An O-ring is a round-shaped joint or gasket that creates a seal between two surfaces. It is one of the most common sealing solutions used for various mechanisms, such as pumps, valves, and cylinders, to prevent leaks or contamination.

O-rings are used in several industries, including transportation, medical, electronics, and even food and beverage. They are also made from different materials depending on their purpose, and some are made with nitrile, silicone, or thermoplastic polyurethane, each with its own beneficial properties.

Because of the numerous applications of O-rings, you can expect an extensive range of sizes to meet different needs. There are several O-ring size charts available that you can refer to. Read on to learn about the most commonly used ones.

AS 568 or AS 568-A

This is the Aerospace Size Standard for O-rings issued by the Society of Automotive Engineers or SAE. Also known as the American standard, the AS 568 or AS 568-A chart is the most widely used reference in the United States for O-ring sizing.

There are 369 standard sizes available in the AS 568 standard that are categorized into six sections, showing measurements in inches and millimeters. Here, the cross-sectional diameters range from 0.040 to 0.275 inches, while the internal measurements go from 0.029 to 25.490.

ISO 3601

The ISO 3601 standard was published by the International Organization for Standardization, which includes two groups of O-rings—Class A and B. While the Class A sizes utilize the AS 568 standard, Class B O-rings may be produced within the satisfactory and economical sizing that suit metric grooves.

BS 1806 and BS 4518

The British Standards Institution created two O-ring sizing charts—BS 1806 and BS4518. Like Class A O-rings, the BS 1806 conforms to the AS 568 standards, the five main cross-section groups, and additional sizes. The BS 1806 standard has been replaced by the ISO 3601 but is still used for cross-referencing.

Meanwhile, the BS 4518 states the British Standard metric sizes. So, it is most commonly used by industries in the European region. The BS 4518 O-ring sizing code includes four numbers (O-ring ID), followed by a hyphen and two digits that specify the cross-section measurement. Both numbers are shown in tenths of a millimeter.

DIN 3771

Published by the German Institute of Standards, the DIN 3771 sizing standard is identified by its internal diameter and cross sections. Besides that, it specifies two quality levels: N or S. N refers to an O-ring with a normal or 1.0 acceptable quality level (AQL), while S pertains to those with a special or 0.65 AQL.

NF T 475-01

Another common O-ring standard used is the NF T 475-01, issued by the French Standards Institute and is similar to the ISO 3601. For this, the sizing code includes the letter of one of the five cross-section groups (A-E) and the four numbers indicating its internal diameters.

It also specifies the visual (N or S) and precision class—the letter A for aviation applications and G for general use.

JIS B 2401

Most Japanese industries’ O-ring sizes are measured using the JIS B 2401 standard. Also known as the Japanese Metrics, this sizing chart is divided into groups based on its application.

This includes aerospace (A) and general (G), which both follow the ISO 3601 standard with some additional sizes. Meanwhile, the traditional group has a total of 194 O-ring sizes and is divided into four sections based on their application, namely:

• P – mobile or dynamic

• G – fixed or static

• S – special sizes

• V – vacuum flange

With this O-ring size chart guide, you will better understand the different sizing standards of this sealing component. Using the right kind, material, and size of O-rings is crucial to ensure your piping, machinery, and other equipment will function properly. It will also prevent any issues, such as leaks or spills.

If you are unsure about choosing O-ring sizes, it’s best to consult a professional. Contact a trusted sealing company near you to learn more about O-rings and other gasket solutions and get expert advice on which one best fits your needs.