Volume Formulas
Rectangular: V = L x W x H. Vertical Cylinder: V = pi x (D/2)^2 x H. Horizontal Cylinder: V = pi x (D/2)^2 x L. All values converted to cubic feet, then x 7.48052 for gallons or x 28.3168 for liters. Water weight = gallons x 8.34 lbs (or liters x 1 kg).
Frequently Asked Questions
Volume of a cylinder = pi x radius^2 x height. To convert cubic feet to gallons, multiply by 7.48052. For example, a 4-foot diameter (2-foot radius) x 6-foot tall tank: pi x 4 x 6 = 75.4 cubic feet x 7.48 = 564 gallons.
Volume = length x width x height (in feet), then multiply by 7.48 for gallons. A 4 x 4 x 3 foot tank: 48 cubic feet x 7.48 = 359 gallons. For partial fill, multiply by the fill fraction.
Water weighs approximately 8.34 pounds per gallon at 60°F. A 500-gallon tank full of water weighs 500 x 8.34 = 4,170 pounds plus the weight of the tank itself. Always verify your support structure can handle this load.
A common sizing rule is 1 gallon of storage per square foot of collection area per inch of annual rainfall you want to capture. A 1,000 sq ft roof in a location with 20 inches of annual rainfall could theoretically capture 20,000 gallons. Practical systems capture a fraction of this; a 2,500-5,000 gallon tank is typical for residential irrigation use.
Fill rate depends on your water source pressure and pipe size. At 60 PSI through a 1-inch line, you might achieve 10-15 GPM. To fill a 500-gallon tank at 10 GPM takes 50 minutes. Most residential water service connects at 40-80 PSI.
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