Definition of math constant π (pi)

The symbol (pi) is used in math as a geometric constant in formulas and the radian angle system. The value of is naturally occurring and can be found by dividing the length of a circle’s circumference by the length of its diameter.

Traditionally, (pi) has been used in math as the circle constant. However, this website uses the constant (tau) instead of (pi) as the default circle constant[1]. The substitution can be used to translate between the two constants.


Geometry Formulas

Shown below are some basic geometric formulas in terms of (pi). For advanced formulas, see the page for (tau).

Circumference of Circle π (pi)

Calculate the circumference of a circle given the radius.

Area of Circle π (pi)

Calculate the area of a circle given the radius.

Volume of Cylinder π (pi)

Calculate the volume of a cylinder given the radius and height.


The constant (pi) is used when measuring angles in radians to represent a half rotation. Shown below is the radian angle system labeled using on the left and on the right.

Radian angle system labeled with pi Radian angle system


  1. No, really, pi is wrong: The Tau Manifesto
    Michael Hartl