Two formulas, one measurement
Body surface area estimates the total area of skin covering a person, in square meters, from just height and weight. Nobody wraps a tape measure around a patient before dosing chemotherapy, so BSA is always estimated, and two formulas dominate clinical use. The older one comes from Du Bois and Du Bois, who in 1916 took direct surface measurements of nine people (wrapping strips of paper over their bodies and totaling the area) and fit a power-law equation to that small dataset: BSA = 0.007184 × height(cm)^0.725 × weight(kg)^0.425.
Seventy-one years later, statistician Richard Mosteller proposed a simpler alternative: BSA = √(height(cm) × weight(kg) ÷ 3600), a single square root instead of two fractional exponents. Mosteller validated it by checking how closely it tracked Du Bois's numbers, not by remeasuring anyone directly, and it has since become the more commonly used formula in US clinical practice specifically because it is faster to compute by hand.
Where the two formulas diverge
For a person of average build, the two formulas land within roughly half a percent of each other. A 170 cm, 65 kg adult gets 1.754 m² from Du Bois and 1.752 m² from Mosteller, a difference too small to matter for any practical purpose. Run the same height at 100 kg instead of 65, though, and Mosteller starts running ahead: at 170 cm and 150 kg, Du Bois gives 2.502 m² while Mosteller gives 2.661 m², a gap of about 6%.
That spread traces back to how each formula was built. Du Bois derived his equation from just nine people of unstated build, and Mosteller's formula was fit to track Du Bois's numbers rather than measured against anyone directly, so neither one was calibrated on someone who looks like a 170 cm, 150 kg adult. This calculator shows both results side by side so that gap is visible rather than hidden: when the two formulas land close together, either number is fine to trust; when they start pulling apart, that's a sign the person being measured is outside the range either formula was really built for.
What BSA gets used for
BSA is not a weight-screening number the way BMI is. It does not have "healthy" or "overweight" categories. Its main clinical uses are all about scaling something else to body size more precisely than raw weight allows. Kidney function (glomerular filtration rate, GFR) is conventionally reported "per 1.73 m²" because 1.73 m² was the average adult BSA measured when that convention was set, and normalizing to it lets a small person's and a large person's kidney function be compared on the same scale. National Kidney Foundation guidance notes that this indexed number is what gets compared against normal ranges, while a non-indexed GFR (not divided by 1.73 m²) is what actually gets used for drug dosing, since dosing adequacy depends on absolute filtration capacity rather than filtration capacity scaled to a hypothetical 1.73 m² body.
Chemotherapy is the other major use: many regimens specify a dose "per m²" of BSA rather than a flat milligram amount, on the theory that BSA tracks blood volume and metabolic rate better than weight alone. Cardiac index (cardiac output divided by BSA) is a third, used to compare heart pump performance across differently sized patients. None of these uses require picking one formula as objectively "correct." Mosteller and Du Bois are both accepted, and a clinician's protocol usually specifies which one to use.