How to Fix a Toilet That Keeps Running

A running toilet is usually caused by a warped flapper, faulty chain, or incorrect water level. Most fixes involve adjusting or replacing simple parts inside the tank.

Open the Tank Carefully. Lift the ceramic lid straight up and set it aside on a towel or soft surface. The lid is heavy and can crack easily, so handle it carefully. Look inside the tank to see the flush mechanism.
Identify the Real Culprit. The rubber flapper at the bottom of the tank should sit flat against the flush valve seat. If water is running, the flapper likely isn't sealing properly. Press it down firmly with your hand. If the water stops running, you've found the problem.
Get the Chain Tension Right. The chain connecting the flush handle to the flapper should have slight slack when the flapper is closed. If it's too tight, it prevents the flapper from sealing. If it's too loose, the flapper won't lift when you flush. Adjust the chain length by moving it to a different hole on the flush arm.
Clear Away Buildup. Turn off the water supply valve behind the toilet and flush to empty the tank. Wipe away any mineral buildup or debris from the flapper and the rim it sits on. Even small particles can prevent a proper seal.
Swap in a New Flapper. If the flapper is warped, cracked, or won't seal after cleaning, unhook it from the flush valve ears and remove the chain. Take the old flapper to the hardware store to match the size. Install the new flapper by hooking it onto the ears and reattaching the chain.
Fine-Tune Water Level. The water level should sit about one inch below the rim of the tank. If it's too high, water flows into the overflow tube. Bend the float arm down to lower the water level, or adjust the screw on a float cup mechanism.
Verify Your Fix Works. Turn the water supply back on and let the tank fill. Flush the toilet and watch the flapper operation. It should lift completely when you flush, then drop down to seal tightly. The water should stop running within 30 seconds of flushing.