Pi (л) represents the ratio of the circumference of a circle to its diameter (л = circumference/diameter). Some calculations in which this value is used include: the area of a circle (A=лr^{2} ), the volume of a cylinder (V=лr^{2} h), and the volume of a cone (V=1/3лr^{2}h). Pi is an "irrational number," meaning it extends into infinity and does not have repeating sequences of digits.

Because it's impossible to write out infinite digits, pi is written in an abbreviated form. When doing calculations by hand, pi is generally rounded to 3.14. When working with a calculator, the value for pi is rounded to the number of digits that will fit on the display screen, which is often 12. Rounded to 30 digits past the decimal point, pi equals 3.141592653589793238462643383279.

In 1989, Gregory and David Chudnovsky, at Columbia University in New York City, calculated the value of pi to 1,011,961,691 decimal places. In October 1995, Yasumasa Kanada of the University of Tokyo calculated pi to more than 6.4 billion decimal places. It took Kanada 5 days working on a Hitachi supercomputer to achieve his results.

**Sources:** Cegielski, Charles, ed. 1998 *Yearbook of Science and the Future,* p. 341; Gibson, Carol. *The Facts On File Dictionary of Mathematics,* Rev. ed., p. 139.