1. Find the cause
Take the tank lid off and watch a full flush. If water keeps trickling into the overflow tube, the fill valve is set too high. If the water level drops and the valve refills on its own every so often, the flapper is not sealing.
2. Adjust the water level
Most modern fill valves have an adjustment screw or clip. Lower the float until the water stops about an inch below the top of the overflow tube, then flush and check again.
3. Replace the flapper
Turn off the supply valve, flush to empty the tank, and unhook the old flapper from the ears of the overflow tube. Fit the new one, reconnect the chain with a little slack, and turn the water back on.
4. Replace the fill valve if needed
If the valve hisses, refills slowly or will not shut off, replace it. Disconnect the supply line, unscrew the lock nut under the tank, fit the new valve at the right height and reconnect.
Parts for this job: Toilet Repair.
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