The symbol (pi) is used in math as a geometric constant in formulas and the radian angle system. 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, (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.
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 to represent a half rotation. 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.
Radians are a unit that measure angle using the radius of a circle. One radian is equal to the amount of rotation required to travel the length of one radius along the circumference of the circle.

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