How is body mass index calculated and what ranges define underweight to obesity?

• A. BMI is measured in liters per meter squared.
• B. BMI = weight (kg) / (height in meters)^2; underweight <18.5, normal 18.5-24.9, overweight 25-29.9, obesity ≥30.
• C. BMI = height (cm) / weight (kg); ranges differ by gender.
• D. BMI = weight in pounds divided by height in inches squared.

Answer: (B)

BMI is a simple screening tool that estimates body fatness from height and weight. The calculation uses metric units: weight in kilograms divided by the square of height in meters, written as BMI = weight (kg) / [height (m)]^2. Squaring height normalizes weight because taller people naturally weigh more, but not proportionally to height itself. The standard cutoffs are: underweight below 18.5, normal weight 18.5 to 24.9, overweight 25 to 29.9, and obesity 30 or higher. If you’re using pounds and inches, you’d convert to metric or use the imperial form with a conversion factor, BMI = [weight (lb) / (height in inches)^2] × 703. The other options aren’t correct because BMI isn’t measured in liters per meter squared, it doesn’t use height in centimeters over weight in kilograms, and the simple pounds-per-inches-squared formula omits the necessary conversion factor.