What BMI is
Body mass index compares weight with height: the weight in kilograms divided by the square of the height in metres. It is a quick screening figure for adults, not a diagnosis, and it cannot tell muscle from fat.
BMI = weight (kg) ÷ height (m)²
The bands
The World Health Organization’s bands for adults are: under 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, and 30 or more obese. Indian guidelines suggest lower cut-offs for Asian Indians, who tend to carry more body fat at the same BMI: 23 for overweight and 25 for obesity.
Example
At 175 cm and 70 kg, the BMI is 70 ÷ (1.75 × 1.75) = 22.9, in the normal band. For that height, the normal band runs from about 56.7 kg to 76.3 kg.