O-Ring Durometer Guide: Understanding Shore Hardness and Choosing the Right Seal

When selecting an O-ring for an industrial application, choosing the correct material is only part of the equation. The hardness of the seal—known as its durometer—can significantly impact sealing performance, pressure resistance, service life, and overall system reliability.

A seal that is too soft may extrude under pressure, while one that is too hard may fail to conform to sealing surfaces and allow leaks. Understanding durometer ratings helps engineers and maintenance professionals select O-rings that perform reliably in their operating environment.

In this guide, we'll explain how Shore hardness works, why durometer matters, and how to choose the right O-ring hardness for your application.

What Is O-Ring Durometer?

Durometer is a measurement of an elastomer's hardness, or resistance to indentation. For O-rings and most rubber sealing materials, hardness is measured using the Shore A scale, which ranges from 0 to 100.

The higher the Shore A number, the harder the material:

Shore A Rating	Relative Hardness
40-50	Very Soft
60	Soft
70	Standard
80	Hard
90+	Very Hard

 

Most industrial O-rings are manufactured in a 70 Shore A compound because it provides a balance of flexibility, sealing capability, and durability across a wide range of applications.

However, there is no universal hardness that works for every operating condition. The correct durometer depends on pressure, temperature, movement, media compatibility, and equipment design.

How to Choose the Right O-Ring Durometer

Selecting the proper O-ring hardness starts with understanding the conditions your seal will experience during operation.

Four primary factors influence durometer selection:

1. Operating Pressure

Pressure is often the most important factor when selecting seal hardness.

Higher-pressure systems place greater stress on elastomer seals. If the material is too soft, it can deform and extrude into clearance gaps, leading to premature failure.

As pressure increases, harder compounds are generally recommended.

2. Temperature Range

Extreme temperatures affect the physical properties of elastomers.

At elevated temperatures, rubber compounds may soften and lose mechanical strength. At low temperatures, they can become less flexible and more brittle.

When evaluating durometer, always consider the application's operating temperature range rather than relying solely on room-temperature hardness values.

For applications operating in extremely cold environments, selecting the proper material is equally important. Learn more in our guide to Sealing Solutions for Extreme Cold.

3. Dynamic vs. Static Applications

The amount of movement within the sealing system can influence the ideal hardness.

Static seals experience little or no movement after installation and often allow for softer compounds.

Dynamic applications such as hydraulic cylinders, pumps, and rotating shafts typically benefit from harder compounds that offer improved wear resistance and dimensional stability.

4. Surface Finish and Gland Design

Softer O-rings generally conform more easily to irregular sealing surfaces, while harder compounds require more precise gland design and surface finishes to achieve effective sealing.

Evaluating hardware conditions alongside operating requirements helps ensure optimal performance.

O-Ring Hardness Chart: Common Durometer Ratings and Applications

While every application is unique, the following chart provides general guidelines for selecting O-ring hardness:

Durometer	Typical Applications
40A-50A	Medical devices, food processing equipment, low-pressure systems
60A	Pneumatic equipment, light industrial applications
70A	General-purpose industrial sealing, OEM equipment
80A	Hydraulic systems, pumps, compressors
90A	High-pressure hydraulic, oil & gas, aerospace applications

When pressures become particularly high, backup rings may also be required to prevent extrusion and extend seal life.

Why Durometer Matters in Seal Performance

The hardness of an O-ring affects much more than how the material feels by hand.

Proper durometer selection influences:

• Compression characteristics
• Extrusion resistance
• Wear resistance
• Installation force
• Pressure capability
• Sealing effectiveness
• Long-term durability

An O-ring that is properly matched to the application will provide more reliable sealing performance and a longer service life than one selected solely based on material type.

Understanding Low, Medium, and High Durometer Seals

Low Durometer O-Rings (40A-60A)

Softer elastomers offer increased flexibility and conformability.

Advantages include:

• Improved sealing on rough surfaces
• Lower installation force
• Better sealing performance in low-pressure environments
• Greater flexibility

Common applications include:

• Food and beverage equipment
• Medical devices
• Electronics enclosures
• Low-pressure pneumatic systems

The tradeoff is reduced resistance to extrusion and mechanical wear.

Medium Durometer O-Rings (70A)

Seventy durometer compounds are the most commonly specified hardness across industrial applications.

Benefits include:

• Balanced flexibility and durability
• Good compression characteristics
• Broad application compatibility
• Moderate pressure resistance

Many standard O-rings are manufactured in 70A compounds because they perform well across a wide range of operating conditions.

High Durometer O-Rings (80A-90A)

Harder compounds are designed for demanding environments.

Benefits include:

• Improved extrusion resistance
• Greater wear resistance
• Better dimensional stability
• Enhanced high-pressure performance

These compounds are frequently used in:

• Hydraulic systems
• Oil and gas equipment
• Aerospace applications
• Heavy industrial machinery

How Material and Durometer Work Together

Material selection and durometer selection should never be treated as separate decisions.

For example, a 70A Buna-N O-ring and a 70A Viton® O-ring may have similar hardness ratings, but their chemical compatibility and temperature resistance are very different.

Common O-ring materials include:

Buna-N (Nitrile)

Buna-N offers excellent resistance to petroleum-based fluids and is one of the most widely used elastomers in industrial sealing applications.

Viton® (FKM)

Viton® provides outstanding resistance to fuels, chemicals, and elevated temperatures, making it a popular choice for demanding environments.

EPDM

EPDM performs exceptionally well in water, steam, and outdoor applications where weather resistance is important.

Silicone

Silicone O-rings offer excellent flexibility and broad temperature resistance, making them common in medical, food processing, and electronic applications.

Choosing the right combination of material and hardness helps ensure long-term sealing performance.

Common Durometer Selection Mistakes

Even experienced engineers can overlook factors that impact seal performance.

Some of the most common mistakes include:

Selecting the Softest Compound Available

While softer compounds may initially create a tighter seal, they are often more susceptible to extrusion and wear.

Ignoring Pressure Requirements

System pressure frequently determines hardness requirements more than any other operating factor.

Overlooking Dynamic Motion

Dynamic applications typically require greater wear resistance than static sealing applications.

Focusing Only on Material Compatibility

Chemical compatibility is important, but selecting the wrong hardness can still result in leakage or premature failure.

Not Accounting for Long-Term Aging

Temperature, media exposure, and environmental conditions can alter hardness over time and impact sealing effectiveness.

When to Consider a Custom Seal Solution

Standard O-rings work well in many applications, but some operating environments require customized sealing solutions.

Custom compounds and hardness formulations may be beneficial when:

• Extreme temperatures are present
• High pressures exceed standard capabilities
• Specialized chemical compatibility is required
• Industry-specific compliance standards apply
• Equipment designs have unique gland requirements

Rocket Seals works with OEMs, engineers, and maintenance teams to develop custom sealing solutions tailored to demanding operating environments.

Learn more about our Custom Seals capabilities.

Choosing the Right Seal for Long-Term Performance

Selecting the right O-ring durometer isn't simply about choosing the hardest or softest material available. The ideal hardness depends on operating pressure, temperature, movement, chemical exposure, and equipment design.

By understanding Shore hardness ratings and evaluating the unique demands of your application, you can select a sealing solution that delivers improved reliability, longer service life, and reduced maintenance costs.

Whether you're specifying a standard O-ring or a custom-engineered sealing solution, Rocket Seals can help identify the optimal material and durometer combination for your application.

Need help selecting the right seal? Contact the sealing experts at Rocket Seals today.