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Pi

Pi is a greek letter written as .

It is the ratio of the distance round a circle (its circumference) to the distance across (the diameter) and is the same for all circles. Also the ratio of the area of a circle to its radius squared is always pi.

It has many other applications in mathematics too. The first 100 decimal places are:

3.14159265358979323846264338327950288419716939937510
58209749445923078164062862089986280348253421170679

Pi is not an exact fraction.

²²/₇ =3.142857.. is only approximately equal to .

William Shanks computed the value of pi by hand to 707 decimal places. In 1946 it was checked using desk calculators (mechanical devices to do arithmetic) and it was discovered that he was wrong from the 528th place and afterwards.

The current record is 68,719,470,000 (68 billion) decimal places calculated in 29 hours in Japan and verified on 5 April 1999.