O-Ring Sizes: A Quick Guide to Ensure a Perfect Fit
If installed correctly, the likelihood of an O-Ring malfunctioning in place is low. However, to
avoid some of the causes of O-Ring failure and possible failure modes you must select the
correct size for your application.
If you skip this step and omit to accurately size your O-Ring you could be in for failure and
Fortunately, it is relatively easy to size an O-Ring accurately providing you are familiar with O-
Ring sizing standards as well as how to measure an O-Ring yourself for exact sizing.
Read on to learn how to perfectly select O-Ring sizes.
O-Ring Sizing Standards
Because O-Rings are used in a multitude of engineering applications across the world, multiple
sizing standards exist.
● The USA AS568 O-Ring sizing standard
● The metric O-Ring sizing standard
● The British BS O-Ring sizing standard
● The Japanese JIS O-Ring sizing standard
● The Quad-Ring ® seals X-Ring sizing standard
● The Square Ring sizing standard
The US AS568 sizing standard is the most widespread in the US, however, it is not uncommon
for companies to sell O-Rings that conform to other sizing standards as well. A useful guide for
choosing an O-Ring size and determining the range of standard sizes available per AS568,
check out the Parker O-Ring Handbook ORD 5700.
Independent of the sizing standards are custom O-Rings which can be manufactured if you
required an O-Ring size that does not fit into any of the common standards.
If you need to work back and forth between these standards to locate a size that one standard
does not cover, use an inches-to-mm conversion tool, input your desired size and see if there
are O-Rings available from other standards that you can utilize.
Taking Tolerances into Consideration
When sizing O-Rings it is important that you take into account tolerances. Tolerances are small
variations in O-Ring size that occur due to manufacturing limitations.
For example, all of the AS568 100 Series O-Rings have a tolerance of +-.003". This can
influence the cross-section measurement of .103" and change its size to anything between
.100” and .106”
How to Measure an O-Ring
If you have an O-Ring that you would like to know the size of, you can do this by measuring the
diameter, the cross-section, and the inside diameter.
Start by laying the O-Ring onto a flat surface. Using a ruler or caliper, measure the distance
from one inner edge to the other. This is the inside diameter.
Using the ruler or caliper then measure the distance between one outer corner of the O-Ring to
the other. This is the diameter.
Next, use a caliper to measure the cross-section of the O-Ring. Lightly grip the O-Ring with the
caliper and record the measurement of thickness.
With these measurements, you will be able to determine the exact size of any O-Ring.
Use Your Knowledge of O-Ring Sizes to Ensure Perfect
Seals That Last
Achieving the perfect fit starts with using the perfect sized O-Ring for the job. Having a good
knowledge of O-Ring sizes ensures that you can be precise in O-Ring selections.