What is pi, and why do its digits never end?

Pi is the number of times a circle's diameter fits around its circumference: about 3.14159. Take any circle, a coin, a plate, a running track, and divide the distance around it by the distance across it. You always get the same number, and that number is pi.

Its digits never end because pi is irrational: it can't be written as one whole number divided by another. Fractions like 22/7 or 355/113 come close, but none is exact. As a result, its decimal digits go on forever and never fall into a repeating pattern.

People have chased those digits for about 4,000 years. A Babylonian clay tablet treats pi as 3.125. Around 250 BC, Archimedes squeezed it between two polygons and proved it lies between 223/71 and 22/7. In December 2025, computers reached 314 trillion digits.

The funny part is how few we need. Just 62 decimal places are enough to calculate the circumference of the entire observable universe to within less than the smallest measurable length. The rest is pure curiosity, and a good excuse for Pi Day on March 14.