The radius of a circle is the distance from the center of the circle to the edge. The radius will be the same no matter where on the circle it is taken. The diameter, on the other hand, is the distance from the edge of the circle to the opposite side. So, the diameter is equal to the distance of 2 radii. Therefore, the diameter is equal to 2r. Since 2*8 is 16 the diameter is 16.
