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How to measure an O Ring?

O Ring measuring techniques

O Rings are round mechanical devices that prevent liquid or gas from leaking through an opening. As time passes, these elastomers may wear out and need to be replaced. A good fit is critical when planning to replace an O Ring.

Measuring an O Ring is relatively easy – 3 measurements need to be taken – Internal Diameter (ID), Outside Diameter (OD), and the width or Cross Section (CS) of the ring.

Please see below image to help you locate where to take measurements from.

O Rings are measured in millimetres (mm).

How to measure an O Ring

How to measure an O Ring?

  • Situate an O Ring on a flat, level, and clean surface.
  • Establish the Inside Diameter (ID) by using a ruler to measure from one inner edge to the other inner edge of the ring.
  • Determine the Outside Diameter (OD) by using a ruler to measure from one outer edge of the ring to another outer edge.
  • Measure Cross Section (CS). Situate an O Ring into a vernier caliper. Slightly and gently clamp the jaws of the caliper onto the O Ring (do not compress it to avoid displacement). Note the thickness of the ring.

Useful formulas

In several circumstances, where you only have 2 out of 3 measurements, you can determine the third diameter by using the following formulas:

Outside Diameter (OD) = ID + (2 x CS)

Inside Diameter (ID) = OD – (2 x CS)

Cross Section (CS) = (OD – ID) ÷ 2

If the O Ring is cut, all you need to do is measure the Cross Section (thickness) and the length. To convert Length to Inside Diameter, use the following formula:

Inside Diameter = (Length ÷ 3.142) – Cross Section

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E & OE. M Barnwell Services endeavour to make sure that all content is correct. Information has been gathered from manufacturing partners.