The symbol π is the Greek letter "pi" and in mathematics it represents a special number beginning with 3.14. Hence, Pi Day is celebrated on March 14 (US date 3/14).

Take a circle and divide the circumference of the circle by the diameter of the circle and there you have it: π. If you take another circle and divide the circumference by the diameter again, you get π as well! It is therefore called a constant.

Try it yourself! Measure the diameter of a round object and measure the circumference using a ribbon. Divide the circumference by the diameter and you will get approximately 3.14. Did you calculate π? No! Then what exactly is π? Nobody knows! This is because the constant π has infinitely many decimal places. In other words, the number of decimal places never stops!

No, even your calculator doesn't know! Calculators know an approximation of the constant π to about 30 decimal places. Fortunately, that is more than enough for accurate calculations. The best approximation by a computer now stands at 50 trillion (50000 billion) decimal places. Want to break a world record? Better start learning 70000 decimals by heart!

It may sound crazy, but we really can't live without π. The constant π occurs everywhere around us! From satellites to self-driving cars and from the economy to brewing beer. On this page we explain some of these applications.