Older toilets use between 3.5 and 3.7 gallons of water per flush. Newer toilets use less water, with 1.6 gallons of water being the maximum legal amount they can use per flush. How much water the particular toilet uses for a full flush determines how much water is supposed to be inside the tank between flushes.
Find the Water Line
Remove the lid from the tank and look around the tank's interior walls for a line that is often labeled the "Water Line." Look for it either on the side or back interior wall. This line marks the height at which the water should settle when the tank stops filling up. If the water is below this level, one flush may not remove all of the bowl contents, and you end up wasting water by flushing multiple times.
Follow the Overflow Tube
If you are unable to locate a water line inside the tank, the tank may have never had one. Even on models where the tanks are manufactured with water line marks, some tanks slip through without one. If you can't find a water line mark, the correct height for the water is about 1 inch below the top of the hollow overflow tube located around the middle of the tank. The water level cannot be above this tube, or it will constantly overflow down into the bowl.
Adjusting the Water
Raise or lower the amount of water inside the tank by adjusting the fill valve. The fill valve is the vertical valve on the left-hand side inside the tank. It allows a certain amount of water to refill the tank after every flush. Follow the particular design of your fill valve to adjust the water level. The specific adjustment will vary, from turning an adjustment screw on some models to moving a clip on others. Flush the toilet after you adjust the fill valve and make sure the water cuts off when it reaches the desired height.
Alternative Adjustment Methods
If you choose to leave your fill valve on the same setting and not adjust it, there are products and methods you can use to change how much water is inside the tank. Owners of older toilets whose tanks hold and use more water per flush sometimes choose to do this instead of installing newer toilets. Some homeowners place a brick inside the tank to displace a certain amount of water. If you do this, put the brick in a plastic bag first or you risk pieces of the brick breaking off and clogging the toilet's rim holes. Another method involves filling a plastic bag with water and placing it in the tank. When you flush the toilet, this amount of water stays put.