How to measure and identify an unknown O-ring

An O-ring is defined by two dimensions: its inner diameter (ID) and its cross-section thickness (CS). Once you have these two numbers, you can match almost any ring to a standard and find a replacement. The third dimension, the outer diameter (OD), is merely derived: OD = ID + 2 × CS. What follows is a practical way to measure both dimensions reliably, even without special equipment.

What you need

A caliper (digital or vernier) accurate to at least 0.1 mm is enough. For very small cross-sections a micrometer or thickness gauge helps. Always measure an uncompressed, undamaged ring — a stretched or permanently deformed ring gives wrong numbers.

Step 1: measure the cross-section (CS)

Lay the ring on a flat surface and measure the thickness of the "cord" in section. This is the most important dimension because it unambiguously places the ring into one of a handful of standard cross-section series. In AS568 the common cross-sections are roughly 1.78, 2.62, 3.53, 5.33 and 6.99 mm. If your CS comes out near one of these, you know which series you are in.

Step 2: measure the inner diameter (ID)

Measure the ID across the centre of the ring, ideally with the ring uncompressed and lying flat. For larger rings, measure the outer diameter and subtract twice the cross-section thickness — the result is the ID. With a vernier, use the inner jaws to measure the inside span.

Step 3: match to a standard

With the ID × CS pair, compare the values against a chart. The three most common systems are:

• AS568 (inch-based, the SAE aerospace standard) — dash-number series 001 to 475. The most widespread in North America and in hydraulics.
• ISO 3601-1 (metric) — series A to E; DIN 3771/3770 share the same metric numbering, so ISO doubles as the DIN reference.
• JIS B 2401 (Japanese) — series P (dynamic), S (static) and G (static).

When a measured dimension does not match exactly, look for the nearest standard neighbour — manufacturers stock standardised sizes, not arbitrary in-between values.

What to watch out for

A used ring may have swelled (larger than the original) or shrunk, so measure a fresh sample piece if you have one. Never measure a ring stretched over a finger or shaft — stretching enlarges the ID and reduces the CS. When in doubt, measure the groove the ring belongs in too: its diameter and depth confirm the correct size.

The fast route

Our online size-converter takes your ID and CS and instantly finds the matching dash number in AS568 plus the equivalents in ISO 3601 and JIS B 2401, including the nearest standard neighbour if you measure an in-between value. It saves you leafing through charts.