Pi, also written π, is the Swiss Army knife of numbers. No matter how big or small a circle – from the size of our universe all the way down to an atom or smaller – the ratio of a circle’s circumference (the distance around it) to its diameter (the distance across it) is always equal to pi. The Constant Pi= 3.14 = 22/7, thats 22nd July, is celebrated as the Pi Approximation Day.