Measure the height of the cylindrical tank (the length of its straight part). Write down its value in feet or meters, depending on which unit system you're using.
Measure the circular part's radius (the line connecting the center of the circle with its edge). Your unit of measurement depends on whether you expressed the height in feet or meters.
Multiply pi by the height and the square of the radius (3.14 --- r^2 --- h). For example, if your tank's height is 7 feet and its radius is 4 feet, its volume is: 3.14 --- (4^2) --- 7 = 3.14 --- 16 --- 7 = 351.68 cubic feet.
Do the conversion to calculate the volumetric capacity in liters (L). A cubic foot equals 28.3168 L, while a cubic meter equals 1,000 L. Divide the liters value by 3.785 to find the capacity in U.S. gallons.