What BMI can and cannot tell you
How body mass index is calculated, why it is used for populations, where it misleads individuals, and what to look at alongside it.
· Awaiting human editorial review
A simple ratio
Body mass index divides weight in kilograms by the square of height in metres. Because it needs only two measurements, it is cheap and consistent, which makes it useful for studying large groups of people and spotting population trends.
For adults, the World Health Organization classifies a BMI below 18.5 as underweight, 18.5 to 24.9 as a healthy weight, 25 to 29.9 as overweight and 30 or above as obesity.
Where BMI falls short
BMI cannot tell muscle from fat, so very muscular people can be classified as overweight while carrying little fat. It also says nothing about where fat is stored, and fat around the abdomen is more strongly linked with metabolic risk than fat elsewhere. Some health bodies use lower BMI cut-offs for people of Asian heritage, because risk can appear at lower BMIs.
BMI categories for adults do not apply to children and teenagers, who are assessed with growth charts by age and sex, or during pregnancy.
Useful companions
Waist-to-height ratio is a simple complement: keeping your waist under half your height is a widely used guide. Body-fat estimates from tape measurements give another angle, with their own margin of error.
None of these numbers is a diagnosis. They are prompts for a conversation with a health professional, who can consider blood pressure, blood tests, family history and how you feel.