Body mass index as defined by the WHO, with the healthy weight range.
BMI is body mass in kilograms divided by height in metres squared: BMI = kg / m². For example, 70 kg at 1.75 m gives 70 / (1.75 × 1.75) = 22.9. The formula and the bands used to read it are defined by the World Health Organization (WHO) and are the same for adult men and women.
The World Health Organization (WHO) defines four main adult categories: underweight below 18.5, normal weight from 18.5 to 24.9, overweight from 25.0 to 29.9, and obesity from 30.0. WHO subdivides obesity into three further classes. These cut-offs do not apply to children and adolescents, who are assessed against age- and sex-specific percentiles.
BMI measures mass against height, not body composition, and cannot tell muscle from fat. A very muscular person can score high with low body fat; an older person can score normal with low muscle mass. The World Health Organization (WHO) presents it as a population-level indicator rather than an individual diagnosis.
No. Weight gain during pregnancy is expected and normal, so the World Health Organization's adult cut-offs say nothing useful in that period. For children and adolescents up to 19, separate WHO growth references apply, where the result is read as a percentile for age and sex rather than against a fixed number.