Pi (π) is approximately 3.14159 — the ratio of a circle’s circumference to its diameter, and an irrational number whose decimal digits never end or repeat.
No matter how large or small a circle is, dividing its circumference by its diameter always gives the same constant: pi. It's one of the most important constants in mathematics, appearing throughout geometry, trigonometry, physics, and engineering.
Because pi is irrational, it can never be written exactly as a finite decimal or fraction — supercomputers have calculated it to trillions of digits, but for nearly all practical purposes, a handful of decimal places is more than enough precision.
Last reviewed: September 2026