Pi (p) represents the ratio of the circumference of a circle to its diameter, used in calculating the area of a circle (pr²) and the volume of a cylinder (pr²h) or cone. It is a “transcendental number,” an irrational number with an exact value that can be measured to any degree of accuracy, but that can’t be expressed as the ratio of two integers. In theory, the decimal extends into infinity, though it is generally rounded to 3.1416. The Welsh-born mathematician William Jones (1675–1749) selected the Greek symbol (p) for pi. Rounded to 30 digits past the decimal point, it equals 3.1415926535 89793238462643383279.

In 1989, Gregory (1952–) and David Chudnovsky (1947–) at Columbia University in New York City calculated the value of pi to 1,011,961,691 decimal places. They performed the calculation twice on an IBM 3090 mainframe and on a CRAY–2 supercomputer with matching results. In 1991, they calculated pi to 2,260,321,336 decimal places.

In 1999, Yasumasa Kanada (1948–) and Daisuke Takahashi of the University of Tokyo calculated pi out to 206,158,430,000 digits. Professor Kanada at the University of Tokyo continues to calculate the value of pi to greater and greater digits. His laboratory’s newest record, achieved in 2002 and subsequently verified, calculated pi to 1.2411 ? 10¹² digits (more than one trillion). The calculation required more than 600 hours of computing time using a Hitachi SR8000 computer with access to a memory of about 1 terabyte.

Mathematicians have also calculated pi in binary format (i.e., 0s and 1s). The five trillionth binary digit of pi was computed by Colin Percival and 25 others at Simon Fraser University. The computation took over 13,500 hours of computer time.