Think of it as a cylinder. The volume of a cylinder is its length times its cross-sectional area (a circle), so V = (pi)R2L, where R is the radius of the pipe, L is the length, pi is about 3.1416 and V is the volume of water. Be sure you use the inside radius of the pipe and convert all your units to something you can work with (like inches). Then you will get an answer in cubic inches. If you want to convert that to gallons, use the appropriate conversion ratio:
1 gallon (UK "Imperial units") = 277.42 cubic inches
1 gallon (US) = 231 cubic inches
Hope this helps,
Stephen La Rocque.