A toilet that keeps running after flushing is one of the most common household plumbing issues — and it's usually caused by a faulty flapper or fill valve, both inexpensive and easy to replace.
How a toilet works (briefly): - When you flush, the flapper (a rubber seal at the bottom of the tank) lifts to release water into the bowl. - The flapper then drops back to seal the tank, and the fill valve refills the tank until a float shuts it off. - A running toilet means water is continuously escaping or refilling.
The most common causes:
1. A worn or misaligned flapper (the #1 cause): - Over time, the rubber flapper degrades, warps, or gets mineral buildup, so it doesn't seal properly. - Water leaks continuously from the tank into the bowl, and the fill valve keeps running to compensate. - Fix: Replace the flapper — a cheap part ($5–$15) and an easy DIY job.
2. The flapper chain is too long or too short: - If the chain is tangled, too long, or too short, the flapper can't seal properly. - Fix: Adjust the chain length so the flapper seats fully but isn't held up.
3. The float is set too high: - If the float is too high, water rises above the overflow tube and drains continuously, making the toilet run. - Fix: Lower the float so the water shuts off below the overflow tube.
4. A faulty fill valve: - If the fill valve doesn't shut off properly, the toilet keeps refilling. - Fix: Replace the fill valve ($10–$20).
5. The overflow tube is the wrong height or cracked.
How to diagnose it: - Add a few drops of food coloring to the tank water and wait 15–20 minutes without flushing. If color appears in the bowl, you have a leaking flapper. - Listen and look: if water is trickling into the overflow tube, the float is set too high.
Why fix it promptly: - A running toilet can waste hundreds of gallons of water per day, significantly raising your water bill. - The parts are cheap and most fixes are simple DIY jobs.
The takeaway: A running toilet is almost always a worn flapper, a chain/float adjustment, or a fill valve issue — all inexpensive, easy fixes. The food-coloring test quickly tells you if the flapper is leaking. Replacing these parts costs only a few dollars and stops the water waste.