How to properly size an O-ring

sizing an oring

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Determining the exact size of an O-ring is one of the more challenging parts of ordering O-rings for your application. As you know, O-rings are used to create an impenetrable seal. If the O-ring is too large, fluids, gasses, dirt, water vapor outgassing under vacuum, and more will flow around it. If the O-ring is too small, overstretching it to fit it into place will compromise the O-ring’s integrity and can cause it to fail. Knowing how to properly size an O-ring is important to avoid O-ring failure and avoid wasting time and money. Sizing an O-ring isn’t tricky, but it is crucial that you are precise.

The best way to size an O-ring is to measure the cross-section (CS) as well as the inside diameter (ID) of the O-ring with a caliper or micrometer. The outside diameter (OD) is good to note but is a non-critical measurement. If you do not have these tools, you can use any measuring tool that you have available; a caliper or micrometer will be more accurate. For larger IDs over 6”, it is best to use an O-sizer or Pi-Tape for precise measurements.

Metric VS Standard Sizes

All O-rings either come in metric (mm) or standard (inch) sizes. If your CS matches one of the standard AS586 sizes (such as 0.70”, 0.103”, 0.210”, etc.) then your O-ring is most likely a standard size. If it does not match, then the odds are good that you have a metric O-ring.

Here is how to properly measure an O-ring:

  1. Place the O-ring on a level, flat, and clean surface
  2. Determine the CS by measuring the thickness of the O-ring
  3. Determine the ID by measuring from one inner edge to the other inner edge
  4. Determine the OD by measuring from one outer corner to the other outer corner
how to measure an oring


One important consideration you need to keep in mind when sizing an O-ring is tolerance. Due to manufacturing limitations, there will always be some variance in the O-ring’s dimensions. For example, an O-ring size of 0.103 with a tolerance of ± 0.003’, the cross-section can measure between 0.100” and 0.106”. It is best to consult the manufacturer for their specific tolerance variance based on your selected O-ring material.

UC Components, Inc. RediVac® O-rings are designed for clean-critical applications

Our standard RediVac® O-rings are available in a wide range of standard AS568A inch-measure sizes and are manufactured from top-quality fluoroelastomer materials. Buna, silicone, and other materials are available, and specific chemistries may also be available upon request.

Our O-rings are cleaned and packaged in a certified Class 100/ISO Class 5 Cleanroom, making them suitable for immediate use in most HV, UHV, or EUV applications. Vacuum-baked O-rings are available for reduced outgassing under vacuum.

UC Components, Inc. has been the world leader in high vacuum hardware since 1974. We specialize in O-rings and fasteners for High Vacuum, Ultra High Vacuum, and other critical applications. View our parts catalog online to find the components you need, request a quote, or contact us for more assistance or additional information.

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