The symbol (pi) is used in math as a geometric constant. The value of is naturally occuring and can be found by dividing the length of a circle’s circumference by the length of its diameter. Traditionally, the constant (pi) has been used throughout math in formulas relating to the circle.
Shown below are some basic geometric formulas in terms of (pi). For advanced formulas, see the page for (tau).
Calculate the circumference of a circle given the radius.
Calculate the area of a circle given the radius.
Caclulate the volume of a cylinder given the radius and height.
The constant (pi) is used when measuring angles in radians. Shown below is the radian angle system labeled using on the left and on the right.
The circle constant τ (tau) is a geometric constant approximately equal to 6.283. The numeric value is defined as the length of any circle's circumference divided by the length of its radius.

No, really, pi is wrong: The Tau ManifestoMichael Hartl