Three dimensions describe every O-ring: inner diameter (ID, often written d1), cross-section diameter (CS, often written d2 or W), and outer diameter (OD). Only two of them are independent — the third always follows from:

OD = ID + 2 × CS

Measure two, calculate the third, and use the calculated value as a cross-check.

Tools that work

For most engineering purposes you need only a caliper. A digital caliper reading to 0.01 mm is enough for cross-sections above about 1.5 mm. For small cross-sections or worn rings, a cone gauge (for ID) and a micrometer with a light ratchet stop (for CS) give better repeatability, because rubber deforms under jaw pressure.

Two habits improve every measurement:

  • Take at least three readings around the ring and average them — mould parting lines and slight ovality are normal.
  • Use the lightest possible contact force. If the caliper visibly dents the rubber, the reading is low.

Measuring inner diameter

Lay the ring flat, without stretching it, and measure across the inside of the ring at its widest point. Do not hang the ring on the caliper jaws — its own weight stretches it. For small rings, slide the ring onto a cone gauge until it stops and read the scale at the contact line.

If the ring is used and has taken a compression set, the ID may have grown slightly; treat used-ring measurements as approximate and prefer identifying the original groove dimensions.

Measuring cross-section

Close the jaws gently on the cord at four points around the ring and average. On a used ring, measure away from the flattened contact faces if possible. A cross-section that measures noticeably oval usually indicates compression set rather than a non-standard size.

Measuring outer diameter

OD is the least reliable direct measurement, because the ring flexes away from the jaws. Prefer calculating OD = ID + 2 × CS. When only the OD is measurable (for example, the ring sits in a blind groove), measure OD and CS, then calculate ID = OD − 2 × CS.

Matching to a standard size

With ID and CS in hand, compare them against the common standard families:

  • AS568 (inch-based, used worldwide): cross-sections of 1.78 / 2.62 / 3.53 / 5.33 / 6.99 mm.
  • ISO 3601 / GB/T 3452.1 (metric G-series): cross-sections of 1.8 / 2.65 / 3.55 / 5.3 / 7.0 mm.

Note how close the two families sit — a worn 3.53 mm cord and a 3.55 mm cord are inside each other’s measurement noise. The deciding factor is usually the inner diameter: AS568 IDs follow inch increments, ISO/GB IDs follow metric preferred numbers. Our size recommendation step in each calculator does this comparison automatically and shows the deviation from each candidate.

Common mistakes

  • Stretching the ring while measuring the ID (reads high).
  • Squeezing the cord with the caliper (reads low).
  • Measuring a swollen ring after fluid exposure and treating the result as the original size.
  • Assuming OD measurements are authoritative — derive OD instead.

Standard size data is for preliminary engineering selection only. Final purchasing and design should be verified against the applicable standard edition, supplier data sheets and actual operating conditions.