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What is body surface area, and why is it calculated?
Body surface area (BSA) is the measure of the outer surface of the human body in square meters. Because it's hard to measure directly, it's estimated with validated formulas derived from height and weight. BSA is preferred in medicine because it lines up better with many physiological variables than body weight does.
Its main clinical uses: dosing chemotherapy and some other drugs (the dose is usually given as mg/m²), the cardiac index (cardiac output divided by BSA), normalizing values such as basal metabolic rate and glomerular filtration rate, and calculating the percentage of burns. The average adult BSA is around 1.7 m².
How is BSA calculated?
The two most common methods are the Mosteller and Du Bois formulas. Mosteller (1987) can be worked out even by hand because it involves just one square root, and it's the method most used in the clinic. Du Bois & Du Bois (1916) is the oldest and still a reference formula. In this tool, Mosteller is the primary result and Du Bois is shown for comparison:
BSA (m²) = √( height(cm) × weight(kg) ⁄ 3600 )
Du Bois (comparison):
BSA (m²) = 0.007184 × weight0.425 × height0.725
Example: for someone 170 cm and 70 kg:
Mosteller = √(170 × 70 ⁄ 3600) = √3.306 ≈ 1.82 m².
Du Bois = 0.007184 × 700.425 × 1700.725 ≈ 1.81 m² — the two formulas are very close.
Method: Mosteller RD (1987), New England Journal of Medicine — simplified BSA formula; Du Bois D & Du Bois EF (1916), Archives of Internal Medicine — the classic BSA equation.
Which formula should I choose?
In everyday practice the difference is small; both formulas are clinically accepted:