The Body Mass Index (BMI) is the most widely used measure of whether weight is appropriate for height, adopted by the World Health Organization to classify underweight, normal weight, overweight and obesity in adults.

How the calculation works

The index divides weight in kilograms by height in metres squared: BMI = weight ÷ (height × height). Adult classification: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, and 30+ obese. BMI does not distinguish muscle from fat, so it remains a guide — not a diagnosis.