To calculate the area of a circle, it is necessary to first determine the circle's radius, which is half the distance of a straight line across the center of the circle. The formula used to determine the circle's radius is r = C / 2.

Pi is a mathematical constant representing the ratio of the circumference of a circle to its diameter, and is approximately equal to 3.14. Pi has also been represented by the Greek letter "Π" since the 18th century.

The formula used to determine the area of a circle is:

A = pi * r^{2} or

A = Π * r * r

Where A represents the area, r is the radius, and pi equals 3.14.

For example, the area of a circle with a diameter of 2 inches can be calculated as follows:

A = 3.14 * (2/2)^{2}

A = 3.14 * 1^{2}

A = 3.14

In this case, the area of the circle is 3.14 inches.

The mathematical constant pi is commonly used to determine the of area and volume of geometric shapes, which can be applied to things like floor space and how much fluid a particular barrel holds in everyday life.