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?
Already replaced the flapper and it’s still running? Work through our 7-step post-replacement checklist before buying anything else.
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.
FAQ
How do I know if it’s my flapper or fill valve?
Water leaking into the bowl = flapper. Water running into the overflow tube or constant hissing = fill valve.
Which is cheaper to replace?
The flapper ($5–$12) is cheaper than a fill valve ($10–$20), and it’s the more common cause.
Can I replace both at once?
Yes — a full tank rebuild kit replaces the flapper, fill valve, and overflow together for the most reliable fix.
If the tank takes forever to refill, that points to the fill valve or supply — see a slow-filling toilet.
🔧 Plumber-Recommended Parts & Tools
Tackling this yourself? See a licensed plumber’s picks for the best fill valves and tank repair kits — the parts that get the job done right the first time.
Fix the 12 Most Common Plumbing Problems Yourself — Free Plumber’s Handbook — including the toilet fixes plumbers charge $150+ for
Step-by-step fixes for the 12 most common problems — and exactly when to call a pro. Free and instant, from a licensed plumber. Save hundreds on calls you didn’t need.