Place a measuring tape on the top center of the liquid tank and measure the diameter. Write this measurement down.
Divide the diameter by two to calculate the radius. For example, if the tank is 4 feet in diameter, divide 4 by 2 to get a radius of 2 feet.
Measure the height of the liquid tank from top to bottom. Write the height down in feet. For this example, the tank's height is 6 feet.
Square the radius of the tank. In this example, 2 feet squared is 4 feet.
Multiply the radius squared times the height. Multiply this product times pi (3.14) to obtain the volume of the tank in cubic feet. In this example, 4 times 6 times 3.14 equals 75.36 cubic feet.
Multiply the cubic feet of the tank by 7.48 to convert cubic feet into gallons. For example, 75.36 multiplied by 7.48 equals 563.69 gallons of liquid that the tank will hold.