The pipe's cross-sectional area has a number of applications in fluid dynamics, letting you calculate a pipe's flow rate or pressure drop. The measurement relates directly to the pipe's internal diameter. The factor relating the pipe's diameter and its area is pi, which is also the ratio between the pipe's diameter and circumference. If you divide a circle into a large number of wedges, they can form a quadrilateral whose width is the circle's radius and whose length is half the circumference.
Divide the pipe's diameter by 2 to find its radius. If it measures, for instance, 8 inches in length, divide 8 by 2 to get 4 inches.
Square this radius. In the example, 4^2 = 16 square inches.
Multiply the result by pi, which is approximately 3.142 -- 16 x 3.142 = 50.27 square inches. This is the pipe's cross-sectional area.