O-rings are round mechanical seals that keep gas or liquid from passing through an opening. They're used everywhere from the engine of your car to inside the space shuttle. Over time, these seals may wear out and require replacement. If you're planning to replace an O-ring, a good fit is critical. O-rings are measured by inside and outside diameter, in metric or standard units.
Place the O-ring on a clean, flat, level surface.
Determine the outer diameter. Use a ruler to measure the outside diameter of the O-ring from one outside edge to another. Take this measurement in millimetres.
Measure thickness. Place the O-ring into a vernier caliper, oriented lengthwise. Lightly clamp the jaws of the caliper onto the O-ring, but do not compress it. Record the thickness.
Determine the inner diameter. Measure the O-ring from inside edge to inside edge or subtract twice the thickness of the O-ring from the outside diameter measurement.
Using only a ruler to measure an O-ring is less precise.