The best way to determine what size soccer ball you need is to measure yourself. You can measure by standing straight with your arms out in front. With a tape measure, measure your chest from the bottom of your arms to the top. This measurement will give you the circumference around your torso. Divide this number and multiply it with 5. For example, if your chest is 40 inches long, divide this number by 2, and multiply by 5, which gives you 20. This is the circumference for a 20-inch diameter sphere. This formula can be used to calculate the size of your soccer ball.