If you’re working on a project that involves sealing two surfaces together, you’ll need to use an o-ring. O-rings are small, circular rings made of elastomeric material. They’re commonly used in plumbing and mechanical applications to seal between two surfaces.

To ensure that your o-ring seal is effective, you must use the proper size o-ring for your application. Why is it important to know how to measure o-rings? If the o-ring is too small, it won’t create a good seal. It could cause leaks or damage to the surfaces you’re trying to seal together if it’s too large. You can count on Allied Metrics to guide you on how to size an o-ring.

How To Properly Size An O-Ring the First Time Around?

Know the dimensions of the application

To select the proper size O-ring, you must first know the ring dimensioning of the application. It includes the inside diameter (ID) and outside diameter (OD) of the application and the width of the groove where the O-ring will sit.

Choose the right material

O-rings come from different materials, each with its unique properties. The most common materials used for O-rings are nitrile (NBR), fluoroelastomer (FKM), and silicone. The material you choose should be based on the application’s operating conditions, such as temperature, chemical compatibility, and abrasion resistance.

Determine the proper size

Once you have gathered all the necessary information, you can determine the proper size O-ring for your application. You can grab an O-ring sizing guide, but the most common is the groove design method. This method relies on the groove’s ID, OD, and width to determine the correct size O-ring.

Test the O-ring

After selecting the proper size O-ring, it is essential to test it in the application to ensure it functions as intended, like doing a leak test or a pressure test. Doing these tests will help you to avoid any potential problems down the road.

Methods of sizing an O-ring

The method of sizing an O-ring can differ depending on the application:


This method uses a cone with painted circles along its entire height. Find the largest circle the O-ring can fit in without stretching.

Digital Calipers

A digital caliper may also be used to determine the size of an O-ring. Before you begin, check if the calipers are set to zero. The caliper can be used to measure the distance between the inner edge-to-edge distance across the center. After that, the screen will show the size.

PI tapes

With the Pi Tape Measure, you can measure average diameters quickly and directly. To determine the proper size, wrap the tape around the O-ring and bring the two ends together. You can calculate the average O-ring outside diameter by using the corresponding mark.

Optical Comparators

An optical comparator is an instrument that uses a light source and optics to inspect the surface of an object. The object inspected, such as O-rings, is mounted on a stage, which can be moved in X and Y directions to examine the entire surface.

Factors to consider when choosing the right O-ring

If you’re still unsure which to choose or how to size an o-ring, there are a few other things to remember.

  • O-rings come in a variety of materials, each with its unique properties. Some materials are better suited for high temperatures, while others are better for chemical resistance. If you’re unsure which material is best for your application, consult a knowledgeable source.

  • Choose an o-ring that can withstand more compression than the application requires, ensuring a proper seal and preventing damage to the O-ring.

  • Once you’ve selected the proper size o-ring, it’s crucial to install it correctly. First, ensure the surfaces that the o-ring will seal are clean and debris-free. Next, properly lubricate the o-ring so it can sit in the groove. Finally, install the o-ring carefully, making sure not to damage it in the process.

Do You Need A Hand?

Properly sizing an o-ring can be the difference between a successful seal and a leaky one. If you’re unsure of how to size an o-ring, don’t worry! The experts at Allied Metrics are here to help. We have years of experience sizing o-rings for various applications, and we can get the job done quickly and efficiently so you can move on to the next step in your project. Don’t wait – contact us today to learn more about our products.