Body surface area is one of the most useful single numbers in clinical medicine. It scales drug doses and physiologic measurements to body size more faithfully than weight alone, which is why it underpins chemotherapy dosing and metrics like cardiac index. This guide explains what BSA is used for, why five different formulas exist, which ones clinicians prefer, and why their answers diverge.
What BSA is used for
The most common clinical use of BSA is chemotherapy dosing. Many cytotoxic drugs are prescribed in milligrams per square metre (mg/m²) because surface area correlates better than body weight with metabolic rate, blood volume, and organ function — the physiologic quantities that govern how a drug is distributed and cleared. Multiplying the per-m² dose by the patient's BSA yields a starting dose, which is then adjusted for caps, organ function, and prior tolerance.
BSA also appears throughout cardiology and critical care. Cardiac output is divided by BSA to give the cardiac index (L/min/m²), and many echocardiographic measurements are indexed to BSA so they can be compared across patients of different sizes. Burn assessment, glomerular filtration rate normalisation, and some paediatric fluid calculations all lean on surface area as well.
Why there are five formulas
Directly measuring surface area is impractical, so every formula is a regression fitted to a sample of people whose surface area was measured by coating or photographic methods. Du Bois and Du Bois published the first widely used equation in 1916 from only nine subjects. Later researchers — Boyd (1935), Gehan and George (1970), Haycock (1978), and Mosteller (1987) — refit the relationship using larger or more diverse samples, or simplified it for bedside use.
Because each formula comes from a different dataset and uses different exponents, they give slightly different answers for the same height and weight. Mosteller won wide adoption precisely because it is the easiest to compute — a single square root — while remaining accurate enough for clinical dosing. Du Bois remains the historical reference, and Haycock and Gehan-George were validated with children in the sample, which is why they are favoured in paediatrics.
Why the results differ — and which to use
For a typical adult, the five formulas usually agree to within two or three percent. The differences widen at the extremes: very low or very high body weight, and small children, are where the formulas were least constrained by their original data. Boyd's weight-dependent exponent makes it comparatively stable across a wide weight range, while Du Bois tends to underestimate in obesity.
The practical rule is consistency over precision. No formula is the single 'true' BSA — they are all estimates of an unmeasurable quantity. What matters clinically is that a care team picks one formula and applies it consistently for a given patient and protocol, because switching formulas mid-course would change the per-m² dose without any change in the patient. Use Mosteller as the default for adults, Haycock or Mosteller for children, and follow whatever your institution's protocol specifies.