How do you calculate body mass index (BMI) and what are common categories?

• A. BMI = weight (kg) / height (m)^2; categories: underweight <18.5, normal 18.5–24.9, overweight 25–29.9, obesity ≥30
• B. BMI = weight (kg) / height (m); underweight <18.5; normal 18.5–24.9
• C. BMI = weight (lb) / height (in)^2; normal range 20-25
• D. BMI = weight (kg) * height (m)^2; obesity ≥40

Answer: (A)

BMI estimates body fat by comparing weight to height. The standard calculation uses metric units: weight in kilograms divided by the square of height in meters, written as BMI = kg / m^2. This yields a number used to categorize weight status: underweight if BMI is less than 18.5, normal weight from 18.5 to 24.9, overweight from 25 to 29.9, and obesity at 30 or higher. In practice, some charts use pounds and inches with a conversion factor (703), but the basic principle remains the same: weight relative to height squared.

The other formulations are not correct because they either omit squaring height, multiply instead of divide, or apply nonstandard units and cutoffs.