Body Mass Index (BMI) is the quick screen doctors start with: weight relative to height. Enter your height and weight in feet and pounds and get your BMI, your NIH category, and the weight range that corresponds to a normal BMI for your height.
The NIH categories
Underweight: below 18.5 · Normal: 18.5–24.9 · Overweight: 25–29.9 · Obesity: 30 and above. These are the official US National Institutes of Health cutoffs. The formula in pounds and inches is BMI = 703 × weight ÷ height² — the 703 factor converts the metric formula for American units.
What BMI misses
BMI cannot tell muscle from fat: a muscular athlete can register "overweight" while being perfectly healthy, and older adults with lost muscle can score "normal" while carrying excess fat. Waist circumference (under 40 inches for men, 35 for women) and body fat percentage tell you more. BMI is a screening starting point, not a diagnosis.