What Is an O-Ring? Complete Guide to Types, Materials & Applications (Silicone, EPDM, Viton, NBR)

An O-ring is an elastomeric sealing ring with a circular (O-shaped) cross-section that sits between two mechanical components. When compressed into a machined groove, it blocks the leakage of gas or liquid between the mating surfaces. The O-ring is the most widely used industrial seal in the world, because it's simple to manufacture, inexpensive, and performs well across most pressure and temperature ranges.

This article is part of our Complete Guide to Rubber Seals. If you're still unsure whether an O-ring, oil seal, or gasket is the right fit for your project, we recommend starting there.

Table of Contents

• What Is an O-Ring and How Does It Work?
• O-Ring Structure and Dimensional Standards
• O-Ring Material Types Compared
• Common Industrial Applications
• Advantages and Limitations
• How to Choose the Right O-Ring
• Frequently Asked Questions

What Is an O-Ring and How Does It Work?

An O-ring is molded from an elastomer (industrial rubber) with a perfectly circular cross-section in its relaxed state. During installation, the O-ring sits in a groove between two metal or plastic surfaces and is compressed by roughly 15–30%. That initial compression creates enough contact area to prevent leakage; as system pressure rises, the O-ring is pressed further into the groove and the seal tightens further.

O-rings work in both static sealing (between two fixed parts, such as a tank cover) and dynamic sealing with slow reciprocating or low-speed rotary motion (such as a hydraulic cylinder piston).

O-Ring Structure and Dimensional Standards

For an O-ring to seat correctly in its groove, its size must match a recognized standard. Two standards dominate globally:

• AS568 (US/inch standard): the most common standard in North America, numbered AS568-001 through AS568-906.
• ISO 3601 (international metric standard): more common in Europe and on much of the imported industrial equipment used elsewhere.

When ordering an O-ring, three dimensions must always be specified: inside diameter (ID), cross-section diameter (CS, or "cord"), and the allowable tolerance. An error in any of these — even a few tenths of a millimeter — can cause leakage or premature failure.

O-Ring Material Types Compared

Choosing the right material matters more than the shape itself, since every chemical, temperature, and fluid combination calls for a different compound. The table below compares the most common O-ring materials:

For a deeper look at chemical resistance against acids and petrochemical media, see our dedicated guide, Comprehensive Comparison of Chemical- and Acid-Resistant Rubbers.

Common Industrial Applications

• Sealing covers and fittings on pressurized tanks
• Hydraulic and pneumatic cylinders (piston and shaft sealing)
• Industrial valves (between body and bonnet)
• Pumps and compressors
• Automotive systems (engine, transmission, fuel system)
• Pipe fittings and small flanges

Advantages and Limitations

Advantages:

• Low manufacturing and replacement cost compared with other seal types
• Simple installation, no special tools required
• Reliable performance across a wide pressure and temperature range (depending on material)

Limitations:

• Not suitable for dynamic sealing at high rotational speeds (an oil seal is the better choice)
• At very high pressures, without a backup ring, an O-ring can extrude out of its groove
• Service life depends heavily on operating temperature and compression set (see our guide What Is Compression Set? for more detail)

How to Choose the Right O-Ring

1. 1Identify the fluid or gas in contact with the seal (water, oil, fuel, acid, or steam).
2. 2Consider the system's actual operating temperature range (minimum and maximum), not just the average.
3. 3Check the system's working pressure; above roughly 1,500 psi you'll typically need a backup ring.
4. 4Verify the chemical compatibility of the O-ring material against the fluid using a standard reference table (such as ASTM D471).
5. 5For food or pharmaceutical applications, always use an FDA-approved, hygienic-grade O-ring.

Frequently Asked Questions

Sources and References

This article draws on established industry standards — ASTM D2000 (elastomer classification) and ISO 3601 (O-ring dimensional standard) — along with general technical references published by international seal manufacturers, including the Parker O-Ring Handbook and Trelleborg Sealing Solutions.

Summary

Choosing the right O-ring means matching a standard shape with the right material for your temperature, pressure, and fluid conditions. If you're unsure which material or size is right for your project, our technical team is available for a free consultation and custom O-ring quote and manufacturing request.