BMI & Body Surface Area Calculator
One height, one weight, every derived number you actually use: BMI with WHO categories, BSA by Mosteller and Du Bois — and where each shows up in real decisions, from Khorana's BMI item to BSA-indexed echo measurements.
Reviewed by Zaka Ahmed, MD Clinical reference · 3 min read
BMI & Body Surface Area
Enter height and weight; all derived values update live.
BMI category and both BSA estimates appear here.
Derived values
Educational tool only. BMI categorizes populations better than individuals — athletes, sarcopenic elders, and edematous inpatients all break it — and no derived number replaces clinical assessment.
Clinical notes & interpretation Scoring guidance, pitfalls, FAQs, and references
Where each number actually earns its keep
BMI is an epidemiologic tool that clinical medicine borrowed: it feeds risk scores (Khorana's ≥35 item, bariatric and anesthesia thresholds, obesity documentation that changes E/M complexity) and frames counseling, but it cannot distinguish muscle from fat and misclassifies the very patients — sarcopenic elders, edematous inpatients — a neurologist sees most. BSA is the quieter workhorse: chemotherapy dosing, BSA-indexed echocardiographic measurements (aortic root, LV mass, valve areas — relevant every time a stroke workup echo report says "indexed"), and renal-function normalization all run through it. Mosteller's square-root formula has become the default for its accuracy-to-arithmetic ratio; Du Bois persists as the reference standard the others are validated against.
Obesity and stroke, briefly and honestly
Obesity raises stroke risk mostly through its mediators — hypertension, diabetes, AF, and sleep apnea — which is where the treatment leverage lives. The literature's "obesity paradox" (better post-stroke survival at higher BMI in some cohorts) is confounded and not a counseling point. What is a counseling point in 2026: GLP-1 receptor agonists have cardiovascular outcome data including stroke reduction in high-risk patients, making the weight conversation at the secondary-prevention visit more concretely actionable than it has ever been.
Frequently asked questions.
Which BSA formula should I use?
Mosteller for essentially everything at the bedside — it agrees closely with Du Bois across the adult range and is simple enough to sanity-check mentally. Institutions occasionally mandate a specific formula for chemotherapy; follow the protocol.
What are the WHO BMI categories?
Underweight <18.5, normal 18.5–24.9, overweight 25–29.9, obesity class I 30–34.9, class II 35–39.9, class III ≥40 kg/m². Some Asian-population guidance uses lower action thresholds.
Does BMI change any acute stroke decision?
Directly, almost none — thrombolytic dosing uses weight (capped at 90 kg-dose for alteplase's 90 mg maximum), not BMI. Indirectly, obesity flags the sleep-apnea and metabolic workup that belongs in secondary prevention.
References.
- Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317(17):1098. PubMed
- Du Bois D, Du Bois EF. A formula to estimate the approximate surface area if height and weight be known. 1916. Nutrition. 1989;5(5):303–311. PubMed
- Marso SP, Daniels GH, Brown-Frandsen K, et al. Liraglutide and cardiovascular outcomes in type 2 diabetes (LEADER). N Engl J Med. 2016;375(4):311–322. PubMed