How do you fix a running toilet?

Two parts cause the large majority of cases. The flapper, the rubber seal at the bottom of the tank, degrades and stops sealing, so water leaks continuously into the bowl and the fill valve keeps topping it up. Test it by adding a few drops of food coloring to the tank and waiting twenty minutes without flushing; color appearing in the bowl confirms a flapper leak. The other culprit is the fill valve, which fails to shut off at the correct level and sends water down the overflow tube. If the water level sits at or above the top of that tube, the fill valve or its float adjustment is the problem.

Both parts cost little and are sold at any hardware store, and replacement requires shutting the supply valve behind the toilet, flushing to empty the tank, and swapping the component, typically 15 to 30 minutes with no specialized tools. Two adjacent checks: the chain connecting the handle to the flapper should have slight slack, since a taut chain holds the flapper open and a long one gets trapped under it, and both are frequent causes of intermittent running. A silent leak wastes a surprising amount, with EPA estimates putting a running toilet at up to 200 gallons a day, enough to be visible on a water bill. Call a plumber if the leak is at the base rather than inside the tank, which indicates a failed wax ring and a different job entirely.