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.
- Every O-ring size consists of two dimensions: the inner diameter (d1) and the cord thickness (d2), for example 20 x 2 mm.
- 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. The German DIN 3771 was superseded by ISO 3601 in 2010.
- 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.
O-ring dimensions: how to read a size
Every O-ring size is described by two dimensions: the inner diameter d1 and the cord thickness d2, i.e. the diameter of the round cross-section. Both are given in millimeters, and in inch-based standards additionally in inches. The standard defines which combinations of d1 and d2 exist as standard sizes.
Three notations you should know
Inner diameter x cord thickness in millimeters. This is how DIN 3771, SMS 1586 and the generic metric sizes are written.
Size number from the AS568 table. It stands for 12.42 x 1.78 mm.
Inner diameter in four digits, cord thickness in two digits, each in tenths of a millimeter: 3.1 x 1.6 mm.
Which standard fits? The standards guide
Which table is the right one depends above all on where the machine or the drawing comes from. Select the origin, and the guide shows the usual standards and takes you straight to the size table.
The standard that applies today, including in Germany. Since 2008 it has also included the inch sizes from AS568.
The former German standard, superseded by ISO 3601 in 2010. It is still widespread in older plants and drawings.
The comprehensive international standard with five parts, from dimensions to materials.
The inch-based standard of the SAE, established in aerospace and in regions that work in inches. Every size has a three-digit number, the example is AS568-014.
Inch dimensions matching the cross-sections of AS568, with additional intermediate sizes.
Metric British standard. The code is made up of the inner diameter and the cord thickness in tenths of a millimeter, the example is 0296-24.
Metric in the format ID x C/S, plus the designation D for static and dynamic or S for static-only applications.
The Japanese industrial standard with the classic P, G, S and V series, plus sizes to ISO 3601. The example there is G35.
The French dimension table is similar to ISO 3601 and includes parts of DIN 3771.
Only the dimensions are known? Measure the inner diameter and cord thickness and use our standard search to check whether the size is standardized. Many metric sizes are readily available even without a standard.
All standards compared
The table summarizes the origin, measurement system and a size example for each standard. The overview below shows which cord thicknesses the standards have in common.
| Standard | Origin / Field | Measurement system | Size example |
|---|---|---|---|
| AS568 | SAE, aerospace | Inch (leading) | 12.42 x 1.78 mm |
| ISO 3601 | International | metric | 12.42 x 1.78 mm |
| DIN 3771 | Germany, until 2010 | metric | 40 x 3.55 mm |
| BS 1806 | United Kingdom | Inch | 12.42 x 1.78 mm |
| BS 4518 | United Kingdom | metric | 29.6 x 2.4 mm |
| SMS 1586 | Sweden | metric | 17.3 x 2.4 mm |
| JIS B2401 | Japan | metric and inch | 34.4 x 3.1 mm |
| NF T 47-501 | France | metric | 32.5 x 2.65 mm |
| Generic | cross-manufacturer | metric | 20 x 2 mm |
Cord thicknesses compared
The overview shows which standards share cord thicknesses. This helps when a size from one standard is to be replaced by an equivalent size from another.
| Cord thickness d2 in mm | ≈ 1.8 | ≈ 2.6 | ≈ 3.5 | ≈ 5.3 | ≈ 7.0 |
|---|---|---|---|---|---|
| AS568 | 1.78 | 2.62 | 3.53 | 5.33 | 6.99 |
| ISO 3601 | 1.781.80 | 2.622.65 | 3.533.55 | 5.335.30 | 6.997.00 |
| BS 1806 | 1.78 | 2.62 | 3.53 | 5.33 | 6.99 |
| DIN 3771 | 1.8 | 2.65 | 3.55 | 5.3 | 7.0 |
| NF T 47-501 | 1.8 | 2.65 | 3.55 | 5.3 | 7.0 |
| Cord thickness d2 in mm | 1.6 | 2.4 | 3.0 | 5.7 | 8.4 |
|---|---|---|---|---|---|
| BS 4518 | 1.6 | 2.4 | 3.0 | 5.7 | 8.4 |
| SMS 1586 | 1.6 | 2.4 | 3.0 | 5.7 | 8.4 |
| JIS B2401 | not included | 2.4 | not included | 5.7 | 8.4 |
Also: JIS B2401 has its own cord thicknesses (1.5 · 1.9 · 2.0 · 3.1 · 3.5 · 4.0 · 6.0 · 10.0 mm). AS568, ISO 3601 and BS 1806 contain some small sizes from 1.02 to 1.52 mm, BS 1806 also 1.63 mm. Our table of common metric sizes ranges from 1 to 6 mm.
AS568 (SAE, aerospace)
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. Every size has a three-digit number, for example AS568-014 for 12.42 x 1.78 mm.
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)
In its latest version, ISO 3601 comprises far more than just a table of standardized O-ring sizes. Since 2008 it has also included the inch sizes from AS568. 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 1Dimensions
- Part 2Installation spaces
- Part 3Surface defects
- Part 4Back-up rings
- Part 5Materials
These regulated areas cover the essential factors influencing a functioning sealing system. What tolerance classes, grade characteristics and back-up rings mean in detail is explained in our guide to ISO 3601.
DIN 3771 (Germany, until 2010)
For decades, DIN 3771 was the German standard for O-rings in fluid power. It was superseded by ISO 3601 in 2010, but its dimensions are still found in older plants, drawings and parts lists.
The sizes follow the ID x C/S scheme: 40 x 3.55 means an inner diameter of 40 mm and a cord thickness of 3.55 mm. Cord thicknesses range from 1.8 to 7 mm, inner diameters from 1.8 to 670 mm. The structure, tolerances and relationship to ISO 3601 are explained in our article on DIN 3771.
BS 1806 (United Kingdom, inch)
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)
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)
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)
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)
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
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, the designation in the format ID x C/S, i.e. inner diameter x cord thickness, 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.
Finding the size: tables and tools
These tools take you from the standard to the specific size and the right installation space.
Frequently asked questions
Which O-ring standard is the most important?
How do AS568 and ISO 3601 differ?
How do I read an O-ring size such as 20 x 2?
What does AS568-014 mean?
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?
