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How To Measure An O-Ring


Last updated Dec 21, 2023 | Published on May 15, 2017

O-Rings are an essential sealing component prolific in every industry, so there’s shapes and sizes to fit nearly every application.

O-Rings: The right size for every occasion

O-rings come in different shapes and sizes. Knowing the measurements an application needs can make a difference on how well an O-ring works. If the O-ring is too big for the application, it will not seal properly. If the O-ring is too small for the application, the seal might break. The performance of an O-ring seal can be optimized by choosing the most appropriate size, which can be found by measuring an O-ring.

How to measure an O-Ring

O-Rings have three different measurements: inside diameter, outside diameter, and cross-sectional thickness.

Inside diameter (I.D.) is measured from one inner border to the other with a measuring device (ruler, measuring tape, PI tape, etc.)

Outside diameter (O.D.) is measured from the outer border to another with a measuring device. However, O.D. is not often used.

Cross-sectional thickness (C.S.) can be measured with calipers or a micrometer. Gently clamp the jaws of the device onto the O-ring to avoid warping it.

If the O-ring is not whole, measure the length of the of the O-Ring and divide by π (3.14159), then subtract the cross-sectional thickness.

Length ÷ π = Circumference

Circumference – C.S. = I.D.

Diagram of how to measure the outer dimension and inner dimension of an o-ring

For greater accuracy, it is recommended to use digital measurement equipment. 

For more information about our sealing solutions, check out our Gaskets & O-Rings or contact us for help in your sealing application!

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