Two tests that find the cause
Take the tank lid off and set it somewhere safe; it's heavy and breaks easily.
Look at the overflow tube
If water is flowing into the open top of the tall tube in the middle, the water level is too high. Adjust the fill valve's float down until the water stops about an inch below the top of the tube.
The dye test
Put a few drops of food coloring in the tank and don't flush for 15 minutes. Color in the bowl means the flapper or flush valve seal is leaking.
Check the chain
It should have a little slack with the flapper closed. Too tight and the flapper can't seat; too long and it can catch under the flapper.
What's wrong and the fix
| Cause | How you know | Fix |
|---|---|---|
| Worn or warped flapperMost common | Dye test shows color in the bowl; the toilet refills briefly every so often (a "phantom flush"). | Replace the flapper with one matched to the flush valve size, usually 2 or 3 inches. |
| Fill valve set too high or failingMost common | Water trickles into the overflow tube, or the valve hisses and never fully shuts off. | Adjust the float; replace the fill valve if adjusting doesn't stop it. |
| Chain or handle problemMost common | Running starts after a flush and stops when you jiggle the handle. | Adjust chain length or replace the handle assembly. |
| Cracked overflow tube or worn flush valve seatLess common | New flapper doesn't fix the dye test; visible crack. | Replace the flush valve, which means removing the tank. |
Why it's worth fixing this week
A toilet that runs continuously can waste more water in a day than a dripping faucet wastes in a month, and the cost shows up on the next water bill, sometimes as the only sign. A silent flapper leak is easy to miss because the sound stops between refills.
If your bill jumped and you can't find why, test every toilet first. See why is my water bill so high.
When to call a plumber
- The shutoff valve behind the toilet won't turn or leaks when you turn it.
- Parts are replaced and it still runs.
- The tank or bowl is cracked, or the toilet is leaking at the base. That's a toilet repair or replacement job.
More questions
Can a running toilet raise my water bill a lot?
Yes. A steady run into the overflow tube can use hundreds of gallons a day. It's one of the most common causes of an unexplained high bill.
Do I need to turn off the water to replace a flapper?
Yes. Close the valve behind the toilet, flush to empty the tank, then unhook the old flapper from the pegs on the overflow tube.