What Is Body Surface Area, and Why Does Medicine Care About It?
Body Surface Area (BSA) is an estimate of the total external surface area of the human body, expressed in square metres. It sounds like an unusual thing to measure, but it turns out to be one of the more clinically useful numbers in medicine, because it correlates more closely with metabolic rate, cardiac output, blood volume, and kidney function than body weight alone does. Two people of very different heights can weigh the same, yet have meaningfully different BSA — and because many physiological processes scale with surface area rather than mass, BSA often gives doctors a more accurate basis for calculations than weight-based dosing would.
Where BSA Is Actually Used
The most well-known clinical application is chemotherapy dosing, where many cancer drugs are prescribed in milligrams per square metre of BSA rather than per kilogram of body weight, because tumour response and drug toxicity have historically been found to correlate better with BSA across a wide range of body sizes. It's also used in calculating cardiac index (a measure of heart function relative to body size), assessing burn severity as a percentage of total body surface, and in some paediatric and critical-care drug dosing protocols, particularly for medications with a narrow therapeutic window where getting the dose right matters enormously.
The DuBois Formula — A Century-Old Standard That Still Holds Up
BSA (m²) = 0.007184 × Height(cm)^0.725 × Weight(kg)^0.425
This formula was developed in 1916 by Delafield DuBois and Eugene DuBois, based on direct surface measurements of a small number of subjects — a study that would raise eyebrows by today's research standards, yet the resulting formula has proven remarkably durable and remains one of the most widely used BSA equations in clinical practice over a century later. For example, imagine a person who's 170cm tall and weighs 65kg. Plugging those numbers in gives a BSA of roughly 1.76 square metres — a figure a doctor might then use directly in a chemotherapy dosing calculation.
Other BSA Formulas You Might Encounter
DuBois isn't the only formula in clinical use. The Mosteller formula, valued for its simplicity, calculates BSA as the square root of (height in cm × weight in kg, divided by 3600) and is popular in fast clinical settings because it's easy to compute even without a calculator. The Haycock formula is often preferred in paediatric contexts, since it was derived using data that included infants and children, populations where the DuBois formula (based on adult subjects) may be somewhat less precise. In practice, these formulas typically agree within a few percent of each other for most adult body types, so the differences rarely change clinical decisions meaningfully, though a treating clinician will typically specify which formula a given dosing protocol expects.
Typical BSA Ranges
Average adult BSA typically falls somewhere around 1.6 to 1.9 square metres, with men generally trending toward the higher end of that range and women toward the lower end, reflecting average differences in height and build. Children have proportionally much smaller BSA relative to their weight compared to adults, which is one of several reasons paediatric drug dosing requires such careful, size-specific calculation rather than simple proportional scaling from adult doses.
An Important Caveat
This calculator is intended for general educational and informational purposes — to help you understand how BSA is estimated and why it matters in medicine. It is not a substitute for clinical calculations performed by a qualified healthcare provider, particularly for anything related to medication dosing, where errors can have serious consequences. Always defer to your treating physician or pharmacist for any dosing decision.