An O-ring is specified by two dimensions, and an AS568 dash number encodes neither of them arithmetically. It is an index into a table. That one fact explains most of what surprises people ordering a seal: why a dash number from one series is unrelated to the same number in another, why the inside diameters do not step evenly, and why the size you need may not be in the series you assumed.
The number unlocks a row, it does not encode a size
On a thread page an M8 tells you the major diameter before you open anything, and on a bearing page the last two digits of a 6205 give the bore. AS568 has no such arithmetic. The digits after the dash are a row number: every one of the 369 sizes in this database is a separate row, and the number tells you which row to read, not how big the ring is.
That is why two neighbouring dash numbers are not neighbours in size, and why a dash number from one series says nothing about the same number in another.
The series fixes the cross-section
An AS568 number is two parts. The hundreds digit is the series, and it sets the cross-section — the cord diameter that decides how far the ring is squeezed in its groove. The last two digits pick the inside diameter within that series.
| Series | Cross-section (mm) | Inside diameter (mm) | Sizes in this database |
|---|---|---|---|
| 000 Series | 1.02–1.78 (4 distinct) | 0.74–133.07 | 50 |
| 100 Series | 2.62 | 1.24–247.32 | 77 |
| 200 Series | 3.53 | 4.34–456.06 | 84 |
| 900 Series | 1.42–3 (9 distinct) | 4.7–59.36 | 20 |
| 300 Series | 5.33 | 10.46–658.88 | 87 |
| 400 Series | 6.99 | 113.67–658.88 | 51 |
4 of the 6 series carry exactly one cross-section each, which is the pattern people generalise from. 2 do not: 000 Series (4 sections) and 900 Series (9 sections). In 000 Series the 3 extra sections belong to its smallest sizes, where the full cord would leave almost no hole; the 900 Series is cut by purpose rather than by cord, so its section changes with what the ring is for. Read the cross-section off the row, not off the series code.
The inside diameter is not evenly spaced
If the sizes stepped by a constant increment, the table would be redundant — you could interpolate to any diameter you wanted. They do not. In the 100 Series the first steps run 0.82, 0.78, 0.79, 0.79 mm; in the 200 Series they run 1.6, 1.58, 1.6, 1.57 mm. Close to a millimetre in one series and a third of that in the other, and never quite constant in either.
The practical consequence: a shaft measured with a caliper does not lead to a dash number by calculation. Open the series page and find the row whose inside diameter brackets your measurement, then decide whether the next size up is the safer fit.
One inside diameter, more than one cross-section
48 inside diameters in this database appear in more than one series. The first is 6.07 mm, offered as AS568-902 (1.63 mm section) and AS568-010 (1.78 mm section).
This matters because the shaft diameter and the seal section are separate decisions that are easy to conflate. A groove cut for the thinner cord will not accept the thicker one, but the same shaft can be sealed by either — what changes is the groove depth and width, the squeeze percentage and the force the ring puts on the shaft. Choosing between them is a groove design question, and the groove dimensions are not in this database.
Millimetres here are a conversion
AS568 is an inch standard. Every millimetre figure on these pages is the inch dimension multiplied by 25.4, and the pages say so where they print it. The tolerances are converted the same way: the 0.08 mm cross-section tolerance on AS568-902 is an inch tolerance restated, not a rounded metric limit from a different standard.
If your drawing is metric, that distinction is the one to check before ordering: an AS568 ring is the right part only when the groove was cut to an AS568 table.
Two rows where the sources disagree
2 sizes carry no inside-diameter tolerance here — AS568-469 and AS568-470. The two source tables give different values for that dimension and the database prints neither rather than picking one. It is the only place on the site where a figure is missing by policy, and it is missing because a wrong tolerance is worse than an absent one.
Material is a separate choice
The dash number says nothing about what the ring is made of. That is a second, independent decision driven by temperature, the medium being sealed and the movement in the joint — the rubber family page carries the five elastomers in this database with the temperature range and media each one suits, and each entry names its source. Pick the material first: it decides whether the size you found is usable at all.
What this database does not carry
- Groove dimensions. Depth and width for a given section come from AS568's groove tables or ISO 3601-2. Neither is in this database, and no figure here substitutes for one.
- Squeeze and stretch percentages. These follow from the groove and the shaft, not from the ring's own dimensions.
- Hardness. Almost every O-ring is moulded at 70 Shore A regardless of polymer, so it would not separate the entries usefully. The rubber pages explain why hardness is set by the gland rather than by the material.
- Metric (ISO 3601-1) sizes. This library is AS568 only. A metric groove needs a metric table, and this is not one.
Start from the O-ring index to see the six series side by side, or open a series to find the size that brackets your measurement.