How to Stop a Running Toilet
There are a few things to check for a proper diagnosis of a running toilet. Put an end to an unnecessary water bill today.
Check the Lift Chain
Jiggle the lift chain. If this stops the running, adjust the chain until it's properly in place. If the chain is wrapped around the arm, disconnect it from the current slot and try the next arm slot. If the float is disconnected, adjust the arm on the float until it rests where the water level is 0.5" below the top of the overflow pipe.
Use a Dye Tablet to Determine Leaks
If none of those worked, use a dye tablet to check for leaks. Flush the toilet, and as it's refilling, drop the tablet in the tank. If there is a leak, the bowl will start to turn the tablet color.
If the flapper is the culprit, it's an easy fix. Turn off the shut-off valve below the tank, and flush it to empty it. Remove the flapper and replace it with a new one. If the flapper valve seat is cracked, you'll need a whole new assembly, which you can get at a hardware store.
Check the Ball-Cock Assembly
If there's still a problem, you probably have an issue with the ball-cock assembly. Empty the tank, and replace the assembly with a new one.