O-Rings are available in a wide range of standard and non-standard sizes that are suitable for nearly all sealing applications.
Two dimensions describe the size of an O-Ring: it’s inside diameter (ID) and its cross-sectional diameter (CS). The standard sizes are defined by Aerospace Standard AS568B, Aerospace Size Standard for O-Rings. That document, published by the Society of Automotive Engineers (SAE), lists the sizes of O-Rings in six series or groups in both inches and millimetres. The first five series are based on cross-sectional diameter. The sixth series includes 20 sizes for boss seals. The standard sizes are also used for most military specifications. Cross-sectional diameters range from 0.040 to 0.275 inch. Inside diameters range from 0.029 to 25.940 inches.
Standard metric sizes for O-Rings are defined by International Standard ISO 3601-1:2002: Fluid power systems O-Rings Part 1: Inside diameters, cross-sections, tolerances and size identification code. That standard groups metric sizes into two series, G and A. The G series is used for general purpose applications and includes a wide range of inside diameters. The A series is used for aerospace applications where tighter tolerances are recommended.
Dimensional Tolerances in O-Ring Sizes
- The tolerances of AS568B, introduced in 2001, are a combination of the former Class I and Class II tolerances of AS568.
- Class I was for O-Rings made of other elastomers. Class II permitted wider tolerances than Class I. Class II was for O-Rings made of higher-shrinkage elastomers, such as fluorocarbon and fluorosilicone
- O-Ring sealing is based on the volume relationship between the O-Ring and the gland. Slightly wider dimensional tolerances, especially for larger diameter O-Rings, make no significant difference in the O-Ring volume. Therefore, seals made to Class II tolerances have proved as effective as seals made to Class I tolerances.
- For that reason, Aerospace Standard AS568A used a single set of tolerances for inside diameters. For O-Rings with IDs less than 0.5 inches, AS568A used the former Class I tolerances. For larger O-Rings, AS568A used the Class II tolerances.
Choosing Correct Cross-Sectional Diameter for O-Ring Sizes
O-Rings with a larger cross- sectional diameter tend to have better resistance to compression set. They also have less volume swell in fluids and are less likely to leak if their surface is scratched. O-Rings with smaller cross- sectional diameters have better physical properties, are more resistant to explosive decompression, and require less space.
For many dynamic applications, there is some choice of cross-sectional diameters. Larger cross- sectional diameters are more resistant to rolling in a groove but have more friction
In general, tighter tolerance components or rigid components with smooth or ground surface finishes can use O-Rings with the smallest cross- sectional diameter regardless of the pressure. Parts with looser tolerances or less rigid components, such as large plastic housings, should use O-Rings with larger cross-sectional diameters. Those plastic housings could flex under high pressure or expand when heated, reducing the actual squeeze on the seal.
O-Ring Sizes are typically available in standard sizes per industry standards. These include:
- AS 568 – Standard O-Ring Size
AS568 is the most commonly used standard in the United States for defining “inch sizes” of O-Rings. 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
- BS 1806 - Standard O-Ring Size
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.
- ISO 3601 - Standard O-Ring Size
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 millimetre.
- BS 4518 - Standard O-Ring Size
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 millimetres followed by a hyphen and two digits indicating the O-Ring Cross Section, also in tenths of a millimetre.
- SMS 1586 - Standard O-Ring Size
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.
- DIN 3771 - Standard O-Ring Size
The DIN 3771 O-Ring standard is issued by The German Institute for Standards (Duetsches Institute 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.
- NF T 47-501 - Standard O-Ring Size
NFT 47-501 is issued by the Association Françoise 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).
- JIS B 2401 - Standard O-Ring Size
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
- IS0 6149 - Standard O-Ring Size
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.