How large should a septic tank be?

It should be large enough to take care of all the occupants that CAN live in a house (note CAN, as opposed to DO ). If a house can sleep 6 people then the tank must be sized to that number, even if only 2 people live in that house now. Different areas have different septic regulations, but in general, a tank must start at 400 gallon size (that's 4 x 4 x 4 feet) and increase by 100 gallons per person.