This lesson covers the area of a circle. Students learn that the formula for the area of a circle is pi times radius squared, so the area of a circle that has a radius of 5 inches is pi times 5 squared, or 25 pi square inches. And since pi = 3.14, 25 pi square inches can also be written as 25 times 3.14, or 78.5 square inches. Note that the radius of a given circle is half the diameter of the circle. So to find the area of a circle with a given diameter, first find the radius of the circle by dividing the diameter by 2.