The formula for a radius is the diameter of a circle divided by two. The radius of a circle is defined as the distance from the middle of a circle to any point on the edge of the circle.
Another way to calculate the radius of a circle is by using the circumference. The equation using the circumference is the circumference of the circle divided by pi times two. Pi (π) is a number that is approximately 3.14. The radius can also be calculated if the area is known; the radius is equal to the square root of the area of the circle divided by pi.