Selected AS568 O-ring sizes with ID, cross-section and OD in inches and mm, plus notes on standards and measurement.
An O-ring is a torus-shaped elastomer seal fitted into a machined groove and compressed between two surfaces to block the passage of a fluid or gas under pressure. It is one of the simplest sealing devices in engineering and one of the most widely used, appearing in everything from a garden tap washer to an aircraft hydraulic actuator. Its apparent simplicity is deceptive: getting the size, tolerance and material right is what determines whether a seal lasts for years or fails within weeks.
O-ring dimensions are governed by published standards rather than by individual manufacturers, which is precisely the point of standardisation — a −214 O-ring ordered from any conforming supplier should fit the same groove. The standard in dominant use varies by region and industry. AS568, maintained by SAE International, is the standard of American origin and remains dominant in North American hydraulic, pneumatic and automotive equipment. ISO 3601-1 is its international metric counterpart, generally used across Europe and much of Asia. JIS B 2401 is the Japanese standard, and BS 1806, once the British standard, has been formally superseded by BS ISO 3601. This reference covers AS568 dash numbers and their ISO 3601 metric equivalents, and explains where the standards genuinely align and where treating them as interchangeable is a mistake.
A distinction that trips up even experienced buyers: AS568 specifies dimensions only — inside diameter, cross-section, and tolerance for each dash number. It says nothing about material, compound, or hardness. Those are chosen separately according to the fluid being sealed, the operating temperature, and the pressure involved, and are called out through companion specifications rather than the dash number itself. Ordering "a −214" tells a supplier the size; it tells them nothing about whether you need nitrile, fluorocarbon, or silicone.
OD = ID + 2 \times CS \;;\; Squeeze\% = \frac{CS - G}{CS} \times 100
Inputs: AS568 dash number −214, compared against its nearest ISO 3601 metric equivalent
For the great majority of common sizes, dimensionally close enough that engineers do treat them as interchangeable — the difference between AS568 −214 (24.99 mm ID) and its ISO 3601 equivalent (25.00 mm ID) is 0.01 mm, well within normal manufacturing tolerance. The standards share the same underlying dash-number heritage for common cross-sections. Where they genuinely diverge is at the edges: ISO 3601-1 includes metric cross-sections common on European machinery — 2.5 mm, 4.0 mm, 4.5 mm — that have no exact AS568 counterpart, and some AS568 sizes have no clean metric equivalent either. The safe practice is to compare the actual ID and CS figures for the specific size in question, not to assume the two standards line up across their full range. When in doubt, use the standard the original equipment drawing specifies.
This is the detail every quick comparison chart skips, and it matters more than the headline dimensional difference suggests. A groove is machined to produce a specific squeeze percentage — the amount the O-ring cross-section is compressed once installed — and that percentage is what makes the seal work. If a cross-section that looks close on paper (say a 3.0 mm CS in a groove cut for 3.53 mm) goes into the wrong groove, the squeeze can fall outside the working range on either side. Under-squeeze leads to leakage, since the seal never fully closes the gap. Over-squeeze speeds up wear, raises the risk of nibbling or extrusion under pressure, and shortens the seal's working life even when it does not fail immediately. Matching the nominal size is not the same as matching the groove design — always check both.
No. AS568 and ISO 3601 are dimensional standards — they specify inside diameter, cross-section, and tolerance class, and nothing else. Material and hardness are chosen separately based on the fluid being sealed, the temperature range, and the pressure the seal will see, then called out alongside the dash number through a compound specification. Two O-rings can share the identical dash number and dimension while being made from completely different rubber compounds with very different chemical resistance and temperature ratings. Always specify material and hardness explicitly rather than assuming the dash number implies a default compound.
ISO 3601-1 defines two tolerance classes. Class A carries tighter dimensional tolerances, equivalent to those used in AS568, and suits industrial or aerospace applications where the housing or the duty demands closer control. Class B carries slightly wider tolerances and is intended for general-purpose use where the extra precision of Class A is not required. Choosing Class B where Class A is genuinely needed risks a seal that technically meets the nominal dimension but sits outside the tolerance band a precision groove was designed around.
Start from the fluid and the temperature range, since these two factors eliminate most unsuitable compounds immediately. NBR (nitrile) is the general-purpose choice for mineral oils, fuels and greases across roughly −30°C to +120°C, and is the most common seal material by volume. FKM (Viton and equivalents) resists a much wider range of aggressive chemicals and fuels and tolerates higher temperatures, typically up to around +200°C, at a higher material cost. EPDM handles water, steam, and brake fluid well, but degrades rapidly in the presence of mineral oils — a common and costly mistake is specifying EPDM where an oil-based fluid is actually present. Silicone offers an unusually wide temperature range but poor resistance to oils and poor mechanical strength, making it suited to static seals rather than dynamic ones. Always confirm chemical compatibility against the specific fluid formulation, not just the general fluid family, since additives can change resistance significantly.
Lay the O-ring flat and unstressed, then measure the inner diameter across the inside of the ring and the cross-section — the thickness of the rubber cord itself — with a digital calliper for accuracy. A used O-ring may have taken a compression set or stretched slightly from service, so measure a new or lightly-used example where possible, and cross-check the reading against the groove dimensions in the equipment if the O-ring itself is questionable. Once you have ID and CS, compare against the standard chart to find the matching dash number, remembering that a used seal's measurements are a guide rather than a certainty.
The first digit of the three-digit dash number identifies the cross-section family — 0 for 1.78 mm, 1 for 2.62 mm, 2 for 3.53 mm, and so on through the standard cross-section groups. The remaining digits are a sequential identifier within that cross-section family and do not encode the inside diameter directly; the ID has to be looked up against the specific dash number in the published chart. This is why two dash numbers that look numerically close, such as −214 and −215, can have meaningfully different inside diameters — the numbering is sequential within a family, not a direct measurement code.
BS 1806, the original British O-ring standard, has been formally superseded by BS ISO 3601, meaning the UK adopted the international metric standard rather than continuing to maintain its own. This matters mainly for older British equipment with drawings still referencing BS 1806 part numbers — the physical sizes generally carry across to their ISO 3601 equivalents, but confirming the actual dimension against the current standard, rather than assuming a straight part-number substitution, avoids ordering the wrong seal for a legacy drawing.
Extrusion is the failure mode where system pressure forces part of the O-ring material into the clearance gap between the mating surfaces, and nibbling is the progressive erosion that follows as small fragments are sheared off with each pressure cycle. Both are strongly linked to squeeze and gap sizing — an undersized cross-section for the groove, an oversized clearance gap, or excessive squeeze from a mismatched standard all raise the risk. Harder compounds and back-up rings are the usual remedies at higher pressures, but the starting point is always getting the dimensional match between O-ring and groove correct in the first place, which is the reason for treating standards as distinct rather than loosely interchangeable.
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