For any circle,

Credit: © NPG

This number is so special that it is given its own symbol (the Greek letter pi).

The value is a constant, but is called an irrational number as an exact value for does not exist. In non-calculator working out, approximate values are used, of which and are probably the most common. Other, less accurate approximations are and .

All scientific calculators have a button. You can use this to make your calculations more accurate.

The earliest known use of the Greek letter , to represent the ratio of a circle's circumference to its diameter, was by the Welsh mathematician William Jones in his 1706 work ‘*Synopsis Palmariorum Matheseos*’ or, ‘*A New Introduction to the Mathematics*’.