Determining the volume of a cylindrical water tank can be a challenge. This requires you to use the mathematical figure known as pi in computing the volume. Completing the calculation in several steps will help limit your risk of making a mistake in computing the volume of a water tank. The math to compute the volume of a square or rectangular water tank is simpler.

Measure the height of your water tank. This should be from the bottom of the tank to the top of the tank, regardless of the amount of water in the tank. For the purpose of this example the height is 20 feet.

Measure the diameter of your water tank. The diameter of the tank is a straight line that goes from one side of the circle forming the base of the tank to the other side, passing through the exact centre of the circular base. Once you have calculated the diameter of your water tank, divide it by 2 to get the radius. The radius is the measurement from the centre of the circle that forms the base of the tank in a straight line to any edge of the tank. In this example the diameter is 10 feet and the radius is 5 feet.

Calculate the volume of the water tank using the following formula for the volume of a cylinder: Volume = pi times the radius squared (the radius times itself) times the height of the tank, or volume = pi x r squared x height.

Calculate the total volume in our example using the formula in Step 3. For this example multiply pi (which is 3.14159) x 5 x 5 x 20. This equals about 1,570 cubic feet, which is the volume of the water tank. Use a calculator that has the pi symbol on it to get a more accurate solution. The pi symbol is the Greek letter "p."

Measure the length, width and height of the square or rectangular water tank. In this example the length will be 20m, the width 10m and the height 40m.

Calculate the volume of the rectangular tank using this formula: Volume = length x width x height.

Compute the total volume in our example using the formula in Step 2. For this example, multiply 20 x 10 x 40. This would provide a measurement of 8,000 sq. m as the volume of the water tank.