The following information on cisterns comes from Five Acres and Independence by M. G. Kains. Five Acres and Independence is also available to purchase in print.
Capacities of Round and Rectangular Cisterns
An easy way to determine the cistern capacity needed during any period is to multiply the square-foot area of roof by inches of rainfall and divide by 1.6. For instance, a 30′ x 40′ roof and 1 1/2″ rainfall. 1200×1.5÷1.6=1,125 gallons. A family of 5 which uses 5 gallons daily per person will require 9,125 gallons annually, or 4,562 gallons each half year. If a minimum of 1,125 gallons is collected during the half year of low storage, 3,437 gallons will have to be in storage when this period starts; i.e., the cistern would have to be that large or larger. If, however, it is desired to have a 6- months’ reserve, the cistern should hold 4,562 gallons. In this latter case the year’s supply would be collected and stored during half of the year and used during the other.
When the rainfall during a year is only 28″, it would be possible to collect 12″ of water during 6 months of storage period, or 9,000 gallons, as found by the formula above. A circular cistern 8′ deep and 10′ in diameter with a capacity of 4,700 gallons or a rectangular one 8′ square and 10′ deep (Table 14) with a capacity of 4,888 gallons (Table 15) should be ample when the water is collected.
Capacity of Plain Cylindrical Cisterns and Tanks