Measure or somehow get the inside diameter of the pipe, figure the area of the circle out, multiply that by the length and this gives you volume. Keep all your units of length the same....say feet, then you will have cubic feet as the volume. There are 7.48 gallons of water per cubic foot. I am hoping the rest of the math comes easily to you....I rarely use tables since inside diameters of pipe are never that consistant.
BobPE