O-Ring Dimensions and Standards at a Glance
ISO 3601, AS568, DIN and others: which standard defines which O-ring sizes and how to find the right table for your application, in a compact overview.
- Depending on the region, industry and application, different O-ring standards with their own size tables dominate.
- AS568 (SAE, aerospace) is inch-based, while ISO 3601 is the comprehensive international standard with five parts, from dimensions to materials.
- British (BS 1806, BS 4518), Swedish (SMS 1586), Japanese (JIS B2401) and French (NF T 47-501) standards complete the picture, partly in inches, partly metric.
- Generic metric sizes are readily available but do not necessarily conform to a standard.
- Standardized O-rings score better than custom sizes on price, availability and quality.
Why O-ring tables provide certainty
Using O-ring tables brings standardization and therefore certainty. Whether you are designing a technical system or sourcing replacements as part of maintenance and repair, relying on standard-compliant O-rings offers clear advantages. Even converting existing machines or systems to standardized O-rings can pay off through lower follow-up costs.
There is one thing to bear in mind, however: depending on the region of the world, industry or intended application, different O-ring standards with their own size tables dominate. Here you get an overview of the common standards and their particular features.
| Standard | Origin / Field | Measurement system | Size example | Cord thicknesses (mm) |
|---|---|---|---|---|
| AS568 | SAE, aerospace | Inch (leading) | 12.42 x 1.78 mm | 1.78 | 2.62 | 3.53 | 5.33 | 6.99 |
| ISO 3601 | International | metric | 12.42 x 1.78 mm | 1.78 | 1.80 | 2.62 | 3.53 | 5.33 | 6.99 |
| BS 1806 | United Kingdom | Inch | 12.42 x 1.78 mm | 1.78 | 2.62 | 3.53 | 5.33 | 6.99 |
| BS 4518 | United Kingdom | metric | 29.6 x 2.4 mm | 1.6 | 2.4 | 3.0 | 5.7 | 8.4 |
| SMS 1586 | Sweden | metric | 17.3 x 2.4 mm | 1.6 | 2.4 | 3.0 | 5.7 | 8.4 |
| JIS B2401 | Japan | metric and inch | 34.4 x 3.1 mm | 1.5 | 2.0 | 2.4 | 3.1 | 5.7 | 8.4 | 10.0 |
| NF T 47-501 | France | metric | 32.5 x 2.65 mm | 1.8 | 2.65 | 3.55 | 5.3 | 7.0 |
| Generic / metric | cross-manufacturer | metric | 20 x 2 mm | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 | 3.5 |
AS568 (SAE, aerospace)
Size example: 12.42 x 1.78 mm
Cord thickness examples: 1.78 | 2.62 | 3.53 | 5.33 | 6.99
The Society of Automotive Engineers (SAE) first published the dimension table for O-rings in 1971. Since then there have been several revisions, with Revision F currently being the valid edition. In practice, however, this distinction is not particularly relevant, as the O-ring sizes in the table have not changed.
It is a standard for aerospace and covers O-rings for sealing applications and straight-thread tube fittings. O-rings to AS568 are available with cord thicknesses from approx. 1.02 mm to 6.99 mm and inner diameters up to approx. 660 mm. However, the definition in inches is the governing one.
Even in regions with a predominantly metric system, it can make sense to design seals to this standard thanks to the ready availability of the O-rings. O-rings from the AS568 table are available in many different materials. With the latest version of ISO 3601, the cross-sectional diameters to AS568 match apart from deviations in the range of hundredths of a millimeter.
ISO 3601 (International)
Size example: 12.42 x 1.78 mm
Cord thickness examples: 1.78 | 1.80 | 2.62 | 2.65 | 3.53 | 3.55 | 5.30 | 5.33 | 6.99
In its latest version, ISO 3601 comprises far more than just a table of standardized O-ring sizes. While permissible surface deviations (Part 3) have been part of the standard for some time, Part 5 now governs further subject areas as well.
