In order to calculate the gallons in a round tank, the radius and the height of the tank's interior should be measured in feet. The volume is equal to pi times the radius squared times the height times 7.48.
Simply multiplying pi, the radius squared and the height results in the volume in cubic feet. There are 7.48 gallons in a cubic foot, so multiplying the volume in cubic feet by 7.48 results in the volume in gallons.
If the round tank is spherical rather than cylindrical, the radius of the interior of the tank should be determined in feet. The volume in gallons is the product that results from multiplying the radius cubed, pi, 4/3 and 7.48.