Elevated water tanks consist of a raised cistern connected to water mains or plumbing. The elevation means that the weight of gravity presses down on a tall column of water, producing pressure to drive water through pipes. The basic principle is that the water pressure in pounds per square inch (psi) at any point in the system is equal to the weight of a one square inch column of water with a height equal to the vertical distance between the point being measured and the surface of the water in the tank.
Determine the vertical distance between the bottom of the cistern and the point the outlet pipe connects to the water main or plumbing system. This will be stated on the blueprints or building permit documentation (available from the city or county office that enforces building codes).
Measure the depth of the water in the tank. This is the vertical distance between the top of the outlet pipe and the surface of the water collected in the tank. Most tanks have a gauge or markings to show water depth. If not, use a weighted line or measuring rod to find the depth.
Convert the distances you measured from feet to inches. For example, if the height of the outlet pipe is 25 feet, this is 12 inches times 25, or 300 inches. If the water depth in the tank itself is 10 feet, this works out to 120 inches.
Calculate pressure per square inch at the bottom of the tank itself. This is a critical point because the pressure is highest at this point in the tank, so it must be well built and in good condition to bear the weight. Water weighs 62.43 pounds per cubic foot. To find pressure per square inch, multiply the height in inches by 62.43 pounds and divide by 1,728 (the number of cubic inches in one cubic foot). If the water is 120 inches deep, this is (120 inches x 62.43 lbs)/1,728 = 4.34 psi (rounded off to 2 decimal places).
Calculate the pounds per square inch at the point where the outlet pipe connects to the water main or plumbing. To do this, add the vertical height of the pipe to the depth of the water in the tank. For an elevation of 300 inches and a depth in the tank of 120 inches, you have a total of 420 inches. Use this figure to repeat the calculation in Step 4. You have (420 inches x 62.43 lbs)/1,728 = 15.17 psi. This is the water pressure available to push the water through the water main or plumbing.