Body Surface Area Calculator - Mosteller & Du Bois BSA

Why medicine uses body surface area

Body surface area (BSA) is the total area of your skin in square meters. Nobody measures it with a tape anymore; it's estimated from height and weight. Clinicians use it because many body functions scale with surface area better than with weight alone: resting metabolism, heart output, and how fast the kidneys clear some drugs.

You'll see BSA on chemotherapy orders (doses in mg per m²), in cardiology as cardiac index (cardiac output divided by BSA), in kidney function reports that normalize eGFR to 1.73 m², and in burn care. An average adult sits somewhere around 1.6-2.0 m².

Mosteller and Du Bois: the formulas

Du Bois (1916). Delafield and Eugene Du Bois measured the skin of nine people with molds and paper and fit an equation: BSA (m²) = 0.007184 x weight (kg)^0.425 x height (cm)^0.725. It became the reference standard for most of the 20th century, despite the tiny sample.

Mosteller (1987). R.D. Mosteller published a one-line simplification in the New England Journal of Medicine: BSA (m²) = square root of (height in cm x weight in kg / 3600). He showed it tracked Du Bois closely while being easy to punch into a pocket calculator. Today Mosteller is the formula most US hospitals and oncology dosing systems use.

For typical adults the two agree within about 1%. They drift apart at high body weights: Mosteller runs a few percent higher than Du Bois in people with obesity. That's why this calculator shows both and the gap between them.

Worked example

A 5'9" adult weighing 170 lb. Convert first: 69 in x 2.54 = 175.3 cm, and 170 lb x 0.4536 = 77.1 kg.

Mosteller: 175.3 x 77.1 = 13,516; divided by 3600 = 3.754; square root = 1.94 m².

Du Bois: 77.1^0.425 = 6.34 and 175.3^0.725 = 42.3; 0.007184 x 6.34 x 42.3 = 1.93 m². A difference of 0.01 m², about half a percent.

Now a 6'0", 300 lb adult (182.9 cm, 136.1 kg): Mosteller gives 2.63 m² and Du Bois 2.53 m², a gap of almost 4%. On a drug dosed at 100 mg/m², that's a 10 mg difference.

Which formula should you use?

Use whatever your hospital, protocol, or prescriber uses. Consistency matters more than picking the "best" equation, because dosing tables and reference ranges were built around a specific formula. If you're checking a number on a medical record, the chart will usually say which one was applied; Mosteller is the most likely in the US.

For children, many pediatric references use Mosteller or Haycock; Du Bois was fit on adults and can run low in infants. For people with severe obesity, oncologists increasingly dose on actual body weight BSA without capping, following ASCO guidance, but that's a clinical decision.

Common mistakes

Mixing units. Both formulas need centimeters and kilograms. Putting inches and pounds straight into Mosteller's cm/kg version gives a number about 7% too small, an error that is easy to miss because it still looks plausible. (There's a US-unit version that divides by 3131 instead of 3600.) Du Bois fails far worse with the wrong units. The calculator handles the conversion for you in US mode.

Using an old weight. BSA changes as weight changes. For dosing, clinicians use a current measured weight, not the one from last year's chart.

Rounding too early. Round only the final answer. Rounding height and weight first can shift BSA by 0.02-0.03 m².

Treating it as a health score. BSA is not like BMI. A bigger BSA isn't better or worse; it's a scaling factor for dosing and measurement, nothing more.

Doing dose math yourself. If you're checking a chemotherapy or other BSA-based dose, use this to understand the number, then raise any discrepancy with your pharmacist or oncology team. Never change a dose on your own.