how to fix a running toilet

Fix a running toilet by checking the flapper, adjusting the float, or replacing the fill valve.

Inspect and Replace the Flapper. The flapper is the most common culprit. If it is worn, warped, or dirty, it will not seal properly, causing water to leak into the bowl. Clean the flush valve seat or replace the flapper.
Adjust the Float and Water Level. If the water level is too high and spills into the overflow tube, adjust the float. For ball-and-arm floats, bend the arm downward; for modern float cups, use the adjustment screw or clip to lower the water level to about 1 inch below the overflow tube.
Replace the Fill Valve. If the flapper and float are fine but the toilet still runs, the fill valve may be faulty. Replace it by shutting off the water, draining the tank, and installing a new valve according to the manufacturer's instructions.
Check the Flapper Chain Tension. Ensure the chain connecting the flush handle to the flapper has about 1/2 inch of slack. If it is too tight, it prevents the flapper from sealing; if it is too loose, it can get caught under the flapper.