Oops. Cubic feet is a volume measurement so we need to know how long the pipe is to find out what volume it encloses.

On the other hand, a pipe with an 18-inch inside diameter has a cross-sectional area of 1.767-square feet.

The area of a circle is pi*D

where D is the diameter and R is the radius.

On the other hand, a pipe with an 18-inch inside diameter has a cross-sectional area of 1.767-square feet.

The area of a circle is pi*D

^{2}/4 or pi*R^{2}where D is the diameter and R is the radius.