross Section (CS) = OD - ID, Ã· 2. If the o-ring is cut, measure the cross section (thickness) of the O-ring and the length. Converting Length to Inside Diameter: Length Ã· 3.142 = circumference. Circumference - CS = ID.