Find your floating roof storage tank's circumference in inches. Take a tape measure and wrap it around the entire tank and record the measurement. Divide this number by 3.14 or "Pi." Take the number you get and divide it by 2 to find the radius of the tank.
Find the height at which your floating roof is sitting at, in inches. Measure from the floating roof to the bottom of the storage tank.
Multiply the radius by itself. This is also called "squaring" the radius.
Multiply this number by the height of your floating roof. Multiply this number by 3.14 to find the volume of your floating roof liquid storage system.