When the liquid, or half-flush, selection is made, the dual flush toilet uses a little over 3/4 of a gallon of water to flush liquid waste away. Mainly this consists of the small amount of water already in the bowl rather than water released into the bowl.
When the solid, or full-flush, selection is made, the dual flush toilet uses about a gallon and a half of water to flush solid waste away. This includes the small amount of water in the bowl as well as some released in from the tank.
The water used to complete a solid waste flush is approximately double the amount of a liquid waste flush, but is still significantly less than older standard toilets. Many standard toilets use between three and five gallons of water every flush.
Water Carries Waste Away
A dual flush toilet uses the power of water, rather than the combined power of water and air like standard toilets, to wash waste away. This allows the dual flush to use less water.
In order to keep pressure up and allow the water to move fast enough to carry away the waste, the trapway, or hole at the bottom of the bowl, is larger than in a standard toilet.