Daily water usage per person must be determined. The Loomis Tank Center estimates an average rate of 100 gallons per day per person in the United States. Multiply the number of people drawing from the tank by 100, and you have the total daily water usage. For example, if six people are living in the house, 100 times 6 equals 600 gallons per day of water usage.
For a tank that is used intermittently, such as when the water supply is low, a 1,000 gallon tank will accomplish the goal for a family of four. Bear in mind this is for occasional use only. This figure is presuming the water supply will replenish the tank on a daily basis. For continuous use, larger tanks are recommended.
For a continuous use tank, the University of Alaska, Fairbanks recommends a minimum size tank of 2,000 gallons for a family of four. The tank will have to be refilled once a month. However, note this figure is considered a minimum size. Since water usage may be greater, depending on the number of people and conservation practices, it would be wise to upsize this figure, depending on the number of people and the 100 gallon per day guideline.