What Is BMI? The Complete Guide to Body Mass Index
The short answer: BMI (Body Mass Index) is a number calculated from your height and weight. It is used by doctors and health organisations globally as a quick screening tool for weight-related health risk. But it is not a diagnosis — and it has real limitations you need to know.
How BMI Is Calculated
BMI uses a simple formula: divide your weight in kilograms by your height in metres squared. If you are 175cm tall and weigh 75kg, your BMI is 75 ÷ (1.75 × 1.75) = 24.5. In US units: multiply your weight in pounds by 703, then divide by your height in inches squared.
WHO BMI Categories
| Category | BMI Range | Health Risk |
|---|---|---|
| Underweight | < 18.5 | Elevated |
| Normal Weight | 18.5 – 24.9 | Low |
| Overweight | 25.0 – 29.9 | Elevated |
| Obese Class I | 30.0 – 34.9 | High |
| Obese Class II | 35.0 – 39.9 | Very High |
| Obese Class III | ≥ 40 | Extremely High |
The 5 Key Limitations of BMI
1. It cannot distinguish muscle from fat. A professional athlete and a sedentary person at the same height and weight have identical BMIs despite radically different health profiles. Studies show BMI misclassifies body composition in over 54% of people compared to DEXA scanning.
2. It ignores where fat is stored. Visceral fat (around your organs) is far more dangerous than subcutaneous fat (under the skin). BMI measures neither.
3. It varies by ethnicity. People of South and East Asian descent develop metabolic disease at lower BMI thresholds. WHO recommends using ≥23 as overweight and ≥27.5 as obese for Asian populations.
4. It changes with age. Adults over 65 often have better outcomes at BMIs of 25–27, which would be classified as overweight. This is known as the obesity paradox.
5. It treats men and women the same. Women naturally carry 8–12 percentage points more body fat than men at the same BMI due to hormonal differences.
Better Alternatives to BMI
For a more complete picture, use BMI alongside: body fat percentage (use our body fat calculator), waist-to-height ratio (target below 0.5 — use our waist-to-height calculator), and waist circumference (above 102cm for men and 88cm for women indicates elevated metabolic risk).
Frequently Asked Questions
References
- World Health Organization. BMI Classification. WHO Global Database on BMI, 2021.
- Romero-Corral A et al. Accuracy of BMI in diagnosing obesity. Int J Obesity, 2008.