Radius & Diameter
The diameter is the longest distance across the circle passing through the center. The radius is exactly half the diameter.
d = 2r
Circle Formulas
Circumference
The circumference is the perimeter or distance around the circle. It is related to the diameter by the constant Pi (π).
C = πd or C = 2πr
Area
The area is the space occupied by the circle in a 2-dimensional plane.
A = πr²
Finding Radius
If you know the Area or Circumference, you can find the radius using these formulas:
r = √(A / π)
r = C / 2π