Calculate the weight of water in a 6" round pipe. Use 39’ as its length. A: 450 Pounds B: 477 Pounds C: 455 Pounds D: 475 Pounds B: 477 Pounds First the gallons. 6" or .5’ diameter. A= d squared x .7854. .5 x .5 x .7854=.196 SF . Volume of pipe in cubic feet is A x H, or .196 x 39 = 7.65 cubic Feet. 1 CF foot of water = 62.5 pounds. So, 7.65 cubic feet of water weighs 7.65 x 62.5=477#.