It's based on a monthly average of water usage, but we pay a separate bill for the water by the gallon. Not every gallon or tenth of a gallon we use enters the sewage system, but we have to pay at the same rate as people who don't give a fuck. Seriously, all of our water, except for our bathroom lavatories and toilets is recycled into our garden, even our washing machine water, which is really huge in most households. I can not even get anyone to look at the efforts I have made and reestimate our bill. It is very frustrating. I might as well not care, either.