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request a quote on o-ringsAn o-ring is one of the most common types of seals in the world. Due to this popularity and the vast array of applications in which o-rings are used, there is a virtually unlimited number of possible sizes of an o-ring. Thus, the need for size standards which cover the most common sizes used across various industries and geographic regions. Over the years, many government and engineering organizations have produced a multitude of O-Ring standards. Here we present a brief overview of what we believe to be the most often used/referenced standards by our customers in the United States and around the world.


Some of the standards referenced here do specify the acceptable tolerance ranges for the o-rings within that standard. However, With the exception of our AS568 size chart, our other size reference charts do not list tolerances, as these tend to vary between the standards issuing organizations. To view the size tolerances for the o-rings we sell, please refer to either the AS568 size chart for American Standard Sizes, or our Metric O-Ring Tolerances page for all other “metric” o-rings.


**This information is only intended as a guide to identify O-Ring sizes from the various international standards. Although we believe this information to be correct, we cannot guarantee 100% accuracy with each and every standard. Therefore, we recommend referral to the actual specifications before proceeding with design work, etc. **

View all of these size charts online by clicking the links below, or download/view as a PDF.

AS 568

AS568 is the most commonly used standard in the United States for defnining “inch sizes” of o-rings. To see our AS568 size chart in the most common format (showing inch sizes), click here.
While not typically considered “metric sizes”, AS568 O-rings may be referred to by their metric dimensions.

Metric O-Ring Size Cross Reference Table

This chart shows all of the common international size standards and allows you to cross reference with other standards. Please note that no tolerances or materials are taken into consideration in this cross reference chart. View our O-ring Size Cross Reference Table.

BS 1806

The BS 1806 O-Ring size guide was issued by the British Standards Institution and lists all Imperial standard sizes. This standard includes all of the 5 main cross section groups in AS568 plus several sizes that are “between” AS568 Sizes. BS 1806 has been superseded by BS ISO 3601, however, since BS 1806 is still widely referenced in certain industries/regions of the world, it is still relevant for cross reference purposes. View our BS 1806 size chart.

ISO 3601

Issued by the International Organization for Standardization, the ISO 3601 standard contains of two groups of O-Rings, Class A and Class B.

Class A corresponds to the American standard AS 568B in its current format (the 900 series tube fitting O-Rings are not included). The ISO 3601-1 Size Code for these O-Rings is the same as the AS 568 dash number.

Class B allows production of O-Rings in technically acceptable and economical “metric” sizes, which can then fit into metric grooves. The first digit of the Size Code indicates the cross section group (A-E), while the last four digits indicate the O-Ring inside diameter rounded to one-tenth millimeter. View our ISO 3601 size chart.

BS 4518

Issued by the British Standards Institution, Standard BS 4518 identifies British Standard metric sizes. The size code for these O-Rings is a four digit number indicating the O-Ring I.D. in tenths of millimeters followed by a hyphen and two digits indicating the O-Ring Cross Section, also in tenths of a millimeter. View our BS 4518 size chart.

SMS 1586

SMS 1586 refers to Swedish Mechanical Standard for o-rings (Sveriges Mekanstandardisering). In this standard, O-Rings are simply identified by ID and C/S, similar to DIN 3771. Additionally, all SMS 1586 O-Rings are classified into two groups. The first group, intended for dynamic and static installations is indicated in our table by the letter D. The second group, intended mainly for static applications, is indicated with an S. View our SM 1586 size chart.

DIN 3771

The DIN 3771 o-ring standard is issued by The German Institute for Standards (Duetsches Institut für Normung). This standard identifies O-Ring sizes by the ID x C/S, and may be followed by a letter indicating the quality level (N – normal quality; S – special quality), and a code indicating the rubber polymer and IRHD hardness. View our DIN 3771 size chart.

NF T 47-501

NFT 47-501 is issued by the Association Française de Normalisation (French Standards Institute). It is very similar to ISO 3601-1 in both the sizes included and the size reference. The size reference codes are designated with a letter corresponding to each of 5 cross section groups (A-E), 4 digits indicating ID (rounded to 0.1 mm), a 2nd letter indicating Precision Class and a 3rd letter indicating Visual (inspection) Class. The Precision Class is indicated by the letter A (for Aerospace applications) or G (for General Purpose applications). The visual classes are N (1.0 AQL) and S (.65 AQL). View our NFT 47-501 size chart.

JIS B 2401

JIS B 2401 is a Japanese Industrial Standard for O-Ring sizes. This standard, often referred to as “Japanese Metrics”, is organized into four series, based on their application. The O-Rings in each section have ascending number codes to complete the size reference.

  • P Series – Moving (dynamic)
  • G Series – Fixed(static)
  • S Series – Special Sizes
  • V Series – Vacuum Flange

View our JIS B 2401 size chart.

IS0 6149

ISO 6149 specifies O-Rings designed for use in Metric Tube Fittings. This standard includes 13 different sizes and calls out the metric thread size for each o-ring size. View our ISO 6149 size chart.

Generic Metric O-Ring Sizes

We have also included a table showing all of the most common metric sizes that may or may not match up with any of the international standard sizes. We are able to supply most of these sizes, as most factories have established tooling on these generic metric sizes. View our metric o-ring sizes.