The radius of a circle is the distance from the center of the circle to its edge. The diameter of the circle is the distance all the way across it, and is equal to two times the radius. You will often be asked to calculate the radius of the circle based on other measurements.he diameter is the longest line that can be drawn through a circle, and cuts the circle into two equal halves. It is also two times the length of the radius. The formula for the diameter is D = 2r, where “D” stands for diameter, and “r” stands for radius. The same formula in terms of r is r = D/2.