Inside diameter, cord thickness, standards... and then there is matching the groove dimensions. It is important to carefully determine the O-ring dimensions. This will prevent leaks and malfunctioning or non-functioning of your machine or product. You should therefore know what to look out for and how to determine the size so that you choose the right O-ring for an optimal seal in your application.
An O-ring that is used as a seal is always mounted in a groove. That groove must meet certain conditions in terms of dimension and finish, so that the O-ring functions optimally.
If you are developing a new machine or a new product where an O-ring must be used, you usually have various options for determining the correct groove dimensions. Here, you can read more about groove design and also find a link to our O-ring and groove calculator.
However, if you are making a repair, you usually can't adjust anything about the groove dimensions. That is a given. You will then look for a suitable O-ring for an existing groove. In that case, you should first check the existing groove dimension on a drawing or measure the groove (see figure below for explanation). The inner diameter of the groove, the groove width (F), but also the distance (E) is important. So make sure you don't just take the groove depth, but add the groove depth and any gap (S) when in its assembled state. This cannot always be measured and therefore has to be calculated sometimes.
Without knowledge of the groove dimensions, it is difficult, risky or sometimes impossible to determine the correct O-ring size. It is not recommended to determine an O-ring size by measuring a used O-ring.
The ideal inner diameter and cord thickness depend on the application:
The O-ring cross-section (refer to round, black shapes) is pressed radially into the groove between the inside (interior diameter or ID) and the outside (outside diameter or OD).
The O-ring cross-section (refer to round, black shapes) is pressed axially into the groove.
The O-ring cross-section (refer to round, black shapes) is pressed radially into the groove between the inside (interior diameter or ID) and the outside (outside diameter or OD).
The cross-sectional area of the rubber O-ring should be at least 15% smaller than the cross-sectional area of the groove. This is necessary because rubber material cannot be compressed and therefore needs free space when compressed. If this margin is smaller, the risk of damage to the O-ring increases.
When looking for the appropriate O-ring size, it is useful to bear in mind that when assembled, you may stretch the diameter of an O-ring by up to 5% or compress by up to 3%, depending on the application.
Example: Based on your groove dimension in a static application, you are looking for an O-ring with 308 mm inner diameter and 5 mm cord thickness. However, you cannot find a 308 x 5 mm O-ring, or it is not readily available. Since it is possible to stretch the O-ring diameter by up to 5% and compress it by up to 3%, you can choose an alternative size: 305 x 5 mm (with only 1% stretch), or 310 x 5 mm (with only 0.6% compression).
There are many international O-ring size standards, which are often used interchangeably. Choosing an O-ring size from an existing standard is convenient, as you are more likely to have it available as standard. There is also an infinite choice of metric sizes, and you can have O-rings customised in every possible size. These are the most common O-ring standards used internationally:
AS 568 / ISO 3601-1 / DIN 3771 / BS 1806 / BS 4518 / French R / JIS B 2401
AS stands for 'Aerospace Standard' and is often described as 'American Standard' because the US Society of Automotive Engineers (SAE) established the AS568. AS 568, or more specifically AS 568-A, is probably the most widely used standard for O-ring sizes worldwide and is quite recognisable because of the O-ring thicknesses such as 1.78mm, 2.62mm, 3.53mm, 5.33mm, 6.99mm.
View the O-ring size chart of the AS 568 standard
ISO has adopted the dimensions from AS568 in the ISO3601 standard. The standard consists of 4 sections. Section 1 (ISO 3601-1) describes the sizes. The differences from AS568 are, for example, in Section 3 (ISO 3601-3) and deal with tolerance and surface quality.
View the O-ring size chart of the AS 3601 standard
DIN stands for ‘Deutsches Institut für Normung’. The DIN 3771 is an old standard that is less commonly specified but can often be found on older drawings. The O-ring sizes can still be frequently encountered, even outside Germany. Unlike many other O-ring standards, DIN 3771 does not use any additional numbering or code per size.
View the O-ring size chart of the DIN 3771 standard
BS stands for ‘British Standard’. There are two BS standards for O-ring sizes: BS 1806 and BS 4518 O-ring sizes according to the BS 1806 standard are largely in line with ISO 3601-1. The BS 4518 standard is mainly used within the UK.
View the O-ring size chart of the BS 1806 standard
View the O-ring size chart of the BS 4518 standard
The French R standard overlaps with ISO3601 and AS568, but also has some typical dimensions within its range. The type numbering is identified by the R, such as R12
View the O-ring size chart of the French R standard
The JIS standard stands for Japanese Industrial Standard, in which B 2401 is the Japanese O-ring standard. The O-ring sizes are identified by the addition of the letters P, G, S and V which are designated for different applications: P (dynamic), G (static), S (special) and V (vacuum)
View the O-ring size chart of the Japanese JIS standard
Metric sizes are not standardised to the size itself. They are usually rounded off to millimetre dimensions or defined to half-millimetre accuracy.
View the O-ring size chart of a wide range of metric sizes
Do you want to check if the combination of O-ring size and groove dimensions are properly matched? A helpful tool to check this is our O-ring calculator. Keep the groove dimension on the design drawing close at hand when using this.
