A toilet that leaks from the tank into the bowl usually does so quietly. You may notice a faint trickle, a toilet that refills every so often on its own, or ripples in the bowl when nothing has been flushed. The good news is that the cause is most often a cheap, replaceable part inside the tank. This guide helps you work out which one.
Quick answer: the usual suspects
Water that moves from the tank into the bowl without a flush normally means one of three things:
- A worn flapper that no longer seals the opening at the bottom of the tank.
- A fill valve that does not shut off, or a water level set so high that water spills into the overflow tube.
- A worn or damaged flush valve seat, the surface the flapper presses against, or a worn seal on the flush valve.
Parts inside the tank rarely cause leaks onto the floor. For floor leaks, see our article on leaking supply lines and tank bolts.
Confirm the leak with a dye test
- Remove the tank lid and set it somewhere safe.
- Add a few drops of food coloring, or a dye tablet, to the tank water.
- Do not flush. Wait about 10 to 30 minutes.
- Look in the bowl. If color shows up, water is leaking from the tank into the bowl.
Flush afterward so the dye does not linger in the tank.
Work out which part is leaking
Check the tank water level and the timing of the leak.
- Water is at or above the top of the overflow tube, or you can see it flowing down the tube: the fill valve is not shutting off, or the level is set too high.
- Water is well below the overflow tube, yet dye still reaches the bowl: the flapper or flush valve seat is the likely cause.
- The tank refills every few minutes by itself: water is leaking out fast enough that the fill valve keeps topping it up. This points to the flapper or flush valve.
A simple check: turn off the water at the shut-off valve, then press the flapper gently down by hand. Turn the water back on. If the leak stops while you hold the flapper in place, the flapper (or its chain) is the likely culprit.
Fix 1: Replace the flapper
Flappers harden, warp and collect mineral deposits over time. Some in-tank cleaning tablets are reported to speed up that wear, so if you use them, consider whether they are worth it.
- Turn off the water at the shut-off valve and flush to empty the tank.
- Unhook the chain from the lever, and unclip the flapper from the pegs on the overflow tube or lift it off the ears.
- Take the old flapper to the store to match the size and style. Common sizes are 2 inch and 3 inch, but toilet makers vary, so confirm against your model.
- Clip the new flapper in place and attach the chain.
- Turn the water back on and repeat the dye test.
Before installing, wipe the flush valve seat with a cloth or a non-metal scrubbing pad to remove grit and film. A rough or dirty seat can stop even a new flapper from sealing.
Fix 2: Check the chain and handle alignment
The chain should have a small amount of slack, roughly half an inch, when the flapper is closed. A chain that is too short can hold the flapper slightly open. One that is too long may tangle under the flapper. The lever arm should also move freely and return fully.
Fix 3: Adjust or replace the fill valve
If water reaches the top of the overflow tube:
- Look for a water-level mark inside the tank, and check the manufacturer’s guidance if you have it. The water line is generally set below the top of the overflow tube.
- On a cup-style fill valve, turn the adjustment screw or slide the float clip to lower the level. On a ball-and-arm style, gently bend the arm down.
- If the valve still does not shut off, or you can hear hissing or dripping after the tank has filled, the valve is probably worn and should be replaced. Fill valves are inexpensive and usually sold as universal kits.
Replacing a fill valve means disconnecting the supply line at the bottom of the tank, so keep towels and a bucket handy. Once the new valve is installed, check the joints for drips. If a leak shows up there, see toilet supply line and tank bolt leaks.
Fix 4: Flush valve seat or flush valve replacement
If a new flapper does not stop the leak, the flush valve seat may be pitted, cracked or scaled. Light deposits can sometimes be removed by careful cleaning. If the seat is damaged, options include a flapper-compatible seat repair kit or replacing the whole flush valve. Replacing the flush valve usually means removing the tank from the bowl, and that is a bigger job, so this is where some homeowners choose to call a plumber.
Some toilets use a different design, such as a canister-style or siphon flush valve (more common in UK cisterns). In these designs the seal is a diaphragm or washer, and the same logic applies: if water flows into the bowl while the tank level is below the overflow, the flush valve seal is the likely culprit.
Why is the bowl losing water even though there is no leak?
If the water in the bowl drops rather than rises, the problem is a different one:
- A partial blockage in the trap or a vent problem can draw water out of the bowl by siphoning.
- A hairline crack in the bowl can lose water slowly.
- A very slow tank-to-bowl leak may be refilling the bowl so slowly that you do not see it.
If the bowl level keeps dropping, have it checked. A vent or drain issue can let sewer gases into the room, and that is a plumber’s job.
Why is the tank draining slowly?
If the tank level sinks slowly even without a flush, the flapper or flush valve is leaking, which is covered above. If the tank looks fine but refills slowly after a flush, check the fill valve for debris, a partly closed shut-off valve and a kinked supply line.
When to replace the toilet, and how long toilets last
Porcelain can last for decades, while internal parts typically wear out far sooner. Consider replacement if the tank or bowl has cracks, the glaze is badly crazed, or the toilet needs frequent repairs. A cracked tank or bowl will not stay watertight, so replace it rather than patching it.
When to call a plumber
- The leak persists after replacing the flapper and fill valve.
- You need to remove the tank to replace the flush valve and are not comfortable doing so.
- You see water around the base of the toilet or feel the floor is soft.
- Water or sewer smells are present.
Also see how long leaks can affect your water bill, and review how toilet parts fit together if you need a refresher on terminology.