Body Mass Index (BMI) is the most widely used indicator in the world to assess whether a person's weight is appropriate for their height. Although imperfect (it does not distinguish between muscle mass and fat mass), it remains the international medical reference for defining overweight and obesity thresholds.
BMI is calculated by dividing weight in kilograms by the square of height in meters. The WHO (World Health Organization) has defined universal thresholds to classify the result. These thresholds apply to adults aged 18 to 65. For children, elderly people, pregnant women and elite athletes, interpretation should be adapted.
📐 Formula
BMI = Weight (kg) / [Height (m)]² or BMI = Weight (lbs) × 703 / [Height (inches)]²
📊 Reference table
| BMI | WHO Category | Associated risks |
|---|---|---|
| < 16.0 | Severe underweight | Malnutrition, deficiencies |
| 16.0 – 18.5 | Underweight | Nutritional deficiencies |
| 18.5 – 25.0 | Normal weight | Minimal risk |
| 25.0 – 30.0 | Overweight | Slightly increased risk |
| 30.0 – 35.0 | Obesity class 1 | High risk |
| 35.0 – 40.0 | Obesity class 2 | Very high risk |
| > 40.0 | Obesity class 3 (morbid) | Extreme risk |
💡 Practical examples
Example 1: person weighing 154 lbs, height 5'9" (175 cm)
BMI = 154 × 703 / (69)² = 108,262 / 4,761 = 22.7. Category: normal weight. Optimal result according to WHO.
Example 2: person weighing 198 lbs, height 5'7" (170 cm)
BMI = 198 × 703 / (67)² = 139,194 / 4,489 = 31.0. Category: obesity class 1. Ideal weight range for this height: 118–159 lbs.
Example 3: BMI limitations for athletes
An athlete weighing 187 lbs and 6'0" tall has BMI = 25.4 (overweight). But their body fat is low and muscle mass is high. BMI alone is insufficient for athletes.