How is body mass index (BMI) calculated and interpreted?

• A. BMI = height (cm) / weight (kg); interpretation: underweight below 20
• B. BMI = weight (kg) / [height (m)]^2; 18.5–24.9 normal; <18.5 underweight; 25–29.9 overweight; ≥30 obese
• C. BMI = weight (pounds) / height (inches)^2 times 703; classification: normal 15-20
• D. BMI equals body fat percentage score; interpretation: independent of weight

Answer: (B)

BMI estimates body mass relative to height using a simple ratio. The standard calculation multiplies height in meters by itself (squares the height) and then divides weight in kilograms by that square. That yields a number that falls into widely used categories: underweight if below 18.5, normal if 18.5–24.9, overweight if 25–29.9, and obesity if 30 or higher. Using height in centimeters divided into weight in kilograms isn’t the correct form for BMI and would give inconsistent results. Using pounds and inches with the 703 factor is an imperial alternative for the same concept, but the typical interpretation ranges still align with the metric thresholds, not a 15–20 normal range. BMI is not a direct measure of body fat percentage and it depends on weight, so it’s a screening tool, not a precise fat assessment.