The most direct way of measuring the radius of a circle is to measure from the center of the circle to the outside border. If you cannot determine the center of the circle, measure the circumference of the circle and divide that number by 2?.
If you are given the diameter of a circle, you can calculate the radius by dividing the diameter by two, as the diameter is equal to the radius times two. If you are only given the area of a circle, the formula to calculate the radius is to multiply the area by ? (pi) and find the square root of that number.