PI is often used to calculate area of a circle.
A = pi * r * r ( Area of Circle = pi * ( radius * radius) )
Often times, only the first 2 digits of pi is used in calculations, (3.14). So say you wanted to calculate the area of a circle where its radius is 2 inches.
The formula would be A = 3.14 * 2 * 2 = 12.56
However, the more digits of pi you have the more accurate the result will be.
Area = 12.5663706144