Question: how much water should be used to flush?
Answer: Federal law currently mandates that all toilets manufactured in the United States use no more than 1.6 gallons per flush, but WaterSense-labeled models only require 1.28 gallons or less per flush. Toilets made from the early 1980s to 1992 typically used 3.5 gallons per flush (13.2 liters) or more.