Many people still don't know what BMI stands for. This is an extremely important number regarding living a healthy lifestyle. BMI is short for body mass index. Many doctors use this tool to determine if a person is underweight, at a healthy weight, or obese. In other words this is the ratio of height to weight. There is a formula to figure out one's body mass index. Take the weight of the person in pounds, divide by the person's height in inches and multiply by 703. This will then leave a number.