How to measure O-ring size

O-rings are circular, seals that prevent gas or liquid from passing through an opening. After countless operational cycles, O-rings will eventually wear out and fail. When that time comes, it’s essential to know the correct dimensions of your O-ring to source a like for like replacement. In this blog, we’ll explain how to measure an O-ring correctly.

Understanding how to measure an O-ring is important for several reasons. Firstly, finding the right O-ring size will ensure an optimum sealing solution. It sounds obvious, but this is crucial to ensure the effective operation of your sealing application – especially in aggressive industries where perfection is a necessity. Secondly, wrongly sized O-rings can quickly become costly. Having to repeatedly replace worn O-rings can have a detrimental impact in terms of the costly downtime you will incur.

With all that said and done, let’s take a look at how to size your O-rings correctly to maximize performance and cost savings in one fell swoop.

How to measure an O-ring

The good news is that O-rings are relatively easy to measure. Place your O-ring on a level, clean and completely flat surface. Bearing in mind that O-rings are measured in millimetres, begin by determining the inside diameter by measuring from one inner edge to another using a Vernier. Then take measurements of the outside diameter, following the same principles, except measuring to the outer edges.

Next, you’ll want to measure the cross-section. To do this, place the O-ring in a Vernier calliper and lightly clamp the jaws of the calliper onto the O-ring. Taking care not to compress the O-ring, record its thickness (cross-section) as a result. If you only have access to two of the O-ring diameters, then you can calculate the third diameter by using the following equations:

  • Cross Section (CS) = (OD – ID) ÷ 2
  • Inside Diameter (ID) = OD – (2 x CS)
  • Outside Diameter (OD) = ID + (2 x CS)

If the O-ring is cut, then simply measure the cross-section and the length of the outstretched O-ring. Converting the length to Inside Diameter is a simple matter of dividing the length by 3.142 to get the circumference. Then subtract the cross-section from the circumference to get the Inside Diameter.

Of course, O-rings inevitably become misshapen over time, making an accurate measurement of the O-ring itself virtually impossible. In such cases, measuring the width and depth of the groove or shaft where the O-ring will be seated will provide the dimensions needed for your specific O-ring.

Find the right size O-ring with TRP Polymer Solutions

TRP Polymer Solutions can help you to effectively size the correct O-ring for your application. This can be done in various different ways. Firstly, if you know the correct AS-568A O-ring code, then our handy O-ring size guide can help you to pinpoint the correctly sized O-ring that you require.

If you don’t know the precise code, then you can use our size finder. Simply enter an O-ring inside Diameter and/or cross-sectional thickness, to get the results for the precise AS-568A code, O-ring size in millimetres and the tolerance (+- mm ID/Dia).

O-rings available from TRP

At TRP, our O-rings are available to AS-568A / BS1806 / BS ISO 3601-1 / BS ISO 3601-2 standard sizes. Our technical team can help you to find the right size O-ring for your industrial requirements. We can also advise on the best material selection to enhance your O-ring, for the ultimate reliability, durability and performance. Whether that be FFKM, FKM or another rubber material, we can guide you towards the most suitable material, depending on the level of resistance you require.

For more information about how to measure an O-ring, or for further advice and guidance, contact TRP today by calling +44(0)1432 268899 or email us at