Body Mass Index

Body mass index (BMI) is a weight-for-height index defined as body weight in kilograms divided by the square of height in metres (kg/m2). It is the most widely used anthropometric indicator for classifying underweight, normal weight, overweight, and obesity at the population level, valued for being simple, inexpensive, and reproducible.

Definition

Body mass index is the ratio of body weight in kilograms to the square of height in metres (kg/m2), used as a standardized index of relative weight and as a screening indicator of underweight, overweight, and obesity.

Scope

This entry covers the definition and computation of BMI, its origin as the Quetelet index, the standard adult cut-off categories, the use of age- and sex-specific references in children, and the recognized limitations of BMI as a proxy for adiposity. It treats BMI as a measurement concept within nutritional assessment, not as a basis for individual diagnosis or treatment.

Key concepts

• Weight divided by height squared (kg/m2)
• Quetelet index (historical origin)
• Adult cut-offs: underweight, normal, overweight, obesity
• Age- and sex-specific references in children
• BMI as a proxy for adiposity
• Limitations: muscularity, age, ethnicity, fat distribution

Mechanisms

BMI standardizes body weight for height so that individuals of different stature can be compared on a single scale. Keys and colleagues (1972) compared several relative-weight indices and showed that weight divided by height squared correlated well with body fatness while being largely independent of height, which is why this index was favoured over alternatives such as weight/height or weight/height cubed. The index does not distinguish fat mass from fat-free mass, so its relationship to adiposity varies with age, sex, muscularity, and ethnicity, as quantified for example by Gallagher and colleagues (2000).

Clinical relevance

BMI is used as a screening indicator to categorize weight status and to track trends in populations; in adults the conventional thresholds are 18.5, 25, and 30 kg/m2. It describes how weight status is classified rather than prescribing treatment, and because it does not measure body composition directly, its interpretation for an individual requires clinical judgement alongside other measures.

Epidemiology

BMI is the principal index used to monitor overweight and obesity prevalence in national and international surveillance. For children, Cole and colleagues (2000) derived internationally comparable, age- and sex-specific cut-offs anchored to the adult thresholds, enabling consistent estimates of child overweight and obesity across countries.

History

The index originates with the nineteenth-century statistician Adolphe Quetelet, after whom it was long called the Quetelet index. Keys and colleagues (1972) coined the term 'body mass index' and established it as the preferred weight-for-height index after comparing it against competing measures of relative weight, and it was subsequently adopted internationally for classifying weight status in adults and, with age-specific references, in children.

Debates

Is BMI an adequate measure of adiposity?

BMI cannot separate fat mass from lean mass, so at a given BMI body fatness differs by age, sex, and ethnicity; this limits its accuracy as an individual measure of adiposity and motivates complementary measures such as body composition and waist circumference.

Key figures

• Ancel Keys
• Adolphe Quetelet
• Timothy Cole
• Dympna Gallagher

Seminal works

• keys-1972
• cole-2000
• gallagher-2000

Frequently asked questions

How is BMI calculated?

BMI equals body weight in kilograms divided by height in metres squared (kg/m2).

Why is BMI sometimes criticized?

Because it reflects total weight relative to height and does not distinguish fat from muscle or account for fat distribution, so it can misclassify adiposity in muscular individuals or across different ages and ethnic groups.

Is BMI interpreted the same way in children?

No. In children and adolescents BMI is interpreted against age- and sex-specific reference values rather than the fixed adult cut-offs, because body composition changes with growth.