What Is Body Mass Index?

Body mass index, abbreviated BMI, is a simple anthropometric ratio defined as body mass in kilograms divided by the square of height in meters, expressed in units of kg/m squared. It was formulated in the 1830s by the Belgian astronomer and statistician Adolphe Quetelet, who observed that the weight of adults of normal build scaled roughly with the square of stature rather than the cube, and it was known as the Quetelet index until Ancel Keys renamed it in a 1972 comparison of obesity indices. Its purpose is to normalize weight for body size so that individuals and populations of differing height can be compared on a single scale.

The index is a screening measure rather than a diagnosis. It requires only a scale and a stadiometer, produces a number in seconds, and can be applied to millions of records collected for other purposes, which explains its adoption in epidemiology, insurance underwriting, and clinical triage. What it does not do is measure body fat. It measures total mass against height and infers adiposity statistically from that relationship.

Classification Thresholds

For adults, the World Health Organization and the National Institutes of Health apply the same cut points: below 18.5 is classified as underweight, 18.5 to 24.9 as normal weight, 25.0 to 29.9 as overweight, and 30.0 and above as obesity, subdivided into three classes at 35 and 40. These boundaries were set from observed associations between the index and mortality in largely European-ancestry populations, and several health authorities apply lower thresholds for people of South and East Asian descent, where cardiometabolic risk rises at lower index values. In children and adolescents the raw value is not interpreted directly. It is converted to an age- and sex-specific percentile against reference growth charts, since the ratio changes substantially through normal development, a procedure described in the CDC's guidance on using body mass index with growth charts.

Limits as a Measure of Adiposity

The ratio cannot distinguish fat mass from lean mass, nor can it locate where fat is stored. A muscular athlete and a sedentary person of the same height and weight receive identical scores despite very different body composition, and visceral adipose tissue, which carries most of the cardiometabolic risk, is invisible to the calculation. Reference methods including dual-energy X-ray absorptiometry, air displacement plethysmography, bioelectrical impedance analysis, and deuterium dilution measure composition directly, and studies comparing body mass index against percent body fat and leptin as measures of adiposity show substantial misclassification at the individual level. Waist circumference and waist-to-height ratio capture central adiposity and predict cardiovascular outcomes better in many cohorts.

Current Clinical Debate

Clinical bodies have moved toward treating the index as one input rather than a definition. A 2025 Lancet commission proposed distinguishing clinical obesity, defined by organ dysfunction or functional limitation, from preclinical obesity, and recommended confirming excess adiposity with a second measurement before diagnosis. A review of the strengths and limitations of the index in diagnosing obesity weighs its predictive value at population scale against its poor specificity for individuals. The consensus position is that the measure remains useful for surveillance and risk stratification across groups while being insufficient on its own for clinical decisions about a single patient.

Applications

Body mass index is used in a range of fields, including:

• Population health surveillance and epidemiological cohort studies
• Clinical screening, obesity treatment eligibility, and bariatric surgery criteria
• Pediatric growth monitoring against reference percentiles
• Drug dosing and anesthesia risk assessment
• Wearable devices, smart scales, and consumer health applications
• Insurance actuarial modeling and occupational fitness standards