Quick answer from a licensed plumber: If water trickles from the tank into the bowl, it’s the flapper (the rubber seal at the bottom). If water keeps running into the overflow tube and never shuts off, it’s the fill valve (or the float set too high). The flapper is the more common culprit and the cheaper fix.
How to Tell Which One It Is
| Symptom | Likely cause |
|---|---|
| Water seeping into the bowl, “phantom flushes” | Flapper |
| Water running into the overflow tube | Fill valve / float too high |
| Hissing that never stops | Fill valve |
| Tank slow to refill | Fill valve |
Fixing the Flapper
Shut off the water, drain the tank, unclip the old flapper, and snap in a matching one. See the best toilet flappers.
Fixing the Fill Valve
If it’s the fill valve, first try lowering the float. If that doesn’t stop it, replace the valve — a 10-minute job. See the best toilet fill valves.
Still Running?
Check the chain slack and the float height. Our complete running-toilet guide covers every cause.
How to Test Which One It Is
Two quick checks settle it. First, the food-coloring test: add a few drops to the tank and wait 15 minutes without flushing — color seeping into the bowl means the flapper isn’t sealing. Second, just watch and listen: water trickling into the overflow tube (the open vertical pipe) or a fill valve that never fully shuts off points to the valve or a float set too high.
Why It Usually Fails
Flappers are rubber, and rubber hardens, warps, and loses its seal over a few years — faster in chlorinated or hard water. Fill valves fail from mineral grit fouling the seal or simple age. Both are wear items, so a toilet that’s run fine for years can start running with no other change.
The Fix
Either repair is about ten minutes and well under $20. If your toilet is older and you’ve already replaced one part, a full tank rebuild kit (flapper + fill valve + overflow) is the most reliable one-and-done fix. Our complete running-toilet guide covers every cause and the exact parts.