The body weight index is a great way to figure out how much work you're going to need to put into your workout goals since it shows you how far behind you are from where you want to be. However, finding out your body weight index can be a bit difficult if you don't know the formula for it. To find your body weight index, simply multiply your body weight in pounds by 703. Then, divide that number by you height in inches squared. The result will be your body weight index, and you can use that to gauge how much work you have left.