What is body surface area used for?
Chiefly for dosing drugs where the therapeutic window is narrow — chemotherapy above all — because BSA correlates better with metabolic rate, blood volume and renal clearance than body weight does. It is also used to index cardiac output and glomerular filtration rate, and to estimate fluid requirements in burns care. Outside those clinical uses it has little everyday application.
Which BSA formula should be used?
Du Bois is the historical standard and still common, though it was derived from only nine subjects. Mosteller is the most widely used in practice because it is simple enough to do mentally — the square root of height in cm times weight in kg, divided by 3600 — and agrees closely with the others. Haycock and Gehan-George are often preferred in paediatrics. For most adults the formulas agree within a few percent; they diverge more at extremes of size.
What is a typical BSA?
Around 1.7 square metres for an average adult — roughly 1.6 for women and 1.9 for men, though the spread is wide. Children range from about 0.25 at birth to adult values in adolescence. Many chemotherapy protocols historically capped calculated BSA at 2.0 square metres for very large patients, though that practice has been challenged and guidelines now often recommend using actual BSA.
Why is BSA used instead of weight for drug dosing?
Because many physiological processes scale closer to surface area than to mass. Doubling someone's weight does not double their capacity to metabolise and excrete a drug, so weight-based dosing can overdose larger patients. BSA-based dosing was adopted in oncology for that reason. It is an imperfect proxy — it does not capture body composition, organ function or genetics — and there is ongoing debate about better approaches.
How accurate is a BSA calculation?
It is an estimate from two measurements, so treat it as such. The formulas were validated on limited populations and can be less reliable in obesity, oedema, amputation, pregnancy and extreme body proportions. Because a dosing error matters, clinical practice uses measured height and weight taken on the day, recalculates when weight changes materially, and never relies on a patient-reported figure.
Can I use this to work out my own medication dose?
No. This calculator computes a number; it does not determine a dose. Actual dosing depends on the specific drug, the protocol, renal and hepatic function, prior toxicity, concurrent medication and clinical judgement, and is calculated and checked by prescribers and pharmacists. Use this for understanding or verification against a documented figure, never to alter a dose.