O-Ring Stretch and Compression Set Calculator
Stretch tells you how much an O-ring is pulled over its groove, and compression set tells you how much of its squeeze it gives back after sitting under load. This calculator handles both, and then combines them to show how much sealing squeeze is left.
Your numbers
AS568-210 is 0.734 in.
The diameter the O-ring seats on after installation.
ASTM D395 Method B uses a 12.5 mm thick plied button.
9.375 mm gives the standard 25 percent deflection on a 12.5 mm button.
Measured 30 minutes after removal from the fixture.
Your results
Stretch cross-section loss uses a constant-volume approximation. Compression set follows the ASTM D395 Method B equation; confirm test time, temperature and deflection against the specification you are quoting to.
Stretch and effective squeeze vs groove root diameter
Sweeps Groove root diameter (piston or plug groove) from 0.734 to 0.86, holding your other inputs fixed.
Link compound lot data to your ERP
An optional conversation about tying elastomer lot and test results to item and supplier records.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the free ID and cross-section from the AS568 dash size or drawing, then the groove root diameter and depth for a piston or plug seal. For the compression set part, enter the original button thickness, the spacer thickness used in the fixture, and the final thickness measured after recovery. Results show installed stretch, cross-section loss, effective squeeze, compression set percent and the squeeze remaining after that set.
- Stretch applies to piston and plug glands. Rod glands compress the seal, so check bore fit instead.
- Use measured final thickness values from the lab report, not nominal.
- Run the chart to see how stretch erodes effective squeeze.
Formulas and sources
Stretch is (groove root diameter minus free ID) divided by free ID. Stretching an O-ring thins its cross-section; assuming constant volume, the new section is CS divided by the square root of one plus the stretch fraction. Compression set under ASTM D395 Method B (constant deflection) is CB = (t0 - ti) / (t0 - tn) x 100, where t0 is original thickness, ti is final thickness and tn is the spacer height. A value of zero means full recovery; 100 means none.
Reading the result
Many suppliers keep installed stretch under about 5 percent. Beyond that, section loss lowers effective squeeze, the ring can roll or twist, and the compound ages faster. For compression set, a good elastomer after 22 hours at its rated temperature is often below about 25 to 30 percent, while 50 percent or more usually signals a compound that will leak after thermal cycling. Compare the remaining squeeze output against the minimum your application needs.
Common mistakes
Quoting compression set without the test time and temperature makes the number meaningless; always record both. Comparing sets measured at different deflections is another trap, since the formula normalises by deflection but material behaviour does not scale linearly. Finally, do not use stretch on a groove that has the seal fitted on a rod bore; there the seal is under compression and the formula changes sign.
Frequently Asked Questions
What is the formula for compression set?
ASTM D395 Method B defines compression set as (t0 - ti) / (t0 - tn) x 100, where t0 is the original thickness, ti the final thickness after recovery, and tn the spacer thickness during compression. Report it with the test time and temperature.
How much stretch is acceptable on an O-ring?
A common guideline is to stay at or below about 5 percent installed stretch, and many designers aim lower. Higher stretch reduces cross-section, which cuts effective squeeze and can shorten seal life. Follow your supplier recommendation for the specific compound.
Why does the cross-section get smaller when stretched?
Rubber is nearly incompressible, so when the ring circumference grows its cross-section area shrinks to keep volume constant. The diameter of the cross-section falls by roughly one over the square root of one plus the stretch fraction.
What compression set value is acceptable?
It depends on the compound and the application. Many sealing grades report 15 to 30 percent after 22 hours at rated temperature. For critical static seals people often specify a maximum, such as 25 percent, on the drawing or purchase specification.
If lab results for compound lots sit in a quality system apart from your ERP, we can show how a private assistant links lot, compound and test data so these checks are one question.
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