Body mass index

BMI = Body weight (kg) ÷ Height (m)²

For someone who is 170cm tall and weighs 65kg, the calculation is 65 ÷ (1.7 × 1.7), giving approximately 22.5. Directly comparing the weights of people of different heights is difficult, but converting them to BMI makes it easier to compare body size on the same basis.

This measure originated as the “Quetelet index,” proposed by the 19th-century Belgian statistician Adolphe Quetelet. In 1972, American physiologist Ancel Keys and his research team called it “body mass index (BMI)” in a paper comparing several measures of body size, helping establish its current name. It subsequently became a standard measure for examining body size in populations.

Under World Health Organization (WHO) criteria, adults are classified as underweight at a BMI below 18.5, normal weight at 18.5–24.9, overweight at 25–29.9, and obese at 30 or above. Based on reports that people in Asian regions, including Korea, tend to have more body fat at the same BMI, the Korean Society for the Study of Obesity classifies 23–24.9 as pre-obesity and 25 or above as obesity. Because height and weight continue to change with age in children and adolescents, they are assessed using growth-chart percentiles rather than adult ranges.

These ranges are classification tools used in public health to describe different levels of statistical risk. The same cutoff may be interpreted somewhat differently during growth, pregnancy, older age, or in people with substantial muscle mass.

BMI expresses the relationship between weight and height. Even at the same BMI, body composition varies with muscle mass, skeletal build, age, sex, and race. Someone with well-developed muscles from extensive exercise may have a high BMI but little body fat. Conversely, someone with an average weight may have sarcopenic obesity, with low muscle mass and high body fat. For a more detailed understanding of an individual's body, body fat percentage can therefore also be useful.

A cross-sectional study analyzing 13,601 American adults reported that a BMI of 30 or above identified obesity defined by body fat percentage fairly accurately, but did not identify everyone with high body fat. BMI can be viewed as a simple first step that identifies some people with high body fat.

An analysis pooling data from more than 220,000 people across 58 prospective cohorts reported that BMI, waist circumference, and waist-to-hip ratio were each associated with the development of cardiovascular disease. However, adding waist circumference or waist-to-hip ratio after accounting for established risk factors reportedly did not substantially improve risk prediction.

These studies show trends within populations. An individual's health is not determined by BMI alone; many factors work together, including diet, activity levels, sleep, genetics, and family history.

• Try using it to track your own changes. Looking at weight measured on the same scale at a similar time of day, along with BMI, over weeks and months can help reveal long-term trends.
• Try measuring your waist circumference as well. A tape measure alone can help you gauge whether fat is concentrated around your abdomen. See Anthropometry for the method.
• If you exercise, check body fat percentage as well. When strength training increases muscle mass, your body shape can change even if your BMI stays the same.
• Numbers are not the whole picture of health. Considering fitness, how you feel, and eating habits together gives you a much more balanced understanding of your body's condition.