O-Ring 101: The 3 Things You Need to Know
Choosing an O-ring doesn't have to be complicated. Start with three things: size, material, and hardness.
1) Size
O-ring size is primarily determined by its inside diameter (ID) and cross-section (CS). Getting the dimensions right ensures the O-ring fits properly in its groove and creates the seal you need.
2) Material
The material determines how an O-ring handles temperature, pressure, fluids, chemicals, wear, and other environmental conditions. Common materials include Nitrile, FKM/Viton®, EPDM, Silicone, and Polyurethane.
3) Hardness
O-ring hardness is measured using the Shore A durometer scale. A softer compound may conform more easily to sealing surfaces, while a harder compound can provide greater resistance to extrusion and wear in certain applications.
So... Which O-Ring Do You Need?
The right O-ring depends on more than just its size. Think about what the seal will actually be asked to do. Will it be exposed to high temperatures? High pressure? Oil, water, chemicals, or another fluid? Will it be moving, rotating, or sitting stationary?
These details help determine the right combination of size + material + hardness for your application.
Quick O-Ring Selection Checklist
- What size is the O-ring? — Inside diameter and cross-section
- What is it sealing? — Fluid, gas, chemical, or other media
- How hot will it get? — Minimum and maximum operating temperature
- What pressure will it see? — System operating pressure
- Will it move? — Static, reciprocating, or rotary application
- What hardness is required? — Shore A durometer
Need Help Finding the Right O-Ring?
Don't know the material? Can't find the size? Have an old O-ring but aren't sure what you need to replace it? EPM can help.
Send us your dimensions, application details, drawing, or even a sample when available. Our team can help you identify the appropriate O-ring solution for your application.
Have an O-ring question? Contact EPM or call 1-800-659-5050.
Updated 9/28/2026
