The best way to determine what size soccer ball you need is to measure yourself. Stand straight and keep your arms at your sides. Measure around your chest just below the armpits using a tape measure. This measurement will give you the circumference around your torso. Divide this number with 2 and multiply that by 5. Divide this number by 5 and multiply it again. For example, 40 inches is the circumference of your chest. This is the circumference a sphere that has a diameter 20 inches. This formula allows you to determine the approximate size of the ball.