Pi is a number.
The number is a mathematical constant , the ratio of a circle 's circumference to its diameter , commonly approximated as 3.14159 . Being an irrational number , can not be expressed exactly as a fraction ( equivalently , its decimal representation never ends and never settles into a permanent repeating pattern ) . Still , fractions such as 22/7 and other rational numbers are commonly used to approximate . Also , is a transcendental number ; that is , a number that is not the root of any non-zero polynomial having rational coefficients .