Body Mass Index (BMI) is a screening tool that uses height and weight to estimate body fat. While not perfect, it remains the most common method for classifying underweight, overweight, and obesity in adults.
Formula
BMI = Weight (kg) / Height² (m²)
For imperial units: BMI = (Weight in lb / Height² in inches²) × 703
WHO Classification
- <18.5 — Underweight
- 18.5–24.9 — Normal weight
- 25.0–29.9 — Overweight
- 30.0–34.9 — Obesity Class I
- 35.0–39.9 — Obesity Class II
- ≥40.0 — Obesity Class III (severe)
Limitations
BMI does not distinguish between muscle and fat mass. Athletes with high muscle mass may be classified as overweight despite low body fat. BMI is also less accurate for the elderly, pregnant women, and certain ethnic populations.