Body surface area:
the Mosteller formula and why doctors use it

What body surface area measures

Body surface area (BSA) is an estimate of the total skin area of the body in square metres. A typical adult measures between 1.6 and 2.0 m², a newborn about 0.25 m², and a large adult may exceed 2.2 m². Measuring skin area directly is impractical, so doctors rely on formulas built from height and weight.

The reason it matters is that many physiological quantities scale better with surface area than with weight. Heat loss, metabolic rate and cardiac output track body surface more closely than they track kilograms, which is why a 50 kg person is not simply half of a 100 kg person for dosing or cardiac measurements.

Several equations exist. The Mosteller equation, published in 1987, became popular because it fits in a single line and can be done on a basic calculator, with accuracy close to the older and more complex formulas.

The Mosteller formula

The constant 3600 comes from the original fit; with it, a 60 kg person who is 60 cm tall would have exactly 1 m², which is a handy mental check. In practice, multiply the two numbers, divide by 3600 and press the square-root key.

Unit conversion is the part people trip over. A height of 5 ft 7 in is 67 inches, or about 170 cm, and a weight of 154 lb is about 70 kg. Convert first, then use the metric version; mixing a metric height with a weight in pounds is the surest way to land on a nonsense answer.

A smaller adult of 160 cm and 55 kg works out to √(8,800 ÷ 3600) = √2.444 = 1.56 m². Both results sit inside the usual adult range, so the arithmetic is plausible.

Mosteller compared with DuBois and others

The DuBois and DuBois equation (1916) is 0.007184 × weight^0.425 × height^0.725, and it remains common in textbooks. Haycock and Gehan-George formulas were developed with children in mind. For the 170 cm, 70 kg adult above, DuBois gives 1.81 m² against Mosteller's 1.82 m², a difference of about 0.01 m².

A difference of 0.01 to 0.02 m² amounts to about one percent. For most dosing purposes that is below the precision with which the drug or the measurement is controlled, which is why the choice of formula matters less than being consistent within one treatment protocol.

Where BSA is used in practice

• Oncology dosing: many cytotoxic drugs are prescribed in mg per m². A 75 mg/m² regimen at 1.82 m² comes to 136.5 mg, before any local rounding rules or dose caps.
• Cardiac index: cardiac output divided by BSA, so that heart function can be compared between people of different sizes.
• Burns: percentage of body surface affected is used to plan fluid replacement, using charts that scale with age.
• Renal function: glomerular filtration rate is often reported per 1.73 m² so values are comparable between patients.
• Research and physiology: normalising metabolic rate, heat loss and organ size.

In each case the formula feeds into a protocol with its own rounding, caps and safety checks. A hospital pharmacist or oncologist decides the final amount, not a generic number from the web.

When the number misleads

Equations assume a typical shape. Very obese patients are one concern: surface area computed from total weight can overstate the tissue that actually drives drug clearance, and several guidelines suggest using actual weight rather than idealised weight for chemotherapy, yet still apply dose caps in some settings. The opposite problem occurs in very muscular or very frail patients.

Children need special care. Neonates and small children have a higher ratio of surface to mass, and formulas fit on adult data lose accuracy at the extremes. Paediatric services therefore often use Haycock or nomograms. A reading taken from stale height or weight is also a common source of error: weight changes during treatment, so BSA is recalculated at intervals.

Seeing how weight and height move the result

Because the formula takes a square root, BSA changes more slowly than weight does. Doubling weight does not double surface area; it raises it by about 41 percent, since √2 ≈ 1.41. That damping is why a modest weight change during treatment shifts BSA only slightly.

Reading down the table, a 40 kg difference in weight and a 40 cm difference in height together only move BSA from 1.44 to 2.30 m², a factor of 1.6. This is the practical reason doses scaled by surface area vary less between patients than doses scaled by weight alone would.

Checking your own value

You can verify a BSA by hand in under a minute: multiply height and weight, divide by 3600, take the square root. If the answer is below 1.2 or above 2.5 for an adult, recheck the inputs. A calculator is the quickest way to run several heights and weights side by side, for example to see how a 5 kg weight change shifts BSA by only about 0.06 m².

This article explains the arithmetic. It does not recommend a dose or treatment; individual prescriptions are the job of the treating team.