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What Is BMI?
Body Mass Index (BMI) is a simple measurement that estimates body fat based on height and weight. Used worldwide as a health screening tool for nearly 200 years.
The Basics
BMI is a number calculated from a person's weight and height. It provides a reliable indicator of body fatness for most people and is used to screen for weight categories linked to health risks. Use our free online BMI calculator to instantly find your score in metric or imperial units.
The metric formula: weight (kg) divided by height (m) squared. Imperial: 703 × weight (lbs) ÷ height (in)².
History
Developed by Belgian mathematician Adolphe Quetelet (1830–1850). Originally called the Quetelet Index. Renamed to BMI in 1972 by Ancel Keys, who showed it was the best proxy for body fat percentage among simple anthropometric measures.
Why It Matters
BMI is a screening tool used worldwide. High BMI correlates with increased risk of:
- Heart disease and stroke
- Type 2 diabetes
- Certain cancers
- Sleep apnea
- Osteoarthritis
- High blood pressure
WHO BMI Categories
The World Health Organization defines these standard BMI categories for adults (20+):
| Classification | BMI (kg/m²) |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 – 17 |
| Mild Thinness | 17 – 18.5 |
| Normal (Healthy) | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obese Class I | 30 – 35 |
| Obese Class II | 35 – 40 |
| Obese Class III | > 40 |
Child BMI Calculator: Ages 2–19
For children and teens (ages 2–20), BMI is interpreted using age- and sex-specific CDC percentiles rather than fixed thresholds. This accounts for natural growth patterns:
- Underweight: < 5th percentile
- Healthy: 5th – 85th percentile
- Overweight: 85th – 95th percentile
- Obese: ≥ 95th percentile
The BMI Formula
Metric (kg and cm): BMI = weight (kg) / height (m)²
Imperial (lbs and feet/inches): BMI = 703 × weight (lbs) / height (in)²