Stop a Running Toilet

Water runs constantly in the tank, a soft hiss that becomes background noise until you notice the water bill. A running toilet wastes 200 gallons a day, roughly $70 a month down the drain. The mechanism inside a toilet tank is simpler than it looks—three main parts doing one job each—and most failures announce themselves clearly once you lift the lid and watch a flush cycle. The fix is usually obvious, the parts are cheap, and the work requires no special tools. The system works like this: when you flush, the flapper lifts and water rushes into the bowl. The float drops, opening the fill valve to refill the tank. When the float rises back to the set level, the fill valve shuts off. A running toilet means something in that sequence isn't stopping when it should. You find the failure point by watching one flush cycle with the lid off. Everything that goes wrong reveals itself in motion.

1. Watch the Leak Reveal Itself. Lift the tank lid and set it somewhere safe—porcelain breaks easily. Flush once and watch what happens. The flapper should drop and seal completely. The water should refill to about an inch below the overflow tube. The fill valve should shut off with a definite click. If water keeps running into the overflow tube, the fill valve isn't stopping. If the flapper doesn't seal, you'll see ripples in the bowl.
2. Replace the Worn Seal. The flapper is the rubber seal at the bottom of the tank. Feel it—if it's slimy, pitted, or warped, it's done. Unhook it from the posts on either side of the flush valve and take it to the hardware store for a match. Universal flappers fit most toilets, but exact replacements seal better. Snap the new one in place and reconnect the chain with about half an inch of slack when the flapper is closed.
3. Get the Slack Right. Too much slack and the flapper closes before the tank empties, causing weak flushes and constant topping off. Too tight and the flapper can't seal. The chain should have just enough slack to let the flapper rest flat but lift immediately when you press the handle. Move the chain clip up or down on the lift arm until you get it right. Cut off excess chain so it doesn't catch under the flapper.
4. Lower the Water Line. If water runs into the overflow tube, the fill valve is set too high. Look for the adjustment mechanism—usually a screw on top or a clip on the side. Turn the screw counterclockwise or pinch the clip and slide the float down. The water line should stop about an inch below the top of the overflow tube. Flush and verify the fill valve shuts off cleanly.
5. Clear the Jammed Valve. Sediment can jam the fill valve open. Turn off the water at the shut-off valve behind the toilet and flush to empty the tank. Unscrew the cap on top of the fill valve and lift out the float assembly. You'll see a small rubber seal or diaphragm—rinse it under the sink and wipe out any grit from the valve body. Reassemble, turn the water back on, and test.
6. Swap Out the Bad Valve. Fill valves cost $12 to $20 and install in fifteen minutes. Turn off the water and flush. Disconnect the supply line under the tank with an adjustable wrench. Unscrew the locknut holding the old valve and lift it out. Drop in the new valve, tighten the locknut hand-tight plus a quarter turn, and reconnect the supply line. Adjust the height, turn the water on, and set the fill level.
7. Confirm the Fix Holds. Flush three times in a row and watch the entire cycle each time. The flapper should seal immediately, the fill should stop at the right level, and there should be no hissing after the tank refills. Check again in an hour. If everything stays quiet, the job is done. If the problem returns, you missed something—go back to step one.