O-Ring Size Standards — ISO 3601, JIS B 2401, AS568 and How to Cross Them

An O-ring size is defined by two numbers — inside diameter and cross-section — but it is almost always ordered by a code from one of four or five competing standards. This page decodes them, explains where they overlap, and shows how to move safely between metric and imperial series.

The two numbers that matter

Every O-ring, in every standard, is fully described by:

• d1 — inside diameter of the ring in its free state.
• d2 — cross-section (also written W, or CS), the thickness of the rubber.

Outside diameter is simply d1 + 2 × d2 and is never the ordering dimension. If you measure a used ring, measure the inside diameter and the section, and remember that a ring recovered from a hot application has taken set — it will read small on section and large on diameter.

ISO 3601 — the international reference

ISO 3601 is the umbrella standard for fluid-power O-rings and the one to cite on new designs.

ISO 3601-1 defines two families: series A, the metric sizes (broadly aligned with the traditional Japanese metric series), and series G, which reproduces the inch-derived AS568 sizes in millimetres. That matters commercially — a "metric" ring quoted at 3.53 mm section is an inch ring with a metric label.

Quality grades from ISO 3601-3 are worth specifying explicitly: grade N for general industrial duty, grade S where surface imperfection limits are tighter — aerospace, high-vacuum and critical static joints.

JIS B 2401 — the metric P / G / V series

The Japanese standard remains the working language of most hydraulic equipment built in Japan and is widely used across Europe. Sizes are quoted as a letter and a number, where the number is the nominal inside diameter in millimetres.

• An A suffix (P22A, P48A, G150A) marks the start of the next cross-section band, not a variant of the ring below it.
• P-series sizes carry tighter tolerances than G because they are intended for dynamic duty.
• The V series exists for vacuum flanges and uses larger sections for a wider contact band.

AS568 — the imperial standard

AS568 (originally AN6227/AN6230, later ARP568) is the American aerospace-derived series that dominates imperial-built equipment worldwide. Sizes are a three-digit dash number in which the first digit encodes the cross-section.

Other series you will meet

Tolerances and how they are applied

Tolerance on the section is what governs squeeze, and it tightens as the section gets finer in absolute terms while loosening in percentage terms as the ring gets larger.

When you set groove depth, work the squeeze calculation at both extremes of the section tolerance and the groove tolerance together. A design that only achieves its target squeeze at nominal is a design that leaks on a proportion of production.

Crossing between standards safely

What to do when you have a ring but no code

• Measure inside diameter and section on an unstressed, room-temperature ring, with a shadowgraph or a pin gauge rather than callipers where possible.
• Match the section first. The section determines which series you are in.
• Then match inside diameter within that series; a nearby standard size is almost always the intended part.
• If nothing matches within tolerance, you likely have a non-standard or a moulded-to-order ring — send us the dimensions and the application.

What to do when you have a groove but no ring

• Measure groove depth, groove width and both diameters.
• Work back to the section that gives 8–25 % squeeze at that depth.
• Check the resulting groove fill lands near 75 %. If it lands near 50 %, the groove was designed for a different standard's section.