where C is the circumference, d is the diameter and r is the radius.
The diameter of a circle is a line segment that passes through the center of the circle and has its endpoints on the circle. The radius of the circle is a line segment from the center of the circle to a point on the circle. The diameter of a circle is twice the length of its radius.
If you are given the diameter then use the formula C = πd
If you are given the radius then use the formula C = 2πr
