The number π is a mathematical constant, the ratio of a circle 's circumference to its diameter, commonly approximated as 3.14159. It has been represented by the Greek letter  π  since the mid-18th century, though it is also sometimes spelled out as  pi  (/paɪ/).ome people use a different value, τ = 6.283185... = 2 π, arguing that τ, as the number of radians in one turn or as the ratio of a circle's circumference to its radius rather than its diameter, is more natural than π and simplifies many formulas.