The radius extends from the center of a circle to its outer edge. To find the radius of a circle, locate its center. Then, measure the distance from the center to the outer edge of the circle. This distance is the radius.
The circumference of a circle is two times pi times the radius. Algebraically, C = 2 x pi x r, where "C" is the circumference, "pi" is 3.14159 and "r" is the radius.
For a circle with a radius of 4 meters, the circumference is given by 2 x 3.14 x 4 meters, which equals 25.12 meters.