The formula for the area of a circle is pi multiplied by the radius of the circle squared. The radius of the circle is the length of a straight line stretching from the center of the circle to the line of circumference. It is equal to half the diameter.
The approximate value of pi is 3.14. Pi is the ratio of a circle's circumference to the circle's diameter. If a circle has a radius of 5 inches, the area of that circle equals 3.14 x 5^2, or 78.5 square inches. The circumference is equal to pi times the circle's diameter. If the same circle is used, the diameter is equal to 10 (two times the radius), so the circumference equals 3.14 x 10, or 31.4 inches.