How to measure an O-ring: ID, OD and cross-section

The three dimensions and how to take them

Rubber compresses under the jaws, so a heavy hand reads small. Close the calipers until they touch and stop. A measuring cone, a stepped or tapered gauge the ring hangs on, reads the ID directly and is the handiest tool when there are many rings to identify. Read the millimeter scale when the print is metric and the inch scale when it is not; both appear on the AS568 size chart.

The AS568 cross-section series

The cross-section places a ring in one of the AS568 series before the ID is even read. Each series has one nominal cord thickness, and the dash numbers inside it step up in ID.

A worked example: a ring that measures 0.984 in ID and 0.139 in CS sits in the 2xx series, and the chart row with that ID is AS568-214. The dash number guide explains how the numbering was built and why the 0xx series carries four cord sizes.

What a used ring tells you, and what it does not

A ring that has been in service no longer has its original dimensions. Compression set flattens the cord, so the CS reads low and the ring looks oval rather than round. Fluid swell makes the ring larger in every direction, and heat hardening shrinks it and stiffens it. Measure a new ring from the same lot when one exists, or measure the groove instead.

The flattening is worth reading on its own, because it is the most common reason a replacement is ordered. The compression set guide covers what the flat sides mean and the failure modes post covers the other surface signs.

Measuring the groove instead

When the ring is missing or unreadable, the groove gives the same answer. Measure the groove diameter (the rod for a rod seal, the bore for a piston seal, the face groove for a face seal), the groove width and the groove depth. The cord thickness is the groove depth plus the squeeze the design applied, and the ring ID is set by the rod or the groove diameter so that the ring sits with a small stretch on a rod or a small compression in a bore.

The groove design guide gives the gland dimensions by series, and the squeeze guide explains how much the cord is meant to be compressed in each kind of gland. A groove that does not match any AS568 series is usually metric.

Metric or AS568?

The cord thicknesses 1.78, 2.62, 3.53, 5.33 and 6.99 mm are the inch series read in millimeters. A cord that reads a round millimeter figure, such as 2.0, 2.5 or 3.0 mm, is usually a metric ring made to ISO 3601 or to a maker's metric range, and its ID will also be a round figure. Metric rings are covered in the metric size guide and on the metric O-ring page.

What to send with a quote

• The ID and CS with their units, or the dash number if you have it
• The material and hardness the print calls out; the durometer guide explains the Shore A figure
• The quantity and the print revision, when there is a print
• A photo when the ring is not a plain round section: a lobed ring is an X-ring, a ring with a joint is spliced
• Whether the compound name on the print is a brand or a class; the FKM naming post explains the difference
• For a flat gasket rather than a ring, the sheet family and thickness; the gasket material post lists the families by temperature and media

Conquest quotes from the dash number and the compound the print names; when only a used ring exists, the measurements above and a photo are enough to start.

Sources

1. SAE AS568, Aerospace Size Standard for O-rings (title and series structure only)
2. The AS568 size table on this site (349 general-purpose sizes, -001 to -475, plus the 20 boss sizes, -901 to -932: 369 rows), the source of every dimension and count above
3. Parker O-Ring Handbook ORD 5700 (2018), section 3 (gland, squeeze and stretch), read 2026-09-14