Depending on whether the tank is cylindrical or rectangular, the volume of a water tank is found using the formulas (pi x radius^2 x height) or (length x width x height) respectively. For a cylindrical tank's volume, the measurement for the radius is equal to one-half the circular base's diameter, and pi is always equal to 3.14.
If the measurements are in feet, then the volume is given in cubic feet. For example, a rectangular tank that has a length of 6 feet, a width of 10 feet and a height of 3 feet has a volume of 6 x 10 x 3 = 180 cubic feet.
To convert cubic feet to gallons, multiply the tank volume by 7.47. This can be used for both rectangular and cylindrical tanks.