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Practical Guide · Sealing How to Measure an O-Ring Accurately
Do you need to replace an O-ring but don't know its dimensions? To find the correct reference, two measurements are essential: the inside diameter and the cross-section diameter, also known as the thickness of the O-ring.
Using a caliper, you can accurately determine these dimensions. Follow our step-by-step measuring method, discover the common mistakes to avoid and learn how to select the right O-ring for your application.
Discover the Measuring MethodTable of Contents
Understanding the Dimensions Which Dimensions Should You Measure on an O-Ring?
An O-ring is a circular elastomer seal with a round cross-section. It is mainly defined by two dimensions: its inside diameter and its cross-section diameter.
The outside diameter is a third useful measurement, particularly for checking the overall dimensions and geometry of the O-ring.
ID
Inside Diameter
The distance between two opposite inner edges of the ring. This is the first reference dimension used to identify an O-ring.
CS
Cross-Section Diameter
The diameter of the round section of the O-ring, also called its thickness. It directly affects how the seal behaves under compression.
OD
Outside Diameter
The total distance from one outer edge to the opposite outer edge. It can be calculated using the inside diameter and cross-section diameter.
Example: a 20 × 4 mm O-ring has an inside diameter of 20 mm and a cross-section diameter of 4 mm. These dimensions do not indicate its outside diameter.
The Right Equipment Which Tool Should You Use to Measure an O-Ring?
The Caliper
To measure an O-ring accurately, we recommend using a caliper, either a vernier or digital model.
Its smaller jaws are used to measure the inside diameter. The larger jaws can be used to measure the thickness of the cross-section.
Before you start: check that the caliper is set to zero, gently clean the O-ring and avoid stretching or compressing it.
Step-by-Step Measuring Method How to Measure an O-Ring with a Caliper
To identify the correct O-ring dimensions, you need to take two measurements. The O-ring must remain in its natural, unstretched shape throughout the process.
Step 01
Measure the Inside Diameter
Insert the smaller jaws of the caliper into the opening of the O-ring. Gently open them until they touch the two opposite inner edges.
Read the measurement in millimetres: this is the inside diameter (ID).
Important: do not stretch the O-ring while taking the measurement, as this would artificially increase its inside diameter.
Step 02
Measure the Cross-Section Diameter
Place the round section of the O-ring between the larger jaws of the caliper. Gently close the jaws until they make light contact with the rubber.
The measurement corresponds to the cross-section diameter (CS), also referred to as the thickness of the O-ring.
Important: avoid compressing the cross-section. Repeat the measurement at several points to check that its thickness is consistent.
Once both measurements are complete, you have the main dimensions needed to find the corresponding O-ring reference in a size catalogue.
Calculating the Dimensions How to Calculate the Outside Diameter of an O-Ring
The outside diameter can be calculated using the inside diameter and the cross-section thickness. This simple formula also helps you check whether your measurements are consistent.
OD = ID + (2 × CS)
Outside diameter = inside diameter + twice the cross-section diameter
Example: 20 × 4 mm O-Ring
Inside diameter: 20 mm
Cross-section diameter: 4 mm
20 + (2 × 4) = 28 mm
The theoretical outside diameter of the O-ring is therefore 28 mm.
Special Case How to Measure a Worn or Deformed O-Ring
An O-ring that has already been used may no longer have its original dimensions. Prolonged compression, temperature, ageing or contact with certain fluids can change its shape and thickness.
What to Avoid Common Mistakes When Measuring an O-Ring
Even when using a caliper, certain mistakes can lead to choosing the wrong O-ring size. Here are three common errors to avoid.
Stretching the O-Ring
Stretching the O-ring artificially increases its inside diameter and gives an inaccurate measurement.
Compressing the Cross-Section
Excessive pressure from the caliper jaws reduces the measured thickness and can lead to selecting an incorrect size.
Ignoring Wear
An old O-ring may be swollen, compressed or deformed. Its measured dimensions should therefore be checked carefully.
Finding the Right Reference How to Choose the Right O-Ring Size
Once you have taken your measurements, compare them with the manufacturer's specifications. O-rings are available in a wide range of sizes, including dimensions defined by the ISO 3601 and AS568 standards.
To find the correct reference in our catalogue, three criteria are essential: inside diameter, cross-section diameter and material.
3 Criteria for Finding the Right O-Ring
01
Inside Diameter
The first dimension of the O-ring, corresponding to the diameter of its inner opening, expressed in millimetres.
02
Cross-Section Diameter
The second dimension of the O-ring, indicating the thickness of its circular cross-section, also expressed in millimetres.
03
Material
NBR, EPDM, FKM or another material available for the reference. It must be compatible with the fluid and operating conditions.
Which O-Ring Material Should You Choose?
NBR / nitrile rubber is suitable for many mineral oils, while EPDM is often chosen for water applications and resistance to outdoor ageing. FKM / Viton is used for applications involving certain higher temperatures and more demanding chemical environments.
Material compatibility must always be checked against the actual operating conditions of your application: fluid, temperature, pressure and environment.
Already Know Your O-Ring Dimensions?
Explore our range of standard Le Joint Français / Hutchinson O-rings and find the right reference for your needs according to its inside diameter, cross-section diameter and material.
Find Your O-RingPractical Advice How to Store O-Rings Properly
To prevent deformation before use, O-rings should be stored in a clean, suitable environment, protected from direct sunlight, excessive temperatures and mechanical stress.
In particular, avoid keeping O-rings stretched or compressed during storage. Appropriate storage conditions help preserve their original properties and dimensions.
Frequently Asked Questions FAQ: Measuring O-Rings
How Can You Measure an O-Ring Without a Caliper?
A ruler can provide an approximate measurement for a large O-ring, but it is generally less accurate when measuring the cross-section thickness. A caliper is recommended to identify the dimensions correctly.
How Do You Determine the Exact Size of an O-Ring?
Measure its inside diameter and cross-section diameter. For example, a 20 × 4 mm O-ring has an inside diameter of 20 mm and a cross-section of 4 mm. Then compare these measurements with the available standard sizes.
Should You Measure the Inside or Outside Diameter of an O-Ring?
O-rings are generally identified by their inside diameter and cross-section diameter. The outside diameter is an additional measurement calculated using the formula OD = ID + (2 × CS).
How Do You Measure a Worn O-Ring for Replacement?
Measure the O-ring at several points and check whether it has been stretched, flattened or deformed. If its dimensions have changed during use, look for the original reference or have the groove dimensions and installation conditions checked.
Which Criteria Should You Check When Choosing an O-Ring?
The three main criteria for selecting an O-ring from our catalogue are its inside diameter, cross-section diameter and material. The material must also be compatible with the fluid, temperature, pressure and sealing conditions of the application.
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Not sure about your O-ring dimensions or which material to choose?
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