How to dimension O-rings?

How to dimension rubber O-rings? If you need information on how to measure rubber O-rings, see the guide from We explain the measurement of rubber O-rings, i.e. what will enable the correct measurement of O-rings.

In our news, we have repeatedly mentioned the topic ofrubber O-rings offered in our store. We wanted to provide you with knowledge about the use of these rubber gaskets, their proper protection and, finally, how to put them on. Today we would like to develop this thread even more and consider the methods of dimensioning O-rings, without which it will be difficult for us to use them as intended. Before we get down to the merits, however, let’s recall what these O-rings are, so that everyone is aware of how important we are talking about. Rubber O-ringsare a type of seal that, thanks to its extraordinary flexibility, is perfect for a variety of situations. Due to the material, several types are distinguished, but they all share the most important feature, which, together with the rest, owes their name. Of course, we are talking about a round shape.

Correct size

Due to the size, it is required that the variety of O-rings is as large as possible. This is the only way to secure all the required machine components, the number of which is often dizzying. It is believed that in order to meet customer requirements, gaskets with diameters differing by 1 mm should be produced. However, since all O-rings are round and their size can vary only to a very small extent, how do we remember which one is intended for a given place? A classic would say that God gave us eyes to judge the size of the O-rings by eye. It sounds silly, but unfortunately it may not work in this situation. Let us assume that we have already equipped ourselves with the appropriate measuring equipment. What’s next? Which dimensions do you need to measure to know how the O-rings we hold compare to those offered by the manufacturer? Most often, to dimension round gaskets, we enter their inner diameter and the width of the section, sometimes called thickness.

How to measure O-rings?

If we are looking for the password: How to measure the O-rings? we already know the main function of rubber o-rings: they seal, secure, and separate them from various environments. We also know what we need O-rings for, in what environment they work, at what temperature and with what agents they are in contact with. We also know the approximate amount we need, which makes it very easy for us to buy O-rings in the online store or stationary, with us on site.

What we don’t know about these seals is: how to dimension o-rings?

The main rule is to specify the dimensions of the rubber O-rings in the unit: millimeters [mm]. We do not provide the dimensions of the O-rings in centimeters, decimeters or meters. We use mm. Attempting to change the unit may result in a different O-ring and a different price, and it is better to have low prices.

O-rings dimensions and their markingsa

There are various markings on the market for the dimensioning of O-rings. So let’s move on to the first one:

  1. Di. int. on thickness – O-ring 5×1

In this case, the marking is as follows:
O-ring 5×1 means:
5- Wed int. rubber O-ring
1- thickness of the rubber O-ring

How-to-dimension-Oring 5x1 FKM
O-ring 5×1

Let’s look at another example:
O-ring 18×2. 6
18 is Wed inside, and 2.6 is the thickness of our O-ring.

dimensioned O-ring - 18x2.6


For confirmation, the third example:
O-ring 30×5
30 is Wed inside and 5 is the thickness.

oring 30x5 NBR

  1. Di. int. on Wed call. – Oring 5×7

In this case, the O-ring designation means:
5- Wed int. rubber O-ring
7- Wed call. O-ring


As can be seen in the attached drawing, the first is Wed.

For consolidation, one more example from the dimensioning of O-rings:
Oring 18×23.2, which means: 18 Wed ext, a 23.2 wed. call.

Example of dimensioning the O-ring No. 3 for this method:
O-ring 30×40, means that 30 is the inside diameter and 40 is the outside diameter.


  1. Di. call. on Wed int. – Oring 7×5

This model is the opposite record of the second model.
Marking 7 is for Wed. and 5 refer to the dimension for the dia. ext.

The O-ring 23.2×18 is the same O-ring as used in the second model: 18×23.2
This marking starts with the outer diameter, then the inner diameter.


O-ring 40×30 is an O-ring on the diameter call. 40 and Wed. int. 30


  1. Di. call. on thickness – Oring 7×1

An increasingly rare model of markings. The first number corresponds to the dimension dia. and the other on the thickness. 7×1 is the O-ring on the Wed. external 7 and Wed. ext. 1.

O-ring 23.2×2.6 is O-ring on Wed. call. 23.2 and a thickness of 2.6.

O-ring 40×5 is O-ring on Wed. call. 40 and a thickness of 5.


The full range of O-rings – you can find here!


What is worth remembering about O-ring dimensions?

  • There are many methods of measuring O-rings
  • It is always worth mentioning the method we use when specifying the dimensions of the rubber O-rings.
  • It is also worth remembering that the accepted standard is to specify the dimensions of the O-rings in millimeters, which facilitates cooperation. Most often, however, there are mistakes when the given value is given in centimeters, not millimeters. It is worth asking in which units the team cooperates.
  • POWER Rubber advises on the selection of rubber O-rings

O-ring rubber seals

How to calculate the methods of measuring O-rings?

Here we can ask ourselves: how do we go from annotation model 2 to model 1?
In order to specify the dimensions of the O-rings in the first model, what should be done:

From the Outer diameter of the O-ring, subtract the dia. ext .. Divide the given dimension into two. And this is how we gain thickness, known as the O-ring height. Model one is the dimensioning of O-rings in the format: Wed. ext in thickness, so we take the average ext to the calculated thickness and voila.


Model 2 designation: Wed. ext. on Wed external: O-ring 5×7

We calculate the thickness: 7-5 = 2 2/2 = 1.

By inserting the notation into the model, we get: O-ring 5×1

Inch and metric o-rings

Here it is also worth recalling the two units appearing in the O-rings:

  • metric
  • inch

Among the mentioned types, you can read about thenorms and standards in O-rings.

If you are still not sure about the correct dimensions, write to us – We will help you choose the right size of rubber O-rings!

Please contact us!

Also for telephone contact:

+48 505 16 03 03


Which method to choose?

However, it is worth remembering that there are many methods of dimensioning O-rings in nature. One of them is the specification of the inside diameter in relation to the outside diameter, or vice versa. Modern technology suits us to such an extent that we can even completely free ourselves from the obligation to measure the O-rings by ourselves by using a special cone. This neat measuring element will allow us to quickly identify nearly 200 basic sizes of O-rings. It is enough to put the gasket on the cone and it will stop at the place marked with the corresponding size.

However, this is not the end of the problem with the widely understood O-ring measurement. It is because more than once we have to measure old O-rings, which served as a seal for a long time.

In this case, do we proceed in the same way as with newborns?

On the contrary.

First of all, the internal diameter of the worn O-rings must be measured on the sealed element. As for the thickness, you need to take two measurements (each in a different place) and take the average. This also does not give us 100% certainty that we measured the deformed O-ring correctly. So, when you buy a new O-ring in its place, you need to add to your order also O-rings with a larger size and a smaller cross-section size. Unfortunately, in this matter, the size is of great importance!

We invite you to get a Free Catalog: O-rings The catalog in PDF is available here: O-rings catalog

Rubber products

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