Body Mass Index is the most widely used health screening tool in the world. Doctors, insurance companies, and public health organizations rely on it. But it's also widely misunderstood and frequently criticized. Here's what you need to know.
How BMI is Calculated
or
BMI = Weight (lbs) × 703 ÷ Height (inches)²
BMI = 170 × 703 ÷ (70 × 70) = 119,510 ÷ 4,900 = 24.4
BMI Categories
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class 1) |
| 35.0 – 39.9 | Obese (Class 2) |
| 40.0+ | Obese (Class 3) |
A "normal" BMI of 18.5–24.9 is associated with the lowest risk of weight-related health issues at a population level. But — and this is important — BMI was designed as a population-level statistical tool, not an individual diagnostic.
Where BMI Falls Short
It ignores body composition. BMI can't distinguish between muscle and fat. A muscular athlete and a sedentary person of the same height and weight will have identical BMIs despite very different health profiles. Most professional athletes would be classified as "overweight" by BMI alone.
It ignores fat distribution. Visceral fat (around organs) is far more dangerous than subcutaneous fat (under the skin). Two people with the same BMI can have very different health risks depending on where their fat is stored. Waist circumference is a better predictor of metabolic risk.
It varies by age, sex, and ethnicity. The same BMI represents different body fat percentages in men vs. women, younger vs. older adults, and across different ethnic groups. Some health organizations use different BMI thresholds for Asian populations (overweight at 23+ instead of 25+).
Better Alternatives
Waist-to-hip ratio: measure your waist at the narrowest point and hips at the widest. A ratio above 0.90 for men or 0.85 for women indicates higher health risk.
Waist circumference alone: above 40 inches for men or 35 inches for women is associated with increased risk, regardless of BMI.
Body fat percentage: the most accurate measure, typically requiring calipers, DEXA scans, or bioelectrical impedance. Healthy ranges are roughly 10–20% for men and 18–28% for women.