If, based on all the above information, you still cannot find a suitable O-ring size, then customisation is probably the only solution. This is an option even for a few pieces. This is not a cheap solution, but in certain situations it may be the only or best solution.
There are 3 production methods:
All solutions are available upon request.
Still unsure about the right O-ring dimensions for your seal? Or do you have another question about the O-ring in your application? Our product specialist would be happy to take a look and advise you on how to proceed.
Stijn de Cnop
Product Manager of Sealing & Polymer Technology
Product Manager of Sealing & Polymer Technology
O-rings with a smaller cross-section will resist decompression and be more affordable to manufacture and purchase. O-rings with a larger cross-section significantly increase leak protection capabilities. It’s just as important to choose the right size as it is to pick the right material. O-rings come in a wide variety of sizes, including:
The size of an O-ring can be determined by measuring the diameters and the width of the cross-section. The measurement you will need to include are the internal diameter (ID), outside diameter (OD), and the cross-section width (CS). You can then determine the size of your O-ring using the following equation:
Two of the most important characteristics to look at when choosing O-rings include:
Some materials are used throughout general applications, while more extreme applications tend to default to certain materials with time-tested characteristics. Four of the hardiest specialty O-ring materials are PTFE, Viton, silicone, and nitrile.
Most O-rings are considered static axial seals—they create a tight seal between two parts that don’t move in relation to each other and can be made from materials with lower abrasion and tearing resistance.
Dynamic O-rings, however, hold a seal between moving parts. Not only do these O-rings need to be made from more resilient materials, but they also need more frequent maintenance and lubrication. Dynamic O-rings are classified based on the type of motion they need to withstand, such as reciprocating dynamics or rotary motion.
Once you’ve determined whether you need a dynamic or static O-ring, you’ll need to select the appropriate O-ring material.
One of PTFE’s most unique characteristics is its resilience to heat and cold damage. The material can stay chemically inert and resistant to abrasion at temperatures ranging from -73° C to 260° C. However, the material is rigid and can be difficult to apply to dynamic or moving parts.
PTFE is known for its viability in extreme conditions. Its key strengths include:
PTFE O-rings are commonly found in applications such as automotive steering, chemical processing gaskets, chemical storage, and paint guns.
Silicone O-rings are important components in outdoor and plumbing systems. The material can handle a wide temperature range between -84–232° C, and resists damage from water, acids, ozone, UV radiation, and heat. Pure silicone O-rings cannot withstand much in the way of wear, tearing, and other physical damage, so it’s best suited to static applications.
Common industries that rely on silicone O-rings include:
Nitrile is a resilient elastomer used in a wide variety of industries, such as aerospace and heavy duty equipment. Nitrile can be compounded and processed into several different forms. The material has a strong operational temperature range from -50° C to 120° C, and hydrogenated nitrile can withstand temperatures up to 150° C. However, it is vulnerable to higher temperatures and chemicals such as brake fluid and halogenated hydrocarbons.
Buna is for sturdy, general-purpose O-rings. Buna-N is a nitrile-based synthetic product that has excellent abrasion and tear resistance and withstands exposure to some solvents. Its resistance to weather and ozone damage is significant but can be improved with the addition of other compounds. While Buna O-rings are more cost-effective than fluoroelastomers, they are vulnerable to high temperatures above 149° C.
Nitrile can be used in applications that come into contact with varying temperatures, including high temperatures. Kalrez O-rings, in particular, can operate in temperatures as high as 327° C. Due to its chemical compatibility, typical applications of nitrile O-rings include:
Hydrogenated Nitrile Butadiene Rubber (HNBR) O-Rings can be used in extreme industrial environments because of its high durability properties. It withstands high temperatures and oxidation and can simultaneously be exposed to harsh chemicals without degrading. These chemicals include fuel, oils, and exhaust.
Viton O-rings—an alternative to nitrile—provide a reliable seal at high temperatures up to 205° C, or higher for brief intervals. It’s also more resistant to petroleum, acid, and silicone-based fluids than most other materials, and can often be found in oil processing facilities. Viton also features good general wear resistance.
Since Viton can handle systems with a wide range of fluids and temperatures, it is ideal for a wide range of applications, such as:
Ethylene propylene diene terpolymer (EPDM) is a highly durable material that can withstand exposure to common weather elements such as ozone, water, steam, heat, UV radiation, and oxygen. The material also resists chemical damage from alkaline and mildly acidic compounds. Common uses for EPDM O-rings may include:
Neoprene O-rings are particularly resistant to weather damage, including elements such as UV radiation, ozone exposure, and oxygen that may cause oxidation in other materials. It has a wide operational temperature range of -35–250° F and is resistant to flex cracking and permeation. Neoprene features high resistance to refrigerants and some oils, lubricants, and acids.
Neoprene O-rings are ideal for a narrow range of applications, such as air conditioning and refrigerant systems.
Polyurethane withstands extended contact with CO2, is physically tough, and features good extrusion and abrasion resistances. However, it is vulnerable to heat damage and can only tolerate environments that don’t exceed 100° C. Common applications for Polyurethane O-rings include: