White toilet tank with red-wrapped roll of toilet paper on top.

Toilet Tank

Originally contributed by • last updated 2/23/2021

Daria Shevtsova

When it comes to home appliances and fixtures, toilets are amongst the easiest to repair on your own without having to call an expert. After all, there are really only a few things that can go wrong and, unless it's a clogged drain or a leaky toilet seal gasket, most of them will be inside the toilet tank. Be careful not to drop the tank cover when removing it from the back of the toilet. It can be heavy!


The tank on a toilet, also called a flush tank or a cistern, is designed to hold a set amount of water that is used to refill the toilet bowl after it has been flushed. Toilet tanks are commonly found directly above and behind the bowl at the back of the toilet fixture. Although, on some models, the tank may be suspended higher above the toilet or completely hidden in the wall. Toilet tanks also contain several incorporated mechanisms, such as the flush valve and the fill valve, to release the water from the tank and to allow it to be refilled.

What’s Inside The Toilet Tank?

Typically, on most modern toilets, the toilet tank is accessible by lifting the cover off the top of the basin. However, on some models, the tank is hidden behind the wall and is accessible by removing a panel. Depending on the type of toilet, there are a few possible types of flushing systems contained within the tank:

  • Most commonly, located near the top of the tank or off to the side, there is a fill valve that is attached to either a side-float (also called a ballcock) or a concentric-float. Both the side-float and the concentric-float rise as water fills the tank to a predetermined level and close the fill valve, stopping water from entering the tank.
  • Additionally, attached to the backside of the toilet handle by a chain or lift wire, there is a rubber (sometimes plastic) flapper or ball flush valve mechanism connected to the bottom of the tank. The flush valve covers the outlet (hole) and is held in place against a flush valve seat by water pressure.