Measure the distance from the center of the circle to the circumference. This is the radius of the circle.
Multiply the radius of the circle in question by two to get the diameter.
Multiply pi by the diameter to get the circumference of the circle. For instance, if the diameter of the circle is 20 centimeters, the circumference of the same circle is the product of 20 centimeters and 3.14, which is 62.8 centimeters.