What is pi?
Pi, written π, is the ratio of a circle's circumference to its diameter. It is about 3.14159 and is an irrational number, so its decimals go on forever without repeating.
For any circle, no matter how big or small, dividing the distance around it by the distance across it gives the same number, pi. It is used to calculate the circumference (2πr) and area (πr²) of circles, as well as in physics, engineering and statistics.
Ancient Babylonians and Egyptians used values close to 3.1. The Greek mathematician Archimedes showed around 250 BC that pi is between 3 1/7 and 3 10/71. The symbol π was popularized by Leonhard Euler in the 1700s.
Pi Day
Pi is celebrated on March 14 (3/14), which is also Albert Einstein's birthday. Computers have calculated pi to hundreds of trillions of digits, though only about 40 digits are needed to calculate the size of the observable universe to the width of an atom.
Did You Know?
The fraction 22/7 is a common approximation of pi, but it is slightly too large: 22/7 is about 3.142857.