I. How to Use
When to Use
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BMI: This calculation is routinely used during clinical evaluations to estimate the appropriateness of an individual’s weight relative to their height, serving as a preliminary step in identifying patients who are underweight or overweight.
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BSA: This formula estimates an individual’s total external surface area, aiding in calculating dosages for medications, such as chemotherapy, and measuring cardiac index.
Pearls / Pitfalls
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Always use the most recent height and weight measurements to ensure accuracy.
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Remember that both BMI and BSA are part of a larger clinical context and should be used in conjunction with other assessments and patient history.
BMI
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Does not distinguish between fat and muscle mass.
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Tends to overestimate body fat in individuals with very high muscle mass who are otherwise healthy, and can underestimate it in older adults due to age-related muscle loss.
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The typical classification may underestimate the risk of cardiovascular disease associated with obesity in Asian populations. Organizations like the Wold Health Organization (WHO) and National Institutes of Health (NIH) recommend lower BMI thresholds for these individuals. Please see Evidence for more details.
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BMI is based primarily on data collection from non-Hispanic white populations, making its generalizability prone to inaccuracy.
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Some organizations, such as the AMA, have recognized the importance of using a variety of measures to determine an appropriate risk assessment and advise clinicians to eliminate the use of BMI as a sole indicator of risk.
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BMI should be utilized in conjunction with measurements of visceral fat, body adiposity index, waist circumference and other metabolic/genetic factors when diagnosing obesity (Rubino et al 2025).
BSA
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Multiple formulas for BSA are available. This tool utilizes the Mosteller formula.
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May be less precise for neonates and infants, as well as for individuals at extreme values of height and weight.
Why Use
BMI provides a quick estimate of adiposity, while BSA calculation helps estimate an individual’s total external surface area, which is difficult to measure directly.
II. Next Steps
Advice
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For patients identified as having a diagnosis of underweight or overweight, consider further nutritional or physical assessments and referrals as appropriate.
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Individuals with a diagnosis of overweight or obese should be counseled on diet and exercise.
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In cases involving drug dosing, especially chemotherapy, regular monitoring and dose adjustments based on changes in BSA are necessary to optimize therapeutic outcomes.
Management
Management depends not on BMI or BSA measurements alone, but in conjunction with other body measurements and the clinical picture.
Critical Actions
Diagnosis of obesity can be made with BMI measurement of >30 kg/m2 alone, and no further confirmative testing/measurement is required (Rubino et al 2025).
III. Evidence
Formula
Body mass index, kg/m2 = weight, kg / (height, m)2
Body surface area (the Mosteller formula), m2 = [ Height, cm x Weight, kg / 3600 ]1/2
Facts & Figures
Literature
Original/Primary
https://www.ncbi.nlm.nih.gov/pubmed/16768059
Gadzik J. ‘How much should I weigh?’ Quetelet’s equation, upper weight limits, and BMI prime.Connecticut Medicine. (2006). 70 (2): 81–8. PMID 16768059.
Validation
https://www.who.int/bmi/index.jsp?introPage=intro_3.html
BMI Classification. Global Database on Body Mass Index. World Health Organization. 2006. Retrieved July 27, 2012.
Other References
Recent AMA recommendation from 6/14/2023 with regard to BMI measurement and alternate measures for the diagnosis of obesity: https://www.ama-assn.org/press-center/press-releases/ama-adopts-new-policy-clarifying-role-bmi-measure-medicine
Up-to-date recommendation on diagnostic methods for obesity: https://pubmed.ncbi.nlm.nih.gov/39824205/
Rubino, Francesco et al. Definition and diagnostic criteria of clinical obesity. The Lancet Diabetes & Endocrinology, (March 2025) Volume 13, Issue 3, 221 – 262