How to Calculate BMI: Formula, Ranges, and Limitations
BMI is simple to calculate but complex to interpret. Here is what the numbers mean and when they mislead.
BMI = weight (kg) / height (m)². In imperial units: BMI = weight (lbs) × 703 / height (inches)². A 170-pound person who is 5 foot 10 (70 inches): 170 × 703 / 4,900 = 24.4 BMI — at the top of the normal range. The categories: under 18.5 (underweight), 18.5-24.9 (normal), 25-29.9 (overweight), 30+ (obese).
Where BMI Fails
BMI cannot distinguish muscle from fat. A muscular 200-pound athlete and a sedentary 200-pound person at the same height have identical BMIs but completely different health profiles. BMI also misses visceral fat (the dangerous fat around organs) — a normal-BMI person can have dangerous levels of internal fat while looking slim externally. For individuals, body fat percentage and waist-to-height ratio are better health indicators.
Where BMI Works
Population-level health screening and epidemiology. Insurance and medical intake assessments. Quick identification of people who may benefit from further evaluation. It is a starting point, not a diagnosis. A high BMI with normal body fat, blood pressure, cholesterol, and blood sugar does not indicate poor health.