People often use "washer" and "O-ring" interchangeably, but they're different parts that seal in different ways. Using the wrong one is a common cause of leaks.
The core difference
A flat rubber washer is a disc that seals against a flat face — under a bolt head, between two flat surfaces, or on a threaded fitting. An O-ring is a round-cross-section ring that seals inside a machined groove, usually radially. Washers seal axially (by clamping); O-rings seal by being squeezed in a gland.
When to use a flat washer
- Sealing under a bolt, screw or fitting on a flat surface
- Spacing, shimming or load distribution
- Hose, faucet and plumbing connections
When to use an O-ring
- Sealing in a designed groove between mating parts
- Rotating or reciprocating shafts
- Pressurized fluid or pneumatic systems
If your part sits flat against a surface, you want a washer — choose the compound in our material guide and the dimensions in the size guide, then shop by size.
If your part does need an O-ring, our sister store Rubber Feet Warehouse stocks nitrile (Buna-N) O-rings in standard AS568 sizes.