How to read an AS568 o-ring size chart

What an AS568 o-ring size chart shows

A standard size chart built to AS568F lists a dash number in the first column, then the two dimensions that actually control the part: inside diameter and cross-section diameter. Outside diameter is usually shown as a calculated reference rather than a separately controlled dimension. Each dimension carries its own tolerance band, and the cross-section tolerance is normally tighter than the inside diameter tolerance. The print should always call out the dash number directly rather than leaving a buyer to infer it from a drawing.

How AS568 dash numbers are grouped

The 349 general purpose dash numbers in AS568F are organized into families by cross-section, so sizes with smaller cords sit at one end of the numbering and sizes with larger cords sit at the other. Dash numbers -014, -114, -214 and -314 are a common example of this pattern: each sits at roughly the same inside diameter grouping while moving to a progressively larger cross-section family as the first digit climbs. Two dash numbers that are close in number can still carry very different cross-sections, which is why the number alone is not enough to confirm a match. A separate block of 20 dash numbers, including sizes such as -910, covers SAE boss sizes, used with straight thread port fittings rather than as a radial or face seal in a groove.

Example of an AS568 size chart entry

A size chart entry is easiest to understand next to a real dash number. The rows below use the family grouping described above and link to the full specification page for each dash number, where the exact ID, CS and OD figures are listed against their tolerance bands.

AS568 chart vs metric o-ring sizes

AS568F dash numbers are built around inch dimensions, even though most charts also show the millimeter equivalent for reference. A separate set of metric o-ring sizes uses inside diameter and cross-section stated directly in millimeters, with its own numbering rather than an AS568 dash number. A print may call for either system, and the two are not interchangeable just because a millimeter conversion happens to land close to a dash number's listed size. The metric o-ring sizes guide covers how that separate table is organized.

Confirming a size before you order

A size chart only gives dimensions. Durometer, compound, and any special tolerance class are called out separately on the print, and the chart will not tell a buyer which one applies to a given application. Before ordering from a chart, confirm the dash number is one of the 369 standard sizes, since a number outside that set is not a real AS568 size no matter how close it looks to a listed one. The how to measure an o-ring guide covers what to check on hand if no print is available.

Standard sizes and documentation

Dash number and durometer together are not the whole order. A certificate of conformance ships with the part and names the part number, the specification, the manufacturer, the lot, the quantity and the manufacturer's cure date, which is separate information from anything printed on a size chart. Conquest stocks every AS568 size, so a confirmed dash number from the chart can be matched directly against the specification on the print. Questions on a specific dash number, including the pages under the AS568 size chart, can be sent through request a quote.

Sources

1. Parker O-Ring Handbook ORD 5700, section IX size tables (pages 9-2 to 9-11), read 2026-09-06
2. SAE AS568F, Aerospace Size Standard for O-Rings