Body Mass Index (BMI) is a measure of body fat based on height and weight that applies to both adult men and women. Enter your height and weight using English measures. Click on the "Calculate BMI" button, and your BMI will appear. BMI Categories, according to the National Institutes of Health: Click on the link, the results will surprise you. Underweight = Less than 18.5 Normal weight = 18.5 - 24.9 Overweight = 25 - 29.9 Obesity = BMI of 30 or greater