Four published BSA formulas side by side, with the spread between them shown — because which one a clinician uses genuinely changes the answer.
Body surface area is the total external area of the body, in square metres. It matters clinically because a great many physiological quantities scale better with surface area than with weight — cardiac output, metabolic rate, glomerular filtration, and the distribution and clearance of many drugs.
The everyday consequence is dosing. Chemotherapy agents are almost universally prescribed in mg/m² rather than mg/kg, on the reasoning that surface area tracks metabolic capacity more closely than mass does. Renal function is reported normalised to 1.73 m², the surface area of a notional average adult, so that results are comparable between a large adult and a small one.
| Formula | Year | Expression |
|---|---|---|
| Du Bois | 1916 | 0.007184 × W0.425 × H0.725 |
| Gehan & George | 1970 | 0.0235 × H0.42246 × W0.51456 |
| Haycock | 1978 | 0.024265 × H0.3964 × W0.5378 |
| Mosteller | 1987 | √(H × W / 3600) |
Height is in centimetres and weight in kilograms throughout. Du Bois is the original and was derived from just nine subjects, one of whom was a child with a growth disorder — a fact that gets mentioned whenever the formula's continued dominance is questioned. Mosteller, published as a one-paragraph letter to the New England Journal of Medicine, is by far the easiest to compute mentally and has become the most widely used in practice for exactly that reason.
For an adult 180 cm and 75 kg, the four formulas give 1.936, 1.942, 1.938 and 1.944 m². That spread is under half a percent, which is why arguments about which formula to use are usually academic for average-sized adults.
They diverge more at the extremes — in small children, and in patients with obesity — which is where the choice starts to matter and where institutional protocols specify one formula rather than leaving it open. That is also why this page shows all four rather than picking one: the spread is information.
The figure comes from a 1927 study reporting the average body surface area of American adults. It has no particular physiological significance and it no longer matches the average of any real population. It survives purely as a normalising constant — it lets eGFR results be compared across people and across decades, which is the entire point. There have been periodic proposals to update it and none have been adopted, because the disruption to comparability would outweigh the benefit.
The practice is entrenched but the evidence behind it is thinner than its ubiquity suggests. BSA-based dosing was adopted for cytotoxic chemotherapy in the 1950s, partly from animal-to-human dose scaling, and its ability to reduce variability in drug exposure has been repeatedly questioned since. For many agents, the between-patient variation in clearance is driven by liver enzyme activity or renal function rather than by size, and normalising to BSA does not capture either.
It remains standard nonetheless, because it is what the trial evidence was generated with and because changing dosing conventions for narrow-therapeutic-index drugs is not a small undertaking. Newer targeted therapies are increasingly given at flat doses, which is a quiet acknowledgement of the point.
Mosteller is the most widely used in clinical practice, largely because it is simple enough to compute without a calculator and agrees closely with the others for average-sized adults. What actually matters is consistency: an institution or protocol should specify one formula and use it throughout, because switching between formulas mid-treatment changes the dose slightly for no clinical reason. If a protocol names a formula, use that one.
Very little for average adults — typically under 1% between the four. The disagreement grows at the extremes of size, particularly in small children and in patients with obesity, where the underlying datasets each formula was fitted to differ most. This page shows all four and reports the spread precisely so you can see when the choice matters and when it does not.
It normalises kidney function to a standard body size so results can be compared between people. The 1.73 m² figure comes from a 1927 estimate of average adult body surface area in the United States — it has no physiological meaning and no longer matches any actual population average. It survives as a convention because comparability across patients and across decades of published data is worth more than an updated constant would be.
For some drugs, and less clearly than the practice implies. BSA dosing was adopted for cytotoxic chemotherapy in the 1950s and remains standard, but studies since have repeatedly questioned how much it actually reduces variability in drug exposure. For many agents the dominant source of variation is liver enzyme activity or renal clearance, neither of which BSA captures. Many newer targeted therapies are given at flat doses instead.
No. Dose calculation is a clinical task with checks built around it, and the formula used, the rounding convention, dose capping in obesity, and adjustments for organ function all vary by protocol and by drug. A BSA figure from a website is missing all of that context. Use this page to understand what the measure is, and leave the arithmetic that determines what goes into a patient to the prescribing and pharmacy team.
Related: eGFR Calculator · BMI Calculator · Lean Body Mass Calculator · Ideal Weight Calculator