## What is Pi?

Pi ($$\pi$$) is a mathematical constant that is used in many calculations and theorems. It is approximately 3.14159, but the digits go on infinitely. It is mostly used in trigonometry and geometry (mainly circles, spheres, and ellipses). It is usually referred to as $$\pi$$ (pronounced "pie", written pi) in most formulae and in math textbooks.
Pi is the ratio of a circle's circumference to its diameter (you can see how this works at our circumference of a circle calculator). $$\pi = \frac{C}{d}$$ It is an irrational number, meaning it cannot be expressed in the form of a fraction of integers.