A toilet that runs constantly — or refills on its own every few minutes — is almost always a worn flapper or a faulty fill valve inside the tank. It's a small part failing, but a big water bill: a running toilet can waste 200+ gallons a day. The parts cost $10–$25, and this is the most DIY-friendly plumbing fix there is.
Likely causes
Worn flapper — The rubber seal at the tank bottom warps with age and chlorine — water seeps into the bowl silently.
Faulty fill valve — The toilet runs on after flushing or the valve hisses — the shutoff mechanism is worn.
Chain or handle problem — A chain that's too long or kinked holds the flapper open.
Cracked overflow tube — Water spills into the overflow — less common, needs a new flush valve assembly.
What you can check yourself
Drop food coloring in the tank: if color appears in the bowl without flushing, the flapper leaks. Replace it — a 15-minute job, mostly no tools.
Check the water level: it should sit about an inch below the overflow tube; adjust the fill valve if water spills over.
Jiggle test first: if jiggling the handle stops the running, it's the chain or flapper alignment.
Typical cost
DIY parts run $10–$25. A plumber in Austin, TX typically charges $150–$350 for the repair visit; full toilet replacement runs $350–$700 installed.
When to call a pro
Water on the floor at the base, a toilet that rocks, or a sewer smell — that's a wax-ring or flange problem, not a flapper. Call before the subfloor rots.
Data by Servik — verified local pros from official license records and public sources. Cost figures are typical ranges, not quotes; a pro's written estimate is the real number.