A pipe forms a regular cylinder and all cross sections from a cylinder equal one another in area. This makes the volume of the pipe easy to calculate -- it equals the product of its cross-sectional area and the pipe's length. These cross sections are all circles, and each one's area equals the product of its radius and the square of pi, a constant equal to approximately 3.142.

Square the pipe's inner radius, measured in feet. If the pipe has, for instance, an inner radius of 0.4 feet, use the equation: 0.4 ^ 2 = 0.16.

Multiply the answer by pi: 0.16 x 3.142 = 0.503 square feet.

Multiply this cross-sectional area by the length of the pipe. If the pipe measures, for instance, 13 feet in length: 0.503 x 13 = 6.54. The full pipe contains 6.54 cubic feet of liquid.