The circumference of a circle is directly proportional to its radius. The ratio between a circle's circumference and its diameter is pi, a constant that equals approximately 3.142. The circle's diameter, in turn equals twice the radius. A regular solid that has a circle as a face is called a cylinder. Like all geometric prisms, a cylinder's volume is the product of its cross-sectional area and its length. The cross-sectional area is the area of its circle, and you can calculate it from the circle's radius.

Multiply the circle's radius by 2 to calculate its diameter. For instance, if the radius equals 6 inches, compute as follows to determine the diameter: 6 x 2 = 12 inches.

Multiply the diameter by pi to get the circle's circumference: 12 x 3.142 = approximately 37.7 inches.

Find the square of the radius: 6 x 6 = 36 square inches.

Multiply the answer determined in Step 3 by pi to get the circle's area: 36 x pi = 113 square inches.

Multiply the circle's area by the cylinder's length to obtain the volume. If the length is, for instance, 10 inches, then compute as follows: 113 x 10 = 1,130 cubic inches.