A rule for finding the contents, in gallons, of circular tanks is a - follows: First find the contents in cubic inches and multiply by .0036, or in cubic feet and multiply by 6.23. The cubic capacity of a circular tank in cubic inches equals the diameter in inches squared (that is, multiplied by itself) multiplied by .7851 and by the length in inches. For the capacity in cubic feet, take all dimensions in feet. As an example, the contents of a circular tank 1 ft. diameter by 5 ft. high equals 4 x 4 x .7851 x 5 x 6.23 = 391 gal. (roughly).