A toilet that hisses or refills itself every few minutes can waste hundreds of gallons a day. The good news: the culprit is almost always one of two cheap parts inside the tank, and replacing them takes basic hand tools and about half an hour.

Diagnose first Lift the tank lid and watch. There are two common failures:

  • The flapper leaks. Water seeps from the tank into the bowl, so the fill valve keeps topping it off. Test it by adding a few drops of food coloring to the tank and waiting 15 minutes without flushing. If color appears in the bowl, the flapper is your problem.
  • The fill valve runs. Water keeps trickling into the overflow tube because the valve never fully shuts off, often due to a stuck float or worn valve.

Replace a bad flapper Shut the supply valve under the tank and flush to empty it. Unhook the old flapper from the overflow tube and chain, take it to the store to match the size, and clip the new one on. Adjust the chain so it has a little slack but lifts fully when you flush.

Fix the fill valve If the flapper is fine, adjust the float so the water shuts off about an inch below the top of the overflow tube. If the valve still runs, replace the whole fill valve assembly. They are universal, inexpensive, and twist out after you disconnect the supply line.

Confirm the fix Turn the water back on, let the tank fill, and watch for a full minute. A correctly working toilet fills, shuts off completely, and stays silent until the next flush. If yours does that, you just saved a service call and a chunk off your water bill.