Body mass index (BMI)

How BMI is calculated

The BMI value is computed as weight in kilograms divided by height in metres squared (kg/m2). For example, a person weighing 70 kg and 1.75 m tall has a BMI of 70 / (1.75 × 1.75) ≈ 22.9. In countries using pounds and inches, the same result can be obtained with conversion factors or dedicated calculators.

Common adult categories

• Underweight: BMI < 18.5
• Normal (healthy) weight: BMI 18.5–24.9
• Overweight: BMI 25–29.9
• Obesity: BMI ≥ 30 (often subdivided into class I, II and III)

These cutoffs are used for adults and provide a consistent framework for surveillance and risk assessment, but many organizations, including the World Health Organization (WHO), emphasize that BMI is a screening tool rather than a definitive diagnosis.

History and development

The measure originated in the 19th century as the "Quetelet index," named after the Belgian statistician who described a population‑level relationship between weight and height. In the 20th century it became widely adopted in medicine and epidemiology as obesity emerged as a public‑health concern.

Uses, strengths and limitations

BMI is most useful for estimating weight status in populations, tracking trends over time, and as an initial clinical screen. It is inexpensive and noninvasive. However, BMI does not distinguish between fat and lean mass, nor does it show fat distribution. Important limitations include:

• Possible misclassification of very muscular individuals or those with low muscle mass (older adults).
• Variation by age, sex and ethnic groups—risk associated with a given BMI can differ between populations.
• Children and adolescents require age‑ and sex‑specific percentiles rather than adult cutoffs.

Because of these limitations, BMI is often combined with other measures—waist circumference, body composition testing or clinical assessment—to better estimate health risk. Used thoughtfully, BMI remains a practical starting point for discussing weight, risk and lifestyle interventions.