You use the information that you do have, and a formula that relates it to theradius or diameter of the circle. As an example: The circumference is very helpfulif you know it.
Radius The word radius was a Latin word for the spoke of a wheel. The similarity of radius to the Indo-European root werad from which we get radical and rhizome and the image of the roots of a plant...
I assume you mean radius of a circle, which is just half the diameter. So you measure the distance across the circle at the widest point, and then divide by two