The BMI Formula
Body Mass Index (BMI) is calculated as:
BMI = weight (kg) ÷ height² (m²)
In imperial units: BMI = [weight (lbs) × 703] ÷ height² (inches²)
Example: A person who is 5'9" (175cm) and weighs 175 lbs (79.5kg):
BMI = 79.5 ÷ (1.75)² = 79.5 ÷ 3.0625 = 26.0 (overweight range)
Standard BMI Ranges
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Obese (Class III) |
Why BMI Is Flawed: The Core Problem
BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s as a statistical tool for population analysis — not for evaluating individual health. It measures only the ratio of weight to height and says nothing about body composition.
The most significant flaw: BMI cannot distinguish between fat mass and muscle mass. Muscle is denser than fat, so a muscular person and an obese person can have the same BMI.
Real-World Examples of BMI Failure
- Athletes: Many NFL running backs (highly muscular, very low body fat) have BMIs of 28–32 — classified as overweight or obese. They are clearly not.
- Skinny fat: A sedentary person with low muscle mass and high fat percentage may fall in the "normal" BMI range while having metabolic risk factors associated with obesity.
- Ethnic differences: Research shows that Asian populations have higher metabolic disease risk at lower BMI values. Many health organizations now recommend lower BMI thresholds for Asian individuals (23 for overweight instead of 25).
What BMI Is Still Useful For
Despite its limitations, BMI retains value in specific contexts:
- Population-level screening: BMI is a cheap, quick screening tool. In large populations, it correlates reasonably well with metabolic disease risk.
- Tracking trends: For a given individual over time, rising BMI is a useful signal — even if the absolute value is imperfect.
- Medical communication: BMI categories are universally understood and provide a common language for clinicians.
- Research: BMI's simplicity makes it useful for large-scale epidemiological studies.
Better Alternatives to BMI
- Waist-to-height ratio: Keep your waist circumference less than half your height. Simple, accounts for central adiposity (belly fat), stronger predictor of cardiovascular risk than BMI.
- Body fat percentage: Healthy ranges: men 10–22%, women 20–32%. Measured accurately by DEXA scan or hydrostatic weighing; estimated by bioelectrical impedance scales.
- Waist circumference alone: Greater than 35" (women) or 40" (men) is associated with elevated metabolic disease risk.
- DEXA scan: Gold standard for body composition. Measures fat mass, lean mass, and bone density with high accuracy. Costs $50–$150; available at many hospitals and body composition clinics.
How to Interpret Your BMI in Context
Your BMI number is one data point, not a verdict. A BMI of 26 for a strength athlete with 12% body fat means something very different from a BMI of 26 for a sedentary person with 30% body fat.
For most people without extreme muscle mass, BMI is a reasonable rough indicator. If your BMI is in the overweight or obese range and you're not heavily muscled, it's worth discussing with a healthcare provider alongside other measures. Use our CalcPeek BMI calculator to calculate your BMI and see how it compares to population norms.