Toilet tank gasket kit: What is it and how to fix it?

Last updated: November 5, 2026

What is a toilet tank gasket kit?

A toilet tank gasket kit seals the connection between the separate tank and toilet bowl on a two-piece toilet. A typical kit contains a large tank-to-bowl gasket, mounting bolts, rubber washers, metal washers and nuts. Some packages contain only the gasket or hardware, so check what is included before taking the tank apart.

The large gasket surrounds the flush valve outlet underneath the tank. It keeps flushing water inside the passage to the bowl. The smaller rubber washers seal the tank bolt holes; the bolts hold the tank in position. These parts work together, but each seals a different opening.

Where is the toilet tank gasket kit located?

Look at the joint between the tank and the rear of the bowl. The large gasket sits around the outlet at this joint and is usually hidden until you lift the tank off. Use the toilet model number and parts diagram to identify its shape and mounting arrangement.

When you press the handle, the trip lever lifts the flapper through a chain on many gravity toilets. Other toilets use a tower or canister. The tank-to-bowl gasket is outside the tank, below the flush valve assembly, rather than the moving seal that opens for each flush.

A one-piece toilet has an integrated tank and bowl, so it does not have a removable tank-to-bowl joint like a two-piece toilet. It still has internal flush valve seals or mounting gaskets. For that construction, use the model-specific procedure for changing a one-piece toilet flush valve.

Which toilet seals are different?

Tank-to-bowl gasket: The large, often sponge-rubber ring between a separate tank and bowl. A damaged or poorly seated ring can let water escape at this joint during flushing.

Tank bolt washers: Rubber washers at the mounting holes. Deteriorated washers can drip while the tank is full, even without a flush.

Flush valve seal: The flapper or canister seal inside the tank stops water flowing into the bowl between flushes. The flush valve also has a separate mounting seal where its body fits through the tank. Neither should be confused with the external tank-to-bowl ring.

Wax floor seal: The wax ring, or a suitable wax-free alternative, seals the bowl outlet to the floor flange and drain. Replacing it requires lifting the bowl, not just the tank. Water on the floor alone does not identify which seal has failed.

How to choose a compatible toilet tank gasket kit

Match the tank model, flush valve outlet size, gasket profile and bolt arrangement. A nominal 2-inch flush valve and a 3-inch flush valve generally need different gaskets. Do not choose by the gasket’s outside diameter alone: the opening, thickness and shape must suit the flush valve nut and bowl recess. “Universal” still has model exclusions.

For example, the Korky standard 2-inch gasket has the dimensions below. These are dimensions for that gasket, not a sizing rule for every toilet:

Inner diameter: 2.125 inches
Outer diameter: 3.5 inches
Thickness: 1.25 inches

Some toilets require a thicker ring, a shaped gasket or a special mounting system. Korky’s large 3- and 4-inch gasket, for example, excludes Kohler toilets. Check the exact replacement instructions rather than assuming a larger ring will work. Also match two- or three-bolt hardware, bolt length and any original tank supports or spacers.

Why does my toilet tank gasket kit leak?

Age can leave a gasket compressed, split or hardened. A tank that shifts, uneven bolt tension, incorrect washer placement or the wrong gasket can also cause leaks. Before buying parts, dry the tank, connections and floor, then watch for the first wet spot while the tank fills, sits full and flushes.

A drip at a tank bolt points toward its washer or mounting hole. Water appearing at the central tank-to-bowl joint mainly during a flush suggests the large gasket. Inspect the flush valve mounting seal too, especially if the area leaks while the tank is standing full.

If water starts beside the water supply line, check its coupling and internal seal separately from the fill valve mounting washer. A leaking connector does not automatically mean the fill valve needs replacement. Our guide to the tank’s fill valve explains that separate assembly.

Water trickling into the bowl without an external drip usually calls for checking the flapper or canister seal first. Condensation and porcelain cracks can also resemble gasket leaks. If the source remains unclear, compare the symptoms in this guide to common toilet leak locations before dismantling the toilet.

How to fix the toilet tank gasket kit?

These steps cover a conventional two-piece gravity toilet. Follow its own instructions for proprietary brackets, concealed bolts or pressure-assisted equipment. You normally leave the bowl and floor seal in place. Have towels, a bucket, a sponge, a screwdriver and a suitable wrench ready.

Step 1: Shut off the water

Lift off the lid and put it somewhere stable. Close a conventional multi-turn shutoff valve gently clockwise, or turn a quarter-turn valve to its closed position. Do not force a seized valve. Confirm that water stops entering the tank.

Step 2: Empty the tank

Flush and hold the handle down to drain as much water as possible. Sponge out the remainder. Place a bucket under the supply connection to catch trapped water when you disconnect it.

Step 3: Disconnect the supply and remove the tank

Loosen the supply coupling at the tank without twisting the fill valve shank. A normal right-hand coupling loosens counterclockwise when viewed squarely at its end; avoid relying on a direction described from above. You usually do not need to detach the chain just to lift the complete tank.

Support the tank while removing its mounting nuts and bolts. Lift it straight off with help if needed, then rest it on a padded, stable surface. Do not pull against a stuck bolt or use the porcelain as a lever.

Step 4: Replace the gasket and washers

Remove the old ring and clean residue from the mating surfaces without scratching them. Inspect the porcelain, valve outlet and hardware. Replace damaged bolts and washers, and seat the correct gasket in the position shown by its instructions. Do not stack gaskets to compensate for a poor fit.

For conventional through-tank bolts, a rubber washer belongs beneath the bolt head inside the tank. Do not substitute a metal washer against the porcelain there. The washers and nuts below the tank vary by kit, so follow its diagram and preserve any required supports.

Step 5: Reassemble and test

Lower the tank squarely onto the bowl. Start the nuts by hand, then tighten in small alternating increments so the tank stays level. Stop at the manufacturer’s specified seating position; never keep tightening to close a gap or force porcelain surfaces together. Excess force can crack the tank or bowl.

Reconnect the supply using its specified tightening method. Do not add tape or sealant to a gasket-sealed coupling unless its instructions require it. Turn the water on slowly and inspect the supply connection, bolt holes and tank joint. Flush several times, dry the area and check again after the tank has stood full.

If the toilet tank gasket kit still leaks, shut the water off and recheck the fit, seating and washer order. More bolt tension will not fix the wrong gasket or cracked porcelain. A sound repair leaves the tank stable and all connections dry during filling, flushing and standing.