The radius is calculated by taking one-half of the diameter, and then calculating how far it is out from that point to each side of the radius.įor example, if your diameter were 10 units, you would calculate your radius as 5 units on each side. The radius of a circle is the distance from the centre point on its circumference to its edge. In either case, you need a radius as input and an output value for the radius squared equal to the circle's area. There are two possible formulas, one using circumferences and the other using radii. The pattern to find the area is to multiply by pi.