# March 14 — or 3/14 — is known as Pi Day! Happy Pi Day! No, not pie day (although that would be delicious), today is pi day — a special math day to celebrate a very unique number. Read on to find out why.

## What is pi? Pi is the number that you get when you take the distance around the outside of a circle (called the circumference), and divide it by the distance from one side to the other through the middle of a circle (called the diameter). It doesn’t matter how big or how small the circle is, when you divide the circumference by the diameter, you will always get the same number. Because the number is always the same, it’s called a constant. There's over a million digits in pi (wow!) but we just shorten it to 3.14 to make it easier to remember. Cool, right?

## But there's (so much) more! Even this isn't the entire number of pi!

If you want to be more precise, when you divide the circumference by the diameter, you will actually get 3.141592653589…  ("…" means and so on) The number goes on and on and on and on. There is no pattern. There is no end. It’s just a string of numbers that goes to a never-ending (or infinite) number of places. That’s one of the things that makes pi so special! Mathematicians have spent an incredible amount of time calculating pi. As of the end of 2017, it had been calculated to 22,459,157,718,261 digits — that trillions of digits! That’s a lot of pi!

## What's with the little chair? You may see the symbol used for pi in math equations. It looks like this: π (like a little chair, kind of). It’s actually the Greek letter "P," which is called "pi" in the Greek alphabet. Mathematicians didn’t start using the symbol for pi regularly until around the 1730s, although mathematicians have been using the calculation since Ancient Greece!

## So why is pi important? Pi is used whenever you want to do measurements or calculations using circles, such as if you need to know how big the surface of anything shaped like a circle is (that’s called the area). But scientists and mathematicians use pi in lots of equations that do all sorts of things: from calculating distances of planets orbiting the sun, to designing buildings in earthquake zones and even creating systems that help cell phones work.

## So why is pi day on March 14th? Can you guess? When you look at the month (3) and the day (14) you get… pi (3.14), at least if you write the date the American way! Now that you know all about pi, why not get yourself a nice circle-shaped pie, and celebrate!