Why Won't My Toilet Stop Running? The Most Common Cause
25 May 2026 · Plumbers On Duty
A toilet that runs constantly is one of the most common household plumbing complaints, and one of the most wasteful. That endless trickling sound is water, and money, going straight down the drain. The good news is the cause is usually simple and often a DIY fix.
The usual culprit: the flapper
In most cases, a running toilet is caused by the flapper not sealing properly over the flush valve at the bottom of the cistern. If the flapper does not seal, water keeps leaking from the tank into the bowl, so the tank never fills enough to shut off the supply, and the toilet runs.
Sometimes the flapper is simply stuck and a quick jiggle of the handle solves it. If the flapper is worn or perished, it needs replacing, which is an inexpensive part.
Other common causes
- A misadjusted float. If the float sits too high, water rises above the overflow and runs continuously. Adjust to the correct level.
- A faulty fill valve. If the valve fails to shut off, the cistern keeps filling.
- A worn flush valve seat. If the surface the flapper seals against is rough or scaled, a new flapper alone may not seal.
How to fix it
- Lift the cistern lid and watch a flush cycle to see where water is escaping.
- Check the flapper for a clean seal; replace it if it is hard, warped or perished.
- Adjust the float so the water stops below the overflow pipe.
- If these simple checks do not solve it, call a plumber to inspect the fill valve and valve seat.
Frequently asked questions
Why does my toilet keep running after flushing? Most often the flapper is not sealing over the flush valve, so water leaks into the bowl and the tank never refills enough to shut off. Replacing a worn flapper usually fixes it.
How much water does a running toilet waste? A continuously running toilet can waste a large volume of water daily, which shows up directly on your water bill, so it is worth fixing promptly.
Can I fix a running toilet myself? Often yes. Checking and replacing the flapper or adjusting the float are simple DIY tasks. If those do not work, a plumber can replace the fill valve.
Sources
Need a vetted plumber?
Every plumber on our directory is verifiable. Filter by city, service and rating, then call the one you trust. Browse plumbers near you.