Flat gasket vs O-ring —
what's the difference
and which one do you need?
A flat gasket and an O-ring seal in completely different ways, sit in different joint designs and usually fail for different reasons. Knowing which one belongs where saves you from buying the wrong part, wondering why it leaks and starting the job all over again.
Field rule: do not choose by shape from a parts box. Identify the joint geometry first: flat sealing faces mean flat gasket; a groove with controlled radial or axial compression means O-ring. The fitting design decides the seal.
The short answer
A flat gasket is a flat disc or ring that sits between two flat surfaces. In domestic plumbing the small rubber or fibre disc inside a hose connector is often called a washer — same sealing principle (compressed face-to-face), just a smaller, simpler version of what engineers call a flat gasket. You sandwich it between the faces, tighten the fitting, and the compression from both sides creates the seal. (Note: in mechanical engineering "washer" can also mean a metal load-spreading ring under a bolt head — that is a different part with a different job.)
An O-ring is a loop of rubber with a circular cross-section — like a very thin rubber band, but solid all the way through. It sits in a groove machined into the fitting or housing. When the joint comes together, the O-ring is compressed in its groove. Depending on the joint design, that compression may be radial or axial. That compression is the seal.
Neither is better or worse. They are designed for different joints. The joint design determines which one you need — not personal preference, not what is on the shelf.
If the joint has a designed gland, identify whether it is an axial or radial O-ring seal. If it uses an ungrooved flat seating land, it may use a flat gasket. The joint drawing or actual geometry decides.
This article focuses on the two seal geometries people confuse most often: a flat-face seal between two surfaces, and a round seal squeezed in a groove. Many people search for washer vs gasket, rubber washer vs O-ring, or flat seal vs O-ring — this guide clarifies those distinctions and explains where the small rubber or fibre "washer" inside a plumbing fitting actually fits in.
The flexible hose connecting your washing machine to the water supply has a flat rubber disc at each end — that is a flat gasket. The cartridge inside your kitchen mixer tap has small circular rubber rings sitting in grooves — those are O-rings. Same house, same water, different sealing method because the joint geometry is different.
How each one actually works
- Sits between two flat faces
- Seals by being squashed top and bottom
- Works in threaded flat-face fittings, flanges, unions
- Flat shape — like a thin washer or disc
- Usually fibre, rubber or graphite composite
- Can handle higher temperatures and pressures when the material and joint design are specified for that service
- Sits inside a machined groove
- Seals by being compressed in its groove (radially or axially)
- Works in push-fit fittings, valve cartridges, hydraulic connections
- Round cross-section — like a tiny rubber ring
- Usually EPDM or NBR rubber
- Groove must be the right size — too loose or too tight and it fails
Where you find each one — real world examples
Flat gaskets — you have seen these without knowing it
Washing machine hose connections. The rubber disc that sits inside the threaded connector at each end of the hose — that is a flat gasket. When you overtighten it, it crushes and cracks. When you forget to put one in, water goes everywhere the moment you turn the tap.
Under-sink flexible tap connectors. The braided hoses connecting your taps to the supply pipes have a flat rubber washer at each threaded end. Same principle — flat faces, flat seal between them.
Radiator valve connections and heating system unions. Many flat-face threaded connections in heating systems — such as pump unions, some valve unions and gauge fittings — use a flat gasket or washer between the mating faces. These are typically fibre composite material rather than plain rubber, because the temperatures are higher.
Flanged pipe joints. Where two sections of industrial pipe bolt together face-to-face with a ring of bolts, most standard flat-face or raised-face flange joints use a flat gasket between the flanges. The bigger the flange, the more important the gasket material choice — a high-pressure industrial gas line needs a different material to a domestic water pipe.
O-rings — also everywhere, just harder to see
Kitchen mixer tap cartridge. Take a worn-out tap cartridge out of a leaking mixer tap and you will find several small circular rubber rings sitting in grooves around it. Each one seals a different chamber inside the valve. When they harden and crack — usually from age or ozone exposure — the tap drips.
Garden hose quick connectors. The push-fit connector you click onto your garden tap has a small O-ring inside. That ring gets squeezed when the hose pushes in. Pull the hose out and look — it is right there at the front of the connector.
Hydraulic fittings and cylinders. Hydraulic systems run at very high pressure. Hydraulic systems often use O-rings in precision-machined grooves, especially in fittings, ports and static sealing positions. Dynamic cylinder seals may also use other seal profiles such as lip seals or U-cups. The groove dimensions are calculated to give exactly the right amount of squeeze on the O-ring cross-section.
Sprinkler heads and irrigation fittings. The push-in connections on irrigation systems use O-rings. So do hose reel connectors, outdoor tap adaptors and any fitting that clicks or pushes together rather than screwing together face-to-face.
