# O-Ring Stretch and Compression Set Calculator

> Source: https://netray.co/tools/o-ring-stretch-and-compression-set-calculator
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Find installed stretch, the resulting cross-section reduction, compression set from a test button, and how much squeeze remains after set.

## Category

quality

## Industry

Fluid Power and Sealing

## Audience

- Quality engineer
- Design engineer
- Lab technician

## Tool

**Kind:** calculator

**Inputs:**

#### O-ring free inside diameter

**Key:** id_o

**Type:** number

**Default:** 0.734

**Min:** 0.05

**Max:** 40

**Step:** 0.001

**Unit:** in

**Help:** AS568-210 is 0.734 in.

#### O-ring cross-section

**Key:** cs

**Type:** number

**Default:** 0.139

**Min:** 0.02

**Max:** 0.7

**Step:** 0.001

**Unit:** in

#### Groove root diameter (piston or plug groove)

**Key:** groove_dia

**Type:** number

**Default:** 0.75

**Min:** 0.05

**Max:** 40

**Step:** 0.001

**Unit:** in

**Help:** The diameter the O-ring seats on after installation.

#### Groove depth

**Key:** depth

**Type:** number

**Default:** 0.105

**Min:** 0.01

**Max:** 0.6

**Step:** 0.001

**Unit:** in

#### Test button original thickness (t0)

**Key:** t0

**Type:** number

**Default:** 12.5

**Min:** 1

**Max:** 30

**Step:** 0.01

**Unit:** mm

**Help:** ASTM D395 Method B uses a 12.5 mm thick plied button.

#### Spacer thickness (tn)

**Key:** tn

**Type:** number

**Default:** 9.375

**Min:** 0.5

**Max:** 30

**Step:** 0.001

**Unit:** mm

**Help:** 9.375 mm gives the standard 25 percent deflection on a 12.5 mm button.

#### Final thickness after recovery (ti)

**Key:** ti

**Type:** number

**Default:** 11.6

**Min:** 0.5

**Max:** 30

**Step:** 0.01

**Unit:** mm

**Help:** Measured 30 minutes after removal from the fixture.

**Outputs:**

#### Installed stretch

**Key:** stretch

**Expr:** (groove_dia - id_o) / id_o * 100

**Format:** percent

**Primary:** true

**Description:** Stretch = (groove root diameter minus free ID) divided by free ID. Aim to stay at or below about 5 percent.

#### Cross-section after stretch

**Key:** cs_str

**Expr:** cs / sqrt(1 + stretch / 100)

**Format:** decimal

**Description:** Volume-conserving approximation, inches.

#### Cross-section reduction

**Key:** cs_loss

**Expr:** (1 - cs_str / cs) * 100

**Format:** percent

#### Effective squeeze after stretch

**Key:** sq_eff

**Expr:** (cs_str - depth) / cs_str * 100

**Format:** percent

**Description:** Squeeze using the thinned cross-section.

#### Test deflection

**Key:** defl

**Expr:** (t0 - tn) / t0 * 100

**Format:** percent

**Description:** Should read 25 percent for the standard D395 test.

#### Compression set (Method B)

**Key:** cset

**Expr:** (t0 - ti) / (t0 - tn) * 100

**Format:** percent

**Primary:** true

**Description:** CB = (t0 - ti) / (t0 - tn) x 100. Lower is better; 100 percent means no recovery.

#### Remaining squeeze after set

**Key:** sq_res

**Expr:** sq_eff * (1 - cset / 100)

**Format:** percent

**Description:** Effective squeeze scaled by the recovered fraction.

**Chart:**

#### Stretch and effective squeeze vs groove root diameter

**Type:** line

**X:** groove_dia

**From:** 0.734

**To:** 0.86

**Steps:** 30

**Series:**

- stretch
- sq_eff

**Disclaimer:** 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.

## Content

**Intro:** 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.

**Sections:**

#### How to use it

**Body:** 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.

**Bullets:**

- 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

**Body:** 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

**Body:** 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

**Body:** 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.

**Cta:** 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.

**Faq:**

#### What is the formula for compression set?

**Answer:** 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?

**Answer:** 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?

**Answer:** 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?

**Answer:** 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.

## Search terms

o-ring stretch calculator, o-ring compression set calculator, astm d395 method b calculation, o-ring stretch percentage, compression set formula, o-ring cross section reduction stretch, o-ring residual squeeze, o-ring seal life compression set

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