How to accurately measure O-ring dimensions

  • Find out why it is important to accurately measure O-ring dimensions. We’ll show you the correct technique to measure O-ring dimensions
  • In this article we’ll also give you advice on choosing the right size of O-ring for your application.

As engineers, we have all used thread sealing tapes at some point to fix leaks. But how do we fix leaks in applications that involve extreme temperatures and pressure? In such scenarios, the thread sealing tapes would probably just melt or become rigid, rendering them useless. This challenge led to the discovery of the O-ring, sometime in the 19th century.

measure o-ring dimensions

What is an o-ring?

An O-ring is essentially a gasket with a round cross-section that fits between two components or parts, and is compressed during normal operation, thus creating a seal at the joint. It is typically used to minimise leaks. Common examples include mechanical applications like pipe joints, pump shafts, cylinder pistons, and vacuum applications.

O-rings comes in various sizes and materials, considering that they are used in a variety of environments – high-pressure and low-pressure applications, extremely high or low temperatures, usage with chemicals, and hydraulic and pneumatic applications which involve abrasion, vibration, movement and so on. Depending on the application, they are usually made of synthetic rubber or thermoplastics.

Why is it important to measure O-ring dimensions?

O-rings form a mechanical seal between two stationary or moving parts. Over time, they wear out and might need to be replaced. In order to prolong the shelf life and minimize unnecessary wear and tear, a good or near-perfect fit becomes extremely important, which is why measuring O-ring dimensions accurately is critical.

measure o-ring with a sizing cone
An o-ring sizing cone

How to measure O-ring dimensions

In the UK, O-ring sizes are specified by the British Standard (BS), and each ring size has a related BS number. O-ring sizes are specified by the inside diameter, outside diameter (OD) and the diameter of the cross-section (thickness). The measurements are either in inches or in millimeters.

Steps to follow for an accurate measurement of O-ring dimensions: 

  1. Place the O-ring on a level, flat surface.
  2. Measure the inside diameter (ID), from one inner edge to the other, using a ruler.
  3. Using the ruler, measure the outside diameter (OD), which is the distance between the two outer corners.
  4. Using a Vernier Calliper, measure the cross-section – place the O-ring between the jaws of the calliper and press lightly, taking care not to compress it. Record the measurement (W).

Alternatively, if the O-ring is cut, measure the thickness (W) and the length (L) of the ring. The inside diameter is then calculated using the following formula:

ID = Circumference – W

Where, Circumference = L / 3.142

Outside diameter is given by:

OD = ID + 2W

Note: Some manufacturers provide ‘cones’ to determine the size of the O-ring required for a particular application. These cones do not measure actual sizes; instead, each groove on the cone corresponds to the nearest standard BS O-ring size.

Choosing the right O-ring size

Typically, O-rings with a higher cross-sectional diameter have a better resistance to compression. They swell relatively less when used with fluids, and do not leak as much when they have scratches on their surface. On the other hand, O-rings with a lesser thickness are more resistant to decompression.

In general, when using O-rings around rigid parts with smooth or polished surfaces, and tight tolerances, it is advisable to use O-rings with smaller diameters, irrespective of the pressure. Parts that are less rigid, with relatively loose tolerance could use O-rings of a larger thickness.

Experts also suggest a few general considerations while using O-rings in everyday applications. For example, the O-ring must ‘stretch’ slightly when inserted, for best sealing; the O-ring needs to get slightly ‘crushed’ to seal. All these considerations have a bearing on the choice of the size of the O-ring.


O-rings are one of the simplest and most inexpensive, yet highly critical components to ever be developed. Recent advancements in nanotechnology have resulted in nano-carbon materials being molded onto O-rings, making them more resistant to abrasion and tolerant of friction, thus making them fit for use in high-performance applications. Whatever the case may be, the versatile applications of O-rings over a wide range of environments and atmospheric conditions have made them a critical part of most equipment. And the key to their long shelf-life is proper size selection, which in turn requires one to measure O-ring dimensions accurately.



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