Here's the math behind the online calculators for body mass index (BMI). To find your BMI, first multiply your height in inches by your height in inches (you are squaring the number). Then divide your weight in pounds by that square number. Multiply that result by 703. The answer is your BMI. A healthy BMI falls between 18.5 and 24.9. Overweight is 25.0 to 29.9. Obese is 30.0 and greater.