The joint decides — not you
This is the most important point in the whole article. You do not choose between a flat gasket and an O-ring based on preference. The joint was designed for one or the other. Your job is to identify which one it needs and replace it with the correct type.
How to tell which one you need:
- Threaded connection that screws together face-to-face — flat gasket. The seal happens between the two flat faces as you tighten.
- Push-fit or sliding connection with a groove visible inside — O-ring. The groove holds the O-ring and the mating part squeezes it when inserted.
- Flanged joint with bolts around the outside — flat gasket. The bolts clamp the two faces together onto the gasket.
- Valve cartridges, tap cartridges and many hydraulic ports or fittings — often O-rings in grooves inside the assembly.
When in doubt — look for the groove. If there is a groove machined into the fitting, you need an O-ring. If the joint is two flat faces screwing or bolting together, you need a flat gasket. In most cases, the groove tells you what the joint was designed to use.
Quick reference — which seal for which situation
| Situation | Flat gasket | O-ring |
|---|---|---|
| BSP threaded flat-face fitting | ✓ | — |
| Push-fit pipe fitting | — | ✓ |
| Flanged industrial connection | ✓ | — |
| Tap or valve cartridge | — | ✓ |
| Washing machine hose end | ✓ | — |
| Garden hose quick connector | — | ✓ |
| Radiator valve connection | ✓ | — |
| Hydraulic port or static groove seal | — | ✓ |
| Pump housing seal | — | Typically |
| Sprinkler body or irrigation connector | — | ✓ |
Some assemblies vary by manufacturer — always check for a groove or a flat sealing face before replacing a seal.
What happens when you use the wrong one
Material matters too — but only after you know the type
Once you have established whether you need a flat gasket or an O-ring, the next question is material. Both types come in multiple materials for different temperatures, media and applications.
For flat gaskets: fibre and graphite-based materials dominate. In flat gasket applications, cellulose fibre grades are often used in water and heating duties, aramid fibre grades are assessed for higher-load or chemical duties depending on approval and medium, and graphite-based materials are used where temperature demands exceed fibre grades. In domestic plumbing, plain rubber and fibre washers are also common for standard low-pressure connections. The material depends on what flows through the pipe and how hot it gets.
For rubber seals and O-rings: EPDM for water, hot water and outdoor service. NBR for petroleum oils, greases and many mineral-oil hydraulic duties, with fuel compatibility checked against the exact fuel blend and compound data. The material depends on the medium — and getting it wrong causes the seal to swell, harden or crack over time, exactly as described in our EPDM vs NBR guide.
A plumber replaces an outdoor tap connector. The old flat gasket was black rubber — it has hardened and cracked after years of outdoor exposure. He replaces it with a new black rubber gasket from a mixed bag. It looks identical. Within a year, it has hardened again.
The problem: the failed gasket was likely a rubber compound with poor ozone and weathering resistance, such as general-purpose NBR or another unsuitable compound. The fix is an EPDM gasket — the same shape, the same size, different material. EPDM handles outdoor weathering. Generic rubber does not.
The quick-answer guide
Three checks tell you which seal the joint needs. One: is there a groove machined into the fitting, or two flat sealing faces? Two: does the joint screw or bolt together face-to-face, or push and slide? Three: what does the existing seal look like — flat disc or round ring? Three answers, one correct part. Anything else is guessing in front of an open joint.
Frequently asked questions
What is the difference between a flat gasket and an O-ring?
A flat gasket is a flat disc or ring that sits between two flat surfaces and seals by being compressed face-to-face. An O-ring is a circular ring with a round cross-section that sits in a groove and seals by being compressed in its groove. They work differently and fit different types of joints.
Can I use an O-ring instead of a flat gasket?
Usually no. Flat gaskets and O-rings are designed for different joint geometries. A flat gasket fits between two flat faces — flanges, unions, tap connectors. An O-ring fits in a machined groove. Substituting one for the other generally does not work unless the joint was specifically designed to accept both.
Which is better — a flat gasket or an O-ring?
Neither is universally better. The joint design decides which one is correct. Threaded flat-face plumbing fittings and flanged connections use flat gaskets. Push-fit fittings, valve cartridges and hydraulic connections use O-rings. The right seal is the one that fits the joint it was designed for.
The joint tells you which seal it needs. Your job is to read the joint correctly.
Flat gasket: two flat sealing faces, compressed face-to-face, usually in a flat-face threaded or bolted joint. O-ring: a groove, controlled compression in the groove, push-fit, sliding or cartridge-style assembly. Once you can see that distinction, you stop buying the wrong part and start fixing leaks properly the first time.