O-rings are round, typically rubber, seals that keep gas or liquid from passing through an opening. They're used most commonly around the home to seal internal faucet parts. 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 diameter, outside diameter and thickness or cross-sectional width, in metric or standard units.

...
...

Place the O-ring on a clean, flat, level surface.

...

Determine the outside diameter, often abbreviated as "OD." Use a ruler to measure the outside diameter of the O-ring from one outside edge to another, through the center. Take this measurement in millimeters.

...

Measure the thickness, sometimes referred to as the cross section or width. 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 inside diameter, or "ID." Measure the O-ring from inside edge to inside edge, or subtract twice the thickness of the O-ring from the outside diameter measurement.