With this addition, the O-ring standard also covers formulations and quality requirements for O-rings. To this end, the standard focuses above all on the degree of vulcanization, which can be determined via hardness and compression set. This makes the standard a comprehensive set of rules for the use of O-rings:
| Part | Scope |
|---|---|
| Part 1 | Dimensions |
| Part 2 | Installation spaces |
| Part 3 | Surface defects |
| Part 4 | Back-up rings |
| Part 5 | Materials |
These regulated areas cover the essential factors influencing a functioning sealing system.
BS 1806 (United Kingdom, inch)
Size example: 12.42 x 1.78 mm
Cord thickness examples: 1.78 | 2.62 | 3.53 | 5.33 | 6.99
This British standard contains a table of inch dimensions for the inner diameter and cross-sectional diameter of O-rings. The standard comprises five groups of O-ring cross-sections that match AS568. Beyond that, however, the BS 1806 table defines further intermediate sizes.
BS 4518 (United Kingdom, metric)
Size example: 29.6 x 2.4 mm
Cord thickness examples: 1.6 | 2.4 | 3.0 | 5.7 | 8.4
This is likewise a British standard. In this case, however, the O-ring size table refers to metric dimensions. Each O-ring dimension is given an ID as its designation, made up of the respective inner diameter (specified as four digits in tenths of a millimeter) and the cross-section (also in tenths of a millimeter).
The two dimensions are separated by a hyphen, so that the reference 0031-16 denotes an O-ring with an inner diameter of 3.1 mm and a cord thickness of 1.6 mm. In total, the table comprises 203 different sizes. 1.6 mm is at the same time the smallest cord thickness in the table, which extends up to 8.4 mm.
SMS 1586 (Sweden)
Size example: 17.3 x 2.4 mm
Cord thickness examples: 1.6 | 2.4 | 3.0 | 5.7 | 8.4
SMS 1586 is the Swedish mechanical standard (Sveriges Mekanstandardisering) for O-rings. Sizes from the dimension table are designated in the format ID x C/S, where ID stands for the inner diameter in millimeters and C/S for the cross-sectional diameter in millimeters. In this respect, the standard aligns with DIN 3771.
In addition, SMS 1586 introduces two application-related designations for O-rings:
- D: static and dynamic applications
- S: for static applications only
JIS B2401 (Japan)
Size example: 34.4 x 3.1 mm
Cord thickness examples: 1.5 | 2.0 | 2.4 | 3.1 | 5.7 | 8.4 | 10.0
JIS B2401 is the Japanese industrial standard for O-rings. The table contains a series of O-rings in both metric and inch dimensions. As far as the metric dimensions are concerned, the standard includes the O-ring sizes as per the ISO 3601 table.
NF T 47-501 (France)
Size example: 32.5 x 2.65 mm
Cord thickness examples: 1.8 | 2.65 | 3.55 | 5.3 | 7.0
The dimension table of the French standard is similar to ISO 3601. In addition, it incorporates parts of DIN 3771, which relates to O-rings for fluid power applications with particular dimensional accuracy. Going beyond these standards, NF T 47-501 contains some specific designations for classes of O-rings.
Generic and metric dimensions
Size example: 20 x 2 mm
Cord thickness examples: 1.0 | 1.5 | 2.0 | 2.5 | 3.0 | 3.5
In addition to the size tables of specific standards, O-rings can also be obtained in generic metric sizes. These may correspond to a standard size from one or more tables, but they do not have to. Although these are not necessarily O-rings in standardized dimensions, availability is generally good.
This is because O-ring manufacturers widely have the corresponding tooling for production. Here, too, designation in the format C/S x ID is common.
Advantages of standardized O-ring tables
Standardized O-rings should be used wherever possible. Price, availability and reliably high quality speak in favor of standardized sealing elements.
Frequently asked questions
Which O-ring standard is the most important?
How do AS568 and ISO 3601 differ?
What does a designation like 0031-16 under BS 4518 mean?
Are generic metric O-rings standardized?
Why are standardized O-rings worthwhile?
How do I find the right O-ring size?
