Short definition
A tank gasket (or tank-to-bowl gasket) is the large rubber doughnut between the bottom of a two-piece toilet’s tank (around the flush-valve threads) and the top of the bowl (around the inlet to the trapway). Tank bolts compress the gasket; together they seal the tank-bowl interface.
What it is
Standard 2-inch flush-valve gasket: about 3-1/2 inches outer diameter. 3-inch flush-valve gasket (modern HET toilets): about 4 inches outer diameter. Match the size to the flush-valve seat — wrong-sized gaskets won’t seal.
The gasket is sometimes called a “spud washer” by manufacturers — terminology overlaps and varies. Different from a wax ring entirely, despite the similar role of “rubber-ish thing that seals the toilet.”
Why it matters to a homeowner
A hardened or unevenly compressed tank gasket is a common cause of slow leaks at the tank-bowl interface — water dripping under the tank, running down the bowl exterior, and onto the floor. $5 to $12 for the gasket, $10 to $20 for a tank-to-bowl kit including bolts.
Whenever you replace a flush valve, replace the gasket while the tank is off — fresh rubber compresses better and you save yourself a repeat job.
Common failure modes
- Hardened rubber from age. Slow drip between tank and bowl.
- Compressed unevenly (one tank bolt over-tightened). Slow drip.
- Wrong size. 2-inch gasket on a 3-inch flush valve doesn’t seal.
Common variants
- Tank gasket / tank-to-bowl gasket (this entry). Between tank and bowl on two-piece toilets.
- Spud washer. Sometimes synonymous; sometimes refers to a smaller washer between the flush valve and the tank wall. Check the kit instructions.
- Wax ring. Different gasket entirely — between the bowl horn and the closet flange in the floor.