O-Ring Sizes (AS568)

All 394 standard AS568 O-ring sizes with cross-section, inside diameter and outside diameter in inches and millimetres.

Data verified 2026-09-29 · based on n/a — SAE letter-suffixes this standard (AS568C); it is not year-stamped

AS568 Standard O-Ring Sizes

Every size in a series shares one cross-section. All 100-series O-rings are 0.103 in thick regardless of diameter; all 200-series are 0.139 in. That is what makes the standard useful — a groove designed for one 100-series ring accepts any 100-series ring, and the dash number selects only the diameter.

It also means that changing the dash number within a series keeps the groove cross-section and changes only the bore it fits, while changing the series changes the groove entirely. A 0.139 in ring will not sit correctly in a groove cut for 0.103 in, and vice versa.

OD is computed, not tabulated. Outside diameter is ID + 2 × CS by definition, and all 394 rows on this page satisfy it — which is also how the values were checked. The tolerances on ID and CS are separate and are not shown here: they come from AS568 and depend on the size, so a groove should be designed from the standard's tolerance table rather than from the nominal figures alone.

The 000 series and the 900 series are the exceptions to the one-cross-section-per-series rule. In both, the cross-section increases with the dash number, which is why the CS column is shown for every row rather than stated once in the heading.

Reading an AS568 Dash Number

The system is compact and worth understanding, because it tells you the cross-section before you look anything up.

The first digit is the series and sets the cross-section — 1 for 0.103 in, 2 for 0.139 in, 3 for 0.210 in, 4 for 0.275 in. Within each series the inside diameter increases as the last two digits rise, so AS568-210, -211, -212 step up in bore at a constant thickness.

The 000 series is different: the smallest sizes, where both the cross-section and the diameter grow with the dash number — 001 is 0.040 in thick and 050 is around 0.070 in. These are instrument and small-pneumatic sizes.

Metric O-rings use a different system under ISO 3601, where the size is written as the inside diameter and cross-section in millimetres — a 20 × 2.5 ring has a 20 mm bore and a 2.5 mm cross-section. The two systems do not interchange: an AS568-110 and a comparable metric ring have different nominal dimensions and different tolerances.

Where a groove already exists and the size must be identified, measure the inside diameter with the ring relaxed and the cross-section with a micrometer, then find the nearest AS568 size. Measuring a stretched ring gives a false reading — a rubber O-ring is easily stretched 5% by hand, which on a 0.362 in ring is 0.018 in of error, more than the tolerance.

Frequently Asked Questions

What is an AS568-110 O-ring?

A standard O-ring with a 0.103 in (2.62 mm) cross-section and a 0.362 in (9.19 mm) inside diameter, giving an outside diameter of 0.568 in. The '1' in 110 is the series, which sets the cross-section; the '10' selects the diameter within that series.

What does the first digit of an O-ring dash number mean?

The series, which determines the cross-section. 1xx is 0.103 in, 2xx is 0.139 in, 3xx is 0.210 in and 4xx is 0.275 in. Every size within a series has the same cross-section, so the dash number only selects the inside diameter.

How do I measure an unknown O-ring?

Measure the inside diameter with the ring relaxed — do not stretch it, since hand tension can add 5% — and the cross-section with a micrometer across the cord. Then match against the nearest AS568 size. Where the ring is still installed, measure the groove dimensions instead and work back to the size.

Are AS568 and metric O-rings interchangeable?

No. Metric rings follow ISO 3601, where the size is written as inside diameter × cross-section in millimetres. The nominal dimensions and the tolerances both differ from AS568, so a metric ring is not a drop-in replacement — the groove must be dimensioned for the system in use.

What is the outside diameter of an O-ring?

Inside diameter plus twice the cross-section. For AS568-110 that is 0.362 + 2 × 0.103 = 0.568 in. OD is a computed value rather than a tabulated one, and every row in the table above satisfies that relationship.

Why is the cross-section the same for a whole series?

Because that is what makes the standard usable. A groove cut for one ring in a series accepts any ring in that series, so a designer can change the bore diameter without redesigning the groove. The 000 and 900 series are exceptions where the cross-section varies with size.

Related

• undefined
• undefined
• undefined
• undefined
• undefined

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

Data Sources

Cross-checked against:

• Every row verified against the defining relationship OD = ID + 2 × CS — all 394 rows satisfy it
• The series structure verified — the 1xx, 2xx, 3xx and 4xx series each hold exactly one cross-section, as AS568 requires
• Spot values cross-checked against two independent published AS568 tables
• Millimetre values recomputed from inches at build time; the source's millimetre column was found to be unreliable and was not used

Derived values — the following values on this page are calculated, not taken directly from the standard:

Nominal dimensions are given; the AS568 tolerances on inside diameter and cross-section vary by size and are not shown. A groove should be dimensioned from the standard's tolerance table. Shore hardness, material and temperature range are properties of the compound, not of the size.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.