To calculate the area of a circle, use the following formula:

Let's say our circle is 8 inches in diameter. Half of the circle's diameter equals it's radius (8 / 2 = 4). Square the radius (4 * 4 = 16). Multiply that by pi 3.1416 (16 * 3.1416 = 50.2656). So we know our circle has an area of 50.2656 inches.