If the circumference or the area of a circle is known, pi can be used to find the diameter and the radius of that circle. Likewise, if the radius or diameter is known, pi can be used to find out the circumference or the area.
The Greek letter "pi" is used in math and science to stand for the ratio between the circumference of a circle and its diameter. The ratio equals about 3.1415. This value is a constant for any circle. The circumference of a circle is always equal to pi times the diameter. The area of a circle is always equal to pi times the radius squared. These formulas are used constantly by engineers, physicists, designers and mathematicians.
The uses of pi extend beyond merely finding circumferences or radii. NASA uses pi to calculate the trajectories of spacecraft, to determine the sizes of craters and to estimate the sizes of planets outside our solar system.
The curious thing about pi is that it is an "irrational number." Its approximate value has been known since antiquity, and modern computers have calculated its value out to billions of places, but that process is unending. The digits of pi never end, nor do they repeat in a recognizable pattern.