How to Fix a Toilet That Keeps Running

Short answerMost running toilets are caused by a flapper that no longer seals, a float set too high so water spills into the overflow tube, or a worn fill valve. Lower the float or replace the flapper first — both are inexpensive and quick.

Find the cause in two minutes

Lift the tank lid and watch. If water is spilling into the open overflow tube, the fill level is too high. If the water level is fine but the toilet refills on its own every so often, the flapper is leaking into the bowl.

Dye test: a few drops of food coloring in the tank — color in the bowl after 15 minutes means a leaking flapper
Hissing that never stops: the fill valve isn't shutting off

Fixes in order of likelihood

Adjust the float down so water stops about an inch below the top of the overflow tube
Replace the flapper (match the size — most are 2 or 3 inch)
Replace the fill valve if it won't shut off after adjustment
Check that the chain isn't too tight or tangled under the flapper

When a running toilet needs a plumber

Call if parts don't stop it, if the tank or bowl is cracked, if water appears at the base of the toilet, or if the shutoff valve behind the toilet won't close or leaks when turned.

When to call a plumber about a running toilet

Call if the tank is cracked, the shutoff valve under the toilet won't close or leaks, or the toilet still runs after you've replaced the flapper and fill valve. A plumber can also rebuild or replace the tank-to-bowl gasket on toilets that leak at the base of the tank.