A running toilet is a nuisance. However, fixing it doesn't have to be. You'll save money on you water bill and on a plumber by fixing it yourself. Use this guide to find, and fix, the source of the problem.
Check the Guide Rod (or Chain)
If the rod is bent or the chain links are twisted, just straighten them.
Check the Float Mechanism
If lifting the ball up stops the water from running, try to bend the float arm down to get the right buoyancy. If you notice that the ball has water in it, it needs to be replaced. Replace it by unscrewing it from the arm and putting a new one in its place.
Check the Valve Seat and Stopper
If the toilet is still running, the valve seat and stopper may have corrosion or build-up preventing the stopper from closing. Lift the stopper up and check for any objects. Gently scour the seat and the rim. If there is a great deal of damage, replace the stopper and valve seat.
Check the Flush Valve Assembly
It may have to be replaced if the toilet is still running. Take the old parts with you when purchasing new inside gaskets and assemble to ensure a perfect match. If the shaft of the assembly is cracked, the whole shaft and assembly will need to be replaced. Again, take the flush valve assembly with you to get a perfect match.
Now that your toilet is no longer running, your water meter won't be either! You saved big by repairing this yourself.