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A toilet that runs on and off by itself almost always has a leaking flapper — the rubber seal at the bottom of the tank letting water seep slowly into the bowl. As the tank level quietly falls, the fill valve kicks in for a few seconds to top it back up, which is the intermittent running you hear (often called a "phantom flush"). Confirm it with a 15-minute dye test, then replace the flapper, which costs only a few dollars. Less often the cause is a fill valve that will not shut off, or a water level set too high and spilling into the overflow tube.
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How do you confirm a flapper leak?
The dye test settles it in under half an hour and costs nothing. Take the tank lid off, drop about 10 drops of food coloring or a dye tablet into the tank water, and wait 15 to 30 minutes without flushing. Come back and look in the bowl: if the water there has taken on the color, tank water is leaking past the flapper into the bowl — that is your intermittent run. If the bowl stays clear, the flapper is sealing and the problem lies with the fill valve or an overfilled tank instead. This one test tells you which repair you need before you buy a part.
What are the usual causes, in order?
Intermittent running has a short list of culprits, and they are ranked here by how often they are to blame. Most are a cheap DIY fix:
| Cause | Sign | Fix |
|---|---|---|
| Worn or warped flapper | Dye test colors the bowl | Replace flapper (~$5 to $15) |
| Mineral-crusted flapper seat | New flapper still leaks | Clean or replace the seat/flush valve |
| Water level too high | Water trickles into overflow tube | Lower the fill-valve float |
| Failing fill valve | Runs even after flapper swap | Replace fill valve (~$15 to $25) |
| Chain too short/tangled | Flapper held slightly open | Adjust chain slack |
How do you replace the flapper?
It is one of the easiest plumbing repairs there is. Shut off the supply valve at the wall, flush to empty the tank, and unhook the old flapper from the two pegs on the overflow tube and from the flush chain. Take it to the store to match the size (2-inch and 3-inch flappers both exist and are not interchangeable), clip the new one on, reattach the chain with a little slack, turn the water back on, and let the tank refill. Then repeat the dye test to confirm the run is gone. No tools are needed for most flappers.
What if a new flapper still leaks?
If you replaced the flapper and the toilet still seeps, the flapper seat is the problem — the ring molded into the top of the flush valve that the flapper presses against. Hard water pits and crusts this ring so nothing seals against it. Feel it with a finger: a rough or crusty seat needs cleaning with a scouring pad and vinegar, and if the plastic is corroded or cracked, the fix is a flush valve replacement or a seat repair kit. This is the second most common cause after the flapper itself, especially in hard-water homes.
Why fix it quickly?
Because a runner wastes real money. The EPA estimates a leaking toilet can waste up to 200 gallons a day, which shows up on a metered water and sewer bill far in excess of the few dollars a flapper costs. The noise is the annoyance; the bill is the reason to act. For a fully worn-out toilet, see comfort height toilets before you replace, and if you are troubleshooting other bathroom water issues, how to fix low shower pressure and unclogging a toilet without a plunger cover the common ones. More in the bathroom hub.