# τ (tau) The circle constant (tau) is a geometric constant that appears in numerous math formulas relating to circles and angles. The numeric value of is defined as the length of the circumference of a circle divided by the length of its radius and is approximately equal to .

Note: This website uses the constant (tau) instead of (pi) as the default circle constant. The substitution can be used to translate between the two constants.

## Usage In the radian angle system, the circle constant is equal to a full rotation in radians. Measured angles are represented as fractions of the circle constant . For example, shown below are two angles measured using the circle constant.  ### Geometry Formulas

Here are some traditional geometric formulas in terms of the circle constant.

Area of Circle

Calculate the area of a circle given the radius.

Circumference of Circle

Calculate the circumference of a circle given the radius.

Volume of Sphere

Calculate the volume of a sphere given the radius.

Volume of Cylinder

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