Body Surface Area Explained: The Mosteller and Du Bois BSA Formulas

If you have ever read a chemotherapy order and seen a dose written as "75 mg/m²" instead of plain milligrams, you have met body surface area. BSA is the number that turns that "per square metre" into an actual dose, and it quietly underpins a lot of clinical math that has nothing to do with weighing a patient on a scale. This guide walks through what BSA is, how the two formulas you will meet most often actually compute it, and why so many fields prefer it over body weight. It is informational, not a dosing tool.

What body surface area actually measures

Body surface area is exactly what it sounds like: the total external area of a human body, reported in square metres. An average adult lands around 1.7 m², and the 1.73 m² figure printed on dosing nomograms is a long-standing reference body. You cannot wrap a patient in graph paper to measure this directly, so every BSA number you see is an estimate built from two easy inputs, height and weight.

Why bother estimating an area at all? Because surface area tracks metabolic mass better than weight does. Heat loss, fluid requirements, organ size, and the rate at which the body clears many drugs all scale closer to surface area than to raw kilograms. A short, heavy person and a tall, lean person can weigh the same and still have meaningfully different physiology, and BSA captures part of that difference where a single weight reading cannot.

The Mosteller formula

The formula you will see most in modern practice is Mosteller's, published in 1987. It is popular for one blunt reason: you can do it on any pocket calculator without exponent keys.

Mosteller BSA = √(height_cm × weight_kg / 3600)

Multiply height in centimetres by weight in kilograms, divide by 3600, take the square root, and you have BSA in square metres. The 3600 constant does the unit work; the square root is the only operation more exotic than long division.

A worked example

Take a 170 cm, 70 kg adult. Following Mosteller step by step:

170 × 70 = 11,900
11,900 / 3600 = 3.305
√3.305 ≈ 1.82

So this person has a BSA of about 1.82 m². That single number now feeds everything downstream. On a 75 mg/m² regimen, the area-based dose would be 75 × 1.82 ≈ 137 mg before any clinical rounding or capping. The same body has a BMI of roughly 24.2, which shows how differently the two metrics behave: BMI is a unitless ratio flagging weight relative to height, while BSA is a physical area you can dose against.

The Du Bois formula

The Mosteller equation is actually a simplification of older, more elaborate work. The original BSA equation comes from Du Bois and Du Bois in 1916:

Du Bois BSA = 0.007184 × height_cm^0.725 × weight_kg^0.425

Those fractional exponents are why the formula needs a scientific calculator and why Mosteller's algebraic shortcut caught on. The remarkable part is how closely they agree: run our 170 cm, 70 kg adult through Du Bois and you get about 1.81 m², a difference of roughly 0.01 from the Mosteller result. For typical adults the published formulas cluster within about 0.03 m² of each other, which is why clinicians rarely fret over which one a colleague used.

That agreement frays at the edges. Du Bois was fitted on only nine subjects, Gehan and George refit the curve on 401 people in 1970, and Haycock validated his 1978 version down to premature newborns. At the extremes of body size, especially in small children, the formulas diverge enough that the population a formula was built on starts to matter. Comparing them side by side is the fastest way to see that spread, which is one reason the body surface area calculator runs four formulas at once rather than committing to a single output.

Why dose by BSA instead of weight

Here is the clinical heart of it. Many cytotoxic chemotherapy drugs have a narrow therapeutic window: too little and the tumour escapes, too much and the patient suffers serious toxicity. Early oncology research found that normalising dose to BSA produced more consistent blood concentrations across patients of different sizes than dosing by weight alone, because the organs that metabolise and excrete the drug scale with surface area. So the milligram-per-square-metre convention stuck, and most oncology pharmacies compute the BSA with Mosteller.

The same logic shows up well beyond chemo. The cardiac index is cardiac output divided by BSA, which lets you compare heart performance fairly between a small adult and a large one. Glomerular filtration rate is routinely reported "per 1.73 m²" so kidney function can be judged on a common scale. Pediatric dosing leans on BSA precisely because children's body proportions shift so much with age that weight-based rules can mislead, and a surface-area estimate often tracks the underlying physiology better.

A note from working through these by hand

When I first tried to reproduce a textbook BSA answer key, I kept getting numbers that were close but never identical, and I assumed I was making arithmetic mistakes. I was not. The answer key used Mosteller; I had reached for Du Bois because it was the formula my notes listed first. Once I lined up all four formulas for the same 170 cm, 70 kg patient and saw 1.82, 1.81, 1.83, and 1.83 staring back at me, the lesson landed in a way no paragraph had managed: BSA is a model, not a measurement, and "which formula" is a real question with a real, if small, numerical answer. Seeing the spread turned an abstract caveat into something concrete.

Informational, not medical advice

This article and the calculator it describes exist to teach the math, not to set a dose. Real clinical dosing accounts for dose capping, rounding rules, organ function, drug interactions, and the judgement of a qualified prescriber and pharmacist, none of which a height-and-weight formula can supply. Treat any BSA number here as an estimate for learning and cross-checking, accurate to a few percent for typical adults and less so at the extremes. Never use it to make a treatment decision.

Putting it together

BSA condenses two everyday measurements into a single figure that scales chemotherapy doses, the cardiac index, kidney function, and pediatric prescribing. Mosteller gives you a fast, hand-friendly estimate; Du Bois gives you the historical original; for ordinary adults they agree closely, and for unusual body sizes the gap is the interesting part. If you want to practise the arithmetic or check a worked example, the body surface area calculator shows all four formulas together, and when you want the companion ratio that flags weight relative to height, the BMI calculator is one click away.

Made by Toolora · Updated 2026-06-13