To convert between diameter and radius just follow a few steps.
Sometimes we aren't given the number directly so first we need to find the diameter.
First step, take a circle. In that circle, find the middle point and draw a line.
That line is the diameter. Just divide by two; half the diameter is the radius. For example, if you have a diameter of 6cm, just divide by 2. You will get half of the value, which is 3cm; this is the radius.