What is pi ()? Who first used pi? How do you find its value? What is it for? How many digits is it?By definition, pi is the ratio of the circumference of a circle to its diameter. Pi is always the same number, no matter which circle you use to compute it.
For the sake of usefulness people often need to approximate pi. For many purposes you can use 3.14159, which is really pretty good, but if you want a better approximation you can use a computer to get it. Here's pi to many more digits: 3.14159265358979323846.
The area of a circle is pi times the square of the length of the radius, or "pi r squared":
A = pi*r^2A very brief history of pi
Pi is a very old number. We know that the Egyptians and the Babylonians knew about the existence of the constant ratio pi, although they didn't know its value nearly as well as we do today. They had figured out that it was a little bigger than 3; the Babylonians had an approximation of 3 1/8 (3.125), and the Egyptians had a somewhat worse approximation of 4*(8/9)^2 (about 3.160484), which is slightly less accurate and much harder to work with. For more, see A History of Pi by Petr Beckman (Dorset Press).
The modern symbol for pi  was first used in our modern sense in 1706 by William Jones, who wrote:
There are various other ways of finding the Lengths or Areas of particular Curve Lines, or Planes, which may very much facilitate the Practice; as for instance, in the Circle, the Diameter is to the Circumference as 1 to
Pi (rather than some other Greek letter like Alpha or Omega) was chosen as the letter to represent the number 3.141592... because the letter  in Greek, pronounced like our letter 'p', stands for 'perimeter'.
About PiPi is an infinite decimal. Unlike numbers such as 3, 9.876, and 4.5, which have finitely many nonzero numbers to the right of the decimal place, pi has infinitely many numbers to the right of the decimal point.
If you write pi down in decimal form, the numbers to the right of the 0 never repeat in a pattern. Some infinite decimals do have patterns - for instance, the infinite decimal .3333333... has all 3's to the right of the decimal point, and in the number .123456789123456789123456789... the sequence 123456789 is repeated. However, although many mathematicians have tried to find it, no repeating pattern for pi has been discovered - in fact, in 1768 Johann Lambert proved that there cannot be any such repeating pattern.
As a number that cannot be written as a repeating decimal or a finite decimal (you can never get to the end of it) pi is irrational: it cannot be written as a fraction (the ratio of two integers).
Computers have calculated pi to many decimal places. It's easy to find lists of them by Googling 'digits of pi'.
From the Dr. Math archives:
Facts About Pi
History of Calculating Pi
Irrationality of Pi
Pi in real life
Pi and the Area of Circles
Pi's last digit
Proving Pi and Buffon's Needle
Proving Pi is Irrational
Radius of a pizza
What exactly is Pi?
What is the formula for deriving a polygon of degree n?
Where did pi come from?
Are All Digit Strings in Pi?
From the Math Forum and On the Web:
Pi Digits - Eric Weisstein's World of Mathematics
Determination of pi - Alexander Bogomolny
Who Invented Pi? - The Straight Dope
The Pi Pages - Centre for Experimental and Constructive Mathematics (CECM)
Inverse Symbolic Calculator (computing Pi) - Simon Plouffe
A History of Pi - MacTutor Math History Archive
The Joy of Pi - David Blatner
Computing Pi - Harry J. Smith
How to compute digits of pi? - SciMath FAQ
Precomputer History of Pi - Neal Carothers
Pi is Irrational - Keith Lynn
The Math Forum: Internet Mathematics Library: Pi
Math Forum Home || Math Library || Quick Reference || Math Forum Search