   Do the Math Here's how to figure the weight of the water in a center pivot irrigation system. Figure out the volume of one foot of pipe for our center pivot system. The formula for the volume of a cylinder is V = Π r² × L where "r" is the radius of the pipe and "L" is the length of the pipe (in this case 1-foot). In our example, the diameter of the pipe is 10-inches, so the radius is 5-inches. Since our weight of water is 62.5 pounds per cubic foot we have to convert the inches into feet. You should come up with 0.545415391 for the volume of one foot of 10-inch pipe. Multiply the volume of one foot in length ("L") by the number of feet in the system. The largest pivot system can cover an entire section of land, or one square mile. A mile is 5,280 feet and Valmont says their system is 2,600 feet long to cover the section. So multiply the volume of one foot by 2,600 and you should get a little over 1,418 ft³. Then multiply the total volume of the pivot system in cubic feet by the weight of one cubic foot of water, 62.5. You should get 88,630. Now figure out the weight of water in a center pivot system that irrigates only a quarter section of land (180 acres). The pipe in this system has a smaller diameter – only 6.5-inches in diameter – and the length of pipe in the system is 650 feet. Can you come up with the answer?   