An Introduction to O-Ring Sizes and What They Mean

An Introduction to O-Ring Sizes and What They Mean

You can’t choose the right O-ring without considering the size. O-rings are mechanical seals that feature a circular, ring-like shape. They are typically used to seal mating surfaces. Even when exposed to pressurized liquid or gas, O-rings will continue to seal the mating surface with which they are used.

O-rings, however, are available in different sizes. Some of them are bigger than others. Whether you’re planning to use it on a coolant line, gas line, air line or any other pressurized line, you’ll need to choose an O-ring in the right size.

Inside Diameter

Inside diameter is exactly what it sounds like: the inner diameter of an O-ring. All O-rings have an inside diameter. It represents the distance from the inside of an O-ring — at the O-ring’s widest section — to the opposite side.

O-rings with a large inside diameter can accommodate larger lines than those with a small inside diameter. They consist of a larger, wider ring-shaped mechanical seal. You can place these large O-rings around equally large pipes, hoses and lines.

Cross Section

In addition to inside diameter, there’s cross-sectional width. The cross section is the width of an O-ring. It represents the thickness of the tangible part of an O-ring. The cross section doesn’t take into account the inside diameter. Instead, it focuses specifically on the physical part, which is the rubber or elastomeric ring.

The cross section width of an O-ring will almost always be lower than the inside diameter. The inside diameter covers the entire distance across the center of an O-ring. The cross section, on the other hand, only covers the distance across the physical part of an O-ring. Therefore, most O-rings have a much smaller cross section than inside diameter.

Aerospace Standard Sizes

When shopping for O-rings, you may see a dash number for its size. Most O-rings use the Aerospace Standard 568 sizing specification. Published by the Society of Automotive Engineers (SAE), it uses a set of numbers to define O-ring sizes.

The Aerospace Standard 568 simplifies the process of evaluating O-ring sizes. Each O-ring is given a dash number. You can refer to these dash numbers to find the inside diameter and cross section of O-rings.

In Conclusion

O-rings are available in different sizes. When shopping for an O-ring, you should pay attention to the inside diameter and cross section. The inside diameter represents the distance across the inner section of an O-ring at its widest point. The cross section represents the thickness of the elastomeric ring.

USA AS568 Standard O-Ring Sizes

USA AS568 o-ring standard sizes are the most commonly used o-ring sizes in the world. Refer to our standard o-ring size chart to find USA AS568 o-ring dimensions. Order o-rings online from the world’s largest inventory network including custom o-rings typically without tooling fees. O-ring tolerances vary by use, please refer to our o-ring tolerance pages or contact us for responsive expert service.

Choose Marco Rubber & Plastics

For more than 40 years, Marco Rubber & Plastics has been offering custom and standard o-rings and other sealing solutions for multiple industries. With millions of products produced and shipped annually, we are the experts your organization needs to seal the success of your application. Explore standard size o-ring sizes below.

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How to Quickly Measure Your Rubber O-Ring

When finding the right o-ring for your application, size is a critical factor. An ill-fitted o-ring will cause leak paths to form in the sealing application, which will evidently lead to failure. O-Ring seals are engineered in AS568 and ISO 3601 standard sizes, as well as non-standard sizes for custom applications.

To ensure your o-ring is the right fit, there are two important measurements that need to be taken: inside diameter (ID) and cross-section (CS). Here’s a closer look at the steps you need to take when measuring your o-ring seal. 

Step 1: Inside Diameter

The ID measures the space between the inner edges of your o-ring. To find this measurement, lay your o-ring on a flat surface. Use a ruler to measure from one inner edge of the o-ring across to the inner edge. Depending on the size of your o-ring, the ID may be recorded in inches or millimeters. This will help determine how large of an opening your seal should have. 

Step 2: Cross-Section

The CS of your o-ring is otherwise known as the width or thickness of the seal. To measure the CS, you will likely need a vernier caliper. Place the o-ring in the caliper lightly, without compressing it. Measure and record the width of the rubber. This is an important measurement, as the width of the seal plays a major role in a seal’s compression resistance, volume swell and abrasion resistance. 

Step 3: Feel for Hardness

Every rubber material has a unique hardness, which is measured by the Shore A durometer scale. A general rule of thumb is the harder the compound, the higher the durometer. Standard rubber compounds generally have 70 Shore A hardness, but hardness can vary based on the material and the application. You can sort rubber materials by the hardness you’re looking for in our O-Ring Material Selection Guide. 

Next Steps: What to Do With Your Measurements

Once you have recorded the ID and CS of your o-ring, you can use these measurements to formulate the exact o-ring you need for your application. Standard o-ring sizes are often referenced by their ID and CS. For example, a seal formulated with AS568-010 size, 70 shore A hardness and Nitrile material would be referenced as 0.239 x 0.070 70 BN. 

Apple Rubber offers an online O-Ring Size Search tool that allows you to input these measurements to find the seal you need. With over 7,000 standard and non-standard o-ring sizes in stock, it’s likely that we have exactly what you’re looking for on-hand. If the size or material you’re looking for does not appear in the Size Search tool, you can input your o-ring sizes into our Request for Quote form and an Apple Rubber engineer will contact you with more information. 

If you have any questions about o-ring sizes, standards, or are looking for a size in-stock, contact an engineer today. 

How to Quickly Measure Your Rubber O-Ring

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