AS568 — O-ring dimensions
AS568 is the American SAE standard for O-ring sizes in inches. Sizes are grouped into five series by cross-section — from thin (CS 1.78 mm) to thick (CS 6.99 mm). Each size has a dash number (e.g. AS568-214). The withdrawn British standard BS1806 corresponds to it dimensionally in the common sizes.
How to read an AS568 dash number
AS568 is the American SAE standard that grew out of aerospace and defence specifications, and it states sizes in inches. A designation such as AS568-214 is not an arbitrary number — it is the position of that size in the standard table, and it fixes exactly one pair of values: the inner diameter and the cross-section. The dash number alone therefore identifies the ring dimensionally.
- The first digit gives the series, and therefore the cross-section (CS). The 0xx series is 1.78 mm, 1xx is 2.62 mm, 2xx is 3.53 mm, 3xx is 5.33 mm and 4xx is 6.99 mm.
- The remaining two digits give the position within the series. The higher the number, the larger the inner diameter — while the cross-section stays the same.
- The three smallest sizes 001, 002 and 003 are the exception. The standard gives them narrower cross-sections (0.040, 0.050 and 0.060 in); only from 004 does the series settle at 1.78 mm.
Example: AS568-214 belongs to the medium series, has a 3.53 mm cross-section, a 24.99 mm inner diameter and therefore a 32.05 mm outer diameter.
Thin series (001–050, CS 1.78 mm)
50 sizes
Open table
Medium-thin series (102–178, CS 2.62 mm)
77 sizes
Open table
Medium series (201–284, CS 3.53 mm)
84 sizes
Open table
Medium-thick series (309–395, CS 5.33 mm)
87 sizes
Open table
Thick series (425–475, CS 6.99 mm)
51 sizes
Open table
Five series by cross-section — which one is yours
The standard publishes 349 sizes split into five series. The series fixes the profile thickness, while the range of inner diameters within one series is wide. If you only know the cross-section of the old ring, the series is the first step — its table then gives you the inner diameter.
- Thin series (001–050, CS 1.78 mm)
- 001–050, cross-section 1.78 mm, inner diameters from 0.74 to 133.07 mm (50 sizes). The thinnest profile — small static seals, instruments, pneumatics and shallow grooves.
- Medium-thin series (102–178, CS 2.62 mm)
- 102–178, cross-section 2.62 mm, inner diameters from 1.24 to 247.32 mm (77 sizes). A general-purpose profile for static joints of small and medium diameter.
- Medium series (201–284, CS 3.53 mm)
- 201–284, cross-section 3.53 mm, inner diameters from 4.34 to 456.06 mm (84 sizes). The common profile in hydraulic and pneumatic parts, flanges and cylinders.
- Medium-thick series (309–395, CS 5.33 mm)
- 309–395, cross-section 5.33 mm, inner diameters from 10.46 to 658.88 mm (87 sizes). A heavier profile for larger diameters and higher pressures, where more deformation reserve is needed.
- Thick series (425–475, CS 6.99 mm)
- 425–475, cross-section 6.99 mm, inner diameters from 113.66 to 658.88 mm (51 sizes). The thickest profile, meant for large diameters — even the smallest size in the series has an inner diameter above 113 mm.
AS568 and metric sizes
The source values of the standard are in inches. The millimetre figures in the tables come from a conversion (inches × 25.4) rounded to two decimal places, which is why AS568 sizes almost never land on a round metric number. The inch and metric systems are not interchangeable — ISO 3601 and DIN 3771 use their own cross-section range (1.80 to 7.00 mm).
AS568-214 measures 24.99 × 3.53 mm, while the closest metric size, ISO 3601 C0250, measures 25.00 × 3.55 mm. The difference is hundredths of a millimetre and rarely matters on a static joint, but a comparison must always cover both the inner diameter and the cross-section, not one of them.
AS568 corresponds dimensionally to the withdrawn British standard BS1806 — the common sizes match, but BS1806 carries its own tolerance classes and does not cover all 349 AS568 sizes. The metric ISO 3601 and JIS B 2401 ranges have their own tables, linked below; the size converter finds the closest metric counterpart to a dash number and the other way round.
How to get from the tables to an actual ring
If you know the dash number, open the series that matches its first digit and find the row. If you have millimetre readings, the size converter is faster — it resolves the dash number from the inner diameter and cross-section. If all you have is the old ring, it is worth measuring it properly first: a worn ring is often stretched, so the measured diameter points at a different size from the one originally fitted.
Every table row lists the inner diameter, cross-section and outer diameter in millimetres, plus a link into the catalogue. Where the size is offered, the link goes to the size page with its material variants; otherwise to the nearest available ring, shown with its own actual dimensions, or to a catalogue search.
The size is only half of the specification
AS568 describes geometry, not material. Whether a seal survives is decided by the compound and its hardness together with the operating conditions — temperature, medium and pressure. The same AS568 size therefore appears in the catalogue in several materials: NBR, HNBR, EPDM, FKM, VMQ and FFKM. That is why a table link points first at the size page, where the material variants sit side by side.
The chemical-resistance table helps with choosing the compound; for a new groove, the calculator sizes it according to ISO 3601-2.