Pi is approximately equal to 3.14 (3,141 592 653...). Many formulae in mathematics, science, and engineering involve Pi, which makes it one of the most important mathematical constants. For instance, the area of a circle is equal to Pi times the square of the radius of the circle.
Pi began being symbolized by the Pi symbol in the 1706 by the British mathematician William Jones. Jones used 3.14159 as the calculation for Pi.
Pi has been known for nearly 4,000 years and was discovered by ancient Babylonians. A tablet from somewhere between 1900-1680 B.C. found Pi to be 3.125. The ancient Egyptians were making similar discoveries, as evidenced by the Rhind Papyrus of 1650 B.C. In this document, the Egyptians calculated the area of a circle by a formula giving Pi an approximate value of 3.1605. There is even a biblical verse where it appears Pi was approximated:
And he made a molten sea, ten cubits from the one brim to the other: it was round all about, and his height was five cubits: and a line of thirty cubits did compass it about.
I Kings 7:23 (King James Version)
The first calculation of Pi was carried out by Archimedes of Syracuse (287-212 B.C.). One of the greatest mathematicians of the world, Archimedes used the Pythagorean Theorem to find the areas of two polygons. Archimedes approximated the area of a circle based on the area of a regular polygon inscribed within the circle and the area of a regular polygon within which the circle was circumscribed. The polygons, as Archimedes mapped them, gave the upper and lower bounds for the area of a circle, and he approximated that Pi is between 3 1/7 and 3 10/71.
Zu Chongzi of China (429-501) calculated Pi to be 355/113, though how he arrived at this number is a mystery, as his work was lost.