Body mass index

Body mass index (BMI) is a numerical value used to assess the relationship between an individual’s body mass and height. It serves as a widely applied indicator for categorising weight status across populations and is particularly valued for its simplicity. Although BMI does not directly measure body fat, it provides a practical estimate that supports public health assessments and helps identify broad weight-related trends.
BMI is calculated by dividing a person’s mass by the square of their height, with results expressed in kilograms per square metre (kg/m²). This measure can be obtained using a weighing scale and a stadiometer, with calculations performed manually, via calculator, or using lookup tables that display BMI values according to height and weight.

Definition and Method of Measurement

The standard definition of BMI is based on the formula:

• BMI = mass (kg) ÷ height (m)²

This calculation yields a single figure that can be compared against established weight categories. When imperial units are used, adjustments are required:

• BMI = mass (lb) ÷ height (in)² × 703
• When pounds and feet are combined, a conversion factor of 488 applies.

Lookup charts are often used in clinical and educational settings. These charts typically illustrate contours or colour gradients to represent categories such as underweight, normal weight, overweight, and obesity. They may also assist in conversions from non-metric units to metric units for calculation purposes.

Adult BMI Classifications

BMI provides a convenient rule of thumb for broadly classifying adults according to body size, taking tissue mass (muscle, fat, and bone) and height into account. Widely used adult classifications include:

• Underweight: less than 18.5 kg/m²
• Normal weight: 18.5–24.9 kg/m²
• Overweight: 25–29.9 kg/m²
• Obesity: 30 kg/m² or greater

These categories were established for population assessment rather than individual diagnosis. Research indicates that BMIs below 20 and above 25 are associated with increased all-cause mortality, with risks rising as values deviate further from the 20–25 range.

Historical Development

The conceptual foundation of BMI was established by Adolphe Quetelet, a Belgian polymath active in the fields of astronomy, mathematics, statistics, and sociology. Between 1830 and 1850, he developed what was initially termed the Quetelet Index as part of his pursuit to define the characteristics of the “average man” within his theory of social physics. Quetelet never intended the index for medical purposes; it served instead as a statistical tool reflecting normative human attributes.
His ideas influenced later thinkers such as Francis Galton, whose work expanded the study of human averages and contributed to early developments in eugenic thought. The terminology body mass index was introduced in 1972 by Ancel Keys and colleagues, who evaluated multiple indices of relative weight. Keys concluded that BMI was at least as effective as any other simple index for representing relative obesity in population studies, though he stressed its limitations for individual diagnosis. As obesity rates increased in Western societies, BMI became widely used in clinical and public health contexts.

Interpretation, Strengths, and Limitations

BMI is valued for its ease of use, cost-effectiveness, and applicability to large populations. It offers a simple numerical indicator that allows health professionals to discuss weight status in objective terms and helps public health authorities monitor national and global trends.
However, several limitations reduce BMI’s effectiveness when applied to individuals:

• Body composition variation: People with high muscle mass, such as trained athletes, may be classified as overweight or obese despite having low body-fat levels.
• Fat distribution: BMI does not measure abdominal or visceral fat, which is strongly associated with metabolic risk.
• Ethnic variations: The relationship between BMI, body fat percentage, and disease risk varies across populations.
• Special populations: Shorter individuals and bodybuilders often receive misleading BMI assessments.

Because of these constraints, supplementary measurements such as waist circumference, waist-to-hip ratio, and body-fat percentage may provide more accurate assessments of health risks.

BMI for Children and Adolescents

For individuals aged 2 to 20, BMI is calculated in the same manner as for adults but interpreted using age- and sex-specific percentiles. These percentiles reflect typical growth patterns and allow comparison with peers of the same demographic group:

• Below 5th percentile: underweight
• 85th to 95th percentile: overweight
• Above 95th percentile: obese

British studies from 2013 suggest that females aged 12 to 16 generally exhibit higher BMI values than males of the same age by approximately 1.0 kg/m² on average, reflecting differences in growth and body composition during adolescence.

International Variations in BMI Thresholds

BMI cut-off points differ across regions due to variations in body composition, fat distribution, and disease risk. These differences make global comparisons more complex and must be considered in public health planning.

• Asian populations tend to develop metabolic disorders such as type 2 diabetes and atherosclerotic cardiovascular disease at lower BMI thresholds than Western populations. Additional cut-off values have been introduced for at-risk groups, including values of 23, 27.5, 32.5, and 37.5.
• Hong Kong uses BMI ranges adapted to local health trends.
• Japan employs categories defined by the Japan Society for the Study of Obesity.
• Singapore revised its BMI ranges in 2005 to reflect increased health risks in Asian populations with higher body-fat percentages at lower BMI values.
• United Kingdom: NICE guidance recommends different BMI thresholds for early diabetes prevention among various ethnic groups. For example, intervention may begin at BMI 30 for White populations but at 24–27 for several Asian and Black subgroups.
• United States: In 1998, national standards were aligned with WHO guidelines, lowering the overweight boundary from 27.8 (men) and 27.3 (women) to 25. This reclassification increased the proportion of Americans considered overweight and influenced public discourse on obesity. Later surveys documented rising obesity levels, with approximately 37.7% of adults classified as obese by 2014.

These variations reflect an expanding understanding of how genetic, cultural, and environmental factors influence the relationship between BMI and health outcomes.

Public Health Applications

BMI remains an important indicator for population health surveillance. It allows governments, health organisations, and researchers to:

• Identify trends in undernutrition and obesity
• Allocate healthcare resources
• Develop targeted preventative programmes
• Evaluate the impact of nutrition and lifestyle policies