The Body Mass Index has its origins in the work of Adolphe Quetelet, a Belgian astronomer, mathematician, and statistician who in the 1830s sought to define the characteristics of the "average man" (l'homme moyen) through statistical analysis of human physical measurements. Quetelet observed that in adults of normal build, weight tends to vary proportionally to the square of height rather than the cube, which would be expected if humans scaled uniformly in three dimensions. He expressed this as what he called the Quetelet Index: weight in kilograms divided by height in meters squared. This formula lay relatively dormant for over a century until 1972, when American physiologist Ancel Keys published a study comparing various height-weight indices and concluded that Quetelet's index was the best simple proxy for body fat percentage. Keys coined the term "Body Mass Index," and the metric was subsequently adopted by the World Health Organization as a standard screening tool.

The science of body composition is considerably more complex than any single number can capture. The human body consists of fat mass (essential fat and storage fat), lean mass (muscle, bone, organs, and water), and the ratio between these components determines health risk more accurately than total weight alone. BMI does not directly measure body fat; rather, it uses the statistical correlation between the height-weight ratio and body fat percentage observed across large populations. This correlation, while useful for population-level health assessments, breaks down significantly at the individual level in several important cases.

The limitations of BMI as a health metric have been extensively documented in medical literature. Athletes and individuals with high muscle mass often register as overweight or obese by BMI despite having low body fat, because muscle is denser than fat. Conversely, sedentary individuals may have a normal BMI while carrying excess visceral fat (the metabolically dangerous fat surrounding internal organs), a condition sometimes called "normal weight obesity" or colloquially "skinny fat." BMI also fails to account for differences in body composition across age, sex, and ethnicity. Older adults tend to have more fat and less muscle than younger adults at the same BMI. Women typically have higher body fat percentages than men at equivalent BMI values. Studies have shown that BMI thresholds associated with health risks differ across ethnic groups, with some Asian populations showing elevated risk at lower BMI values and some Polynesian populations showing lower risk at higher values.

Despite these limitations, BMI remains widely used because of its simplicity and the strong population-level correlations with health outcomes. The WHO classification system defines BMI below 18.5 as underweight, 18.5 to 24.9 as normal weight, 25.0 to 29.9 as overweight, and 30.0 and above as obese, with further subdivisions for obesity classes. Medical professionals increasingly use BMI as one component of a broader health assessment that includes waist circumference, waist-to-hip ratio, body fat percentage measured by methods like DEXA scans or bioelectrical impedance, blood pressure, blood lipid profiles, and metabolic markers. This multi-metric approach provides a far more accurate picture of individual health than BMI alone.