The calculation of the radius of a circle is expressed by the equation r = C / 2 * pi. This is where r is the radius, C is the circumference and pi is the irrational number beginning with 3.14.
If the diameter of the circle is already calculated, divide the diameter by 2 to find the radius. If the area of the circle is already calculated, the radius is solved using two steps. First, multiply the area times pi. Then, take the square root of the first calculation. The radius is the distance of between the center of the circle to its edge.