Keep it simple guys! I took a 44" piece of scrap type L 3/4" copper, sealed off one end with a hunk of wax and filled it with water. It held 12 fl oz. That means a foot of L 3/4 holds about 3.5 ounces.

3/4 inch PVC sch 40 pipe has an inside dimension of 0.81 inches ID. Area of a circle is radius squared times pi. .405*.405*3.14 = .5153 inch2. A foot of pipe has 12 inches so it holds 6.18 cubic inches. A gallon is 231 cubic inches.

1 foot of 3/4 inch sch40 PVC holds .026 gallons or one gallon for every 37.4 feet of pipe.

Other pipe types have different wall thicknesses and will hold different amounts.

1 foot of 3/4 inch sch40 PVC holds .026 gallons or one gallon for every 37.4 feet of pipe.

Other pipe types have different wall thicknesses and will hold different amounts.