Formulae vital to the geometric aspect of a circle:
Circumfrence: c = {pi} * d
Area: a = {pi} * r^2
Diameter: d = 2r
Arc Length - The distance from one point on the circumference to another point on the circumference traveling along the edge of the circle.
L = m / 360(deg) * c
Where:
r = radius
d = diameter
{pi} = 3.14159265358979323846... ~ 22/7
c = circumference
a = area
m = measure of the central angle