Formula for the radius of a circle depends on what is given to you.

1) Given: Diameter of a circle

Then, Radius  =  Diameter  ÷  2



2) Given: Circumference of a circle

Then, Radius  =  Circumference  ÷  (2 × π)

3) Given: Area of a circle

Then, Radius  =  √(Area  ÷  π)