Fix a Toilet That Keeps Running After Flushing

Water runs. The flush handle returns to rest. Then, ten seconds later, a hiss starts from the tank—sometimes loud, sometimes barely audible, but relentless. A running toilet wastes 200 gallons a day, turning a simple mechanical failure into a quiet drain on your water bill. The good news: toilet tank mechanisms are built around three simple parts, and all of them are designed to be replaced without cutting pipes or calling a plumber. Most running toilets fail at the flapper—a rubber seal at the bottom of the tank that lifts when you flush and reseats when the tank refills. Over time, mineral deposits and chlorine degrade the rubber, leaving it warped or stiff. The flapper fails to seal, water leaks into the bowl, and the fill valve keeps topping off the tank. Fix the flapper, and the running stops. If the flapper's fine, the problem moves to the float or fill valve, both straightforward adjustments.

1. Find the Leak Source. Remove the tank lid and set it somewhere safe. Watch the tank refill after a flush. If water trickles down the overflow tube—a vertical pipe in the center of the tank—the float is set too high. If water trickles around the flapper at the base of the tank, the flapper is worn or seated incorrectly. Listen for a faint hiss from the fill valve itself, which indicates a valve failure.
2. Drain and Isolate. Turn the water supply valve clockwise until it stops—it's located on the wall or floor behind the toilet, where the supply line connects. Flush the toilet to empty the tank. Use a sponge or towel to soak up any remaining water at the bottom. You need a dry workspace to inspect and replace parts.
3. Replace the Flapper. Unhook the flapper chain from the flush lever. Slide the flapper off the pegs on either side of the overflow tube—some flappers have a ring that slips over the tube itself. Bring the old flapper to the hardware store to match the size and mounting style. Install the new flapper by reversing the removal steps, ensuring it sits flat over the flush valve opening. Reattach the chain with about half an inch of slack.
4. Calibrate the Float. If water flows into the overflow tube, the float is telling the fill valve the tank is empty when it's actually full. For a ball float, bend the metal arm downward slightly to lower the water level. For a cylinder float that slides on the fill valve shaft, pinch the adjustment clip and slide the float down one inch. The water line should sit about an inch below the top of the overflow tube.
5. Swap the Fill Valve. Turn the water back on and let the tank refill. If the fill valve hisses continuously or won't shut off even with the float adjusted, the valve itself is failing. Replace it by unscrewing the lock nut under the tank, lifting out the old valve, and installing a new universal fill valve according to package instructions. Adjust the height so the valve's critical line sits one inch above the overflow tube.
6. Verify Full Flush Cycle. Flush the toilet and watch the entire cycle. The flapper should lift, the tank should empty, the flapper should drop and seal, and the fill valve should refill the tank and shut off cleanly. Listen for 60 seconds after the tank refills—any hissing or trickling means something's still out of adjustment. If the toilet runs intermittently, the flapper isn't sealing completely.
7. Tension the Chain Right. If the flapper sometimes sticks open, the chain may be catching under it. Adjust the chain length so it has just enough slack to let the flapper close but not so much that it tangles. If the flush handle feels loose or doesn't return to position, tighten the mounting nut inside the tank—it reverses normal threading and tightens counterclockwise.
8. Let It Settle In. Leave the tank lid off for a day and check periodically. Some flappers take a few hours to fully settle and seal. If the toilet starts running again, the flapper may not be compatible with your flush valve seat—try a different flapper style. If the problem persists after replacing both flapper and fill valve, the flush valve itself may be cracked and require a full tank rebuild.