How to Fix a Toilet That Keeps Running After Flushing

A toilet that keeps running is usually caused by a warped flapper, incorrect chain length, or faulty fill valve that needs simple adjustments or replacement.

1. Spot the Culprit Inside. Lift off the tank lid carefully and set it aside on a towel. Look inside to see if the rubber flapper at the bottom is sealing properly against the flush valve seat, if the chain connecting the flush handle to the flapper has the right tension, or if water is continuously flowing into the overflow tube.
2. Seal It or Swap It. If the flapper isn't sitting flat against the valve seat, try cleaning around the edges to remove mineral buildup. If it's warped or cracked, turn off the water supply valve behind the toilet, flush to empty the tank, then unhook the flapper from its hinges and replace it with an identical model from the hardware store.
3. Fine-Tune the Chain. The chain should have just enough slack to let the flapper close completely but not so much that it gets caught under the flapper. Adjust the chain length by moving the clip to a different link, aiming for about half an inch of slack when the flapper is closed.
4. Replace the Fill Valve. If water keeps running into the overflow tube, the fill valve needs replacement. Turn off water and flush to empty the tank. Disconnect the water supply line, unscrew the locknut under the tank, and pull out the old fill valve. Install the new valve, reconnect everything, and adjust the float or water level according to the manufacturer's instructions.
5. Verify the Fix Works. Turn the water back on and let the tank fill. Flush the toilet and watch the mechanism work. The flapper should close quickly and completely, the tank should fill to the proper level marked inside, and all running should stop within a minute or two of flushing.