HJ, on plumbing issues I respect your vast knowledge and experience, but this is in the area of my expertise. Not that this whole topic is worth getting into a heated debate about, but please follow the problem. The question was, how much water would a 10' length of pipe hold. Now, let's not get into inside or outside diameters, that is not really the question. First, we have to figure the area of a 1" circle. The formula for that is: P (3.14) x radius (.5") squared (.25") That equals .785 sq inches. Next, we have to consider the length of the pipe in inches since all of our measurements thus far have been in inches. 10' = 120". That now is multiplied by the area of .785 sq inches and gives us 94.2 cubic inches. The is 231 cubic inches in 1 gallon, so our final problem is 94.2 cubic inches divided by 231 which equals .4077922+ gallons. This is slightly more than 4 gallons of water in a 10' pipe.