How do I perform a toilet tank leak test?

If a toilet tank's internal parts need to be serviced or replaced, you may hear the tank running when it has not been used. 

You can perform the simple test below to determine what is causing the toilet to run.

Toilet Tank Leak Test: 

1. Start by turning off the water supply and letting the toilet sit overnight.
2. Check inside the tank the next day. 
3. If the tank is leaking, the water level would have dropped.  The level it dropped to will indicate which part will need to be serviced or replaced.
   1. If the water level is flush with the flapper, then replace the toilet's flapper.
   2. If the water level leaked down below the flapper, then the flush valve will need to be changed.
4. If the water level in the tank didn't change, then this indicates there is an issue will the fill valve.  Please see the solution for Flushing Out The Fill Valve.