How to dimension O-rings?
How to measure rubber O-rings? If you need information on how to measure rubber O-rings, consult the guide from POWERRubber.com. We explain measuring rubber O-rings, that is, what enables correct measurement of O-rings.
In our news we have often touched on the subject of rubber O-rings offered in our store. We wanted to provide you with knowledge about the application of these rubber seals, their proper securing and finally the ways of installing them. Today we would like to further expand this topic and consider methods of measuring O-rings, without which it will be difficult to use them according to their purpose. But before we get to the point, let us remind what these O-rings are, so that everyone is aware of how important the issues we are talking about are. Rubber O-rings are a type of seal that, thanks to their extraordinary elasticity, work excellently in various situations. Due to the material there are several types, but they all share the most important feature, which they also owe their name to. This, of course, is the round shape.
Correct size
It is required that, because of size, the variety of O-rings be as large as possible. This is the only way to secure all the required machine elements, the number of which often makes one dizzy. It is believed that to meet customer requirements, seals should be produced with diameters differing by 1 mm. However, since all O-rings are round and their size can differ by a very small amount, how are we to remember which one is intended for a given place? A classic would say that God gave us eyes to judge the size of O-rings by sight. It does not sound foolish, but unfortunately in this situation it may not work. Suppose we have already equipped ourselves with the appropriate measuring equipment. What next? Which dimensions must be measured to know how the O-rings we hold compare to those offered by the manufacturer? Most often, in order to size round seals, we give their inner diameter and the cross-section width, sometimes called thickness.
How to measure O-rings?
If we are looking for the query: How to measure O-rings? we already know the main functions that rubber O-rings perform: they seal, protect, separate different environments. We also know what we need the O-rings for, in what environment they work, at what temperature, with which substances they come into contact. We also know approximately how many we need, which greatly simplifies purchasing O-rings in an online store or in person at our location. What we do not know about these seals is: how to size O-rings? The main rule is to provide O-ring dimensions in the unit: millimeters [mm]. We do not give O-ring dimensions in centimeters, decimeters, or meters. We use mm. Attempting to change the unit may result in receiving a different O-ring and a different price, and it is better to have low prices.
O-ring dimensions and their notations
There are different notations on the market regarding O-ring sizing. Let us move on to the first of them:
- ID by cross-section – O-ring 5x1
In this case the notation looks as follows: O-ring 5x1 means: 5 - inner diameter of the rubber O-ring 1 - thickness of the rubber O-ring. Let us look at another example: O-ring 18x2.6 — 18 is the inner diameter, and 2.6 is the thickness of our O-ring. 
For reinforcement, a third example: O-ring 30x5 — 30 is the inner diameter, and 5 is the thickness. 
- ID by OD – O-ring 5x7
In this case the O-ring notation means: 5 - inner diameter of the O-ring 7 - outer diameter of the O-ring. As shown in the attached drawing, the first is the inner diameter, and the second dimension is the outer diameter. For consolidation, one more example of O-ring sizing: O-ring 18x23.2, which means: 18 inner diameter, and 23.2 outer diameter.
Example of O-ring sizing no. 3 for this method: O-ring 30x40 means that 30 is the inner diameter and 40 is the outer diameter.
- OD by ID – O-ring 7x5
This model is the reverse notation of the second model. The designation 7 refers to the outer diameter, and 5 refers to the inner diameter. O-ring 23.2x18 is the same O-ring as given in the second model: 18x23.2. In this notation the outer diameter is given first, and then the inner diameter.
Fig. O-ring 40x30 is an O-ring with OD 40 and ID 30
- OD by cross-section – O-ring 7x1
An increasingly less common notation model. The first number corresponds to the OD dimension, and the second to the thickness. 7x1 is an O-ring with OD 7 and ID 1.
O-ring 23.2x2.6 is an O-ring with OD 23.2 and thickness 2.6.
O-ring 40x5 is an O-ring with OD 40 and thickness 5.
You will find the full O-ring offer here!
What is worth remembering about O-ring sizing?
- There are many methods of measuring O-rings
- It is always worth specifying the method we use when giving O-ring dimensions.
- It is also worth remembering that the adopted standard is to give O-ring dimensions in millimeters, which facilitates cooperation. Most often, however, mistakes occur when the given value is provided in centimeters rather than millimeters. It is worth asking which units the team works with.
- POWER Rubber advises on the selection of rubber O-rings
How to convert O-ring measuring methods?
Here we can ask ourselves: how to go from notation model 2 to model 1? In order to provide O-ring dimensions in the first model, what you need to do is: subtract the inner diameter from the outer diameter of the O-ring. Divide that value by two. And so we obtain the thickness, called the O-ring height. Model one is O-ring sizing in the format: ID by thickness, so we take the ID and the calculated thickness and voilà. Example: Notation in model 2: ID by OD: O-ring 5x7 We calculate thickness: 7-5=2 2/2=1. Inserting into the notation model we get: O-ring 5x1
Inch and metric O-rings
Here it is also worth recalling the two units occurring in O-rings:
- metric
- inch
Among the listed types you can read about standards and norms occurring in O-rings.
If you are still not sure about the correct dimensions, write to us - We will help you select the correct size of rubber O-rings!
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Which method to choose?
It is worth remembering, however, that there are many methods of sizing O-rings in nature. One of them is giving the inner diameter in relation to the outer diameter or vice versa. Modern technology helps us so much that we can even completely relieve ourselves of the duty of measuring O-rings ourselves by using a special cone. This handy measuring element will allow us to quickly identify nearly 200 basic O-ring sizes. Simply place the seal on the cone, and it will stop at the place marked with the corresponding size. But this is not the end of the problem with broadly understood O-ring metrology. We will often have to measure old O-rings that have served as seals for a long time.
Do we proceed the same way as with new ones in such a case?
Quite the opposite. First, the inner diameter of worn O-rings must necessarily be measured on the sealed element. As for the thickness, two measurements must be made (each in a different place) and an average drawn. This also does not give us one hundred percent certainty that we have correctly measured a deformed O-ring. It is therefore necessary that when purchasing a new O-ring to replace it, you also add to your order O-rings with one size larger and one size smaller cross-section. Unfortunately, size matters significantly in this regard!
