To find your Body Mass Index in pounds; Multiply your weight in pounds by 703 Divide by your height in inches (1 foot = 12 inches) Divide again by your height in inches. For instance, a 210 pound-person who is 70 inch tall has a BMI of about 30. The BMI formula relates the weight of the person to how tall he or she is. The BMI for normal weight ranges from 18.5 to 24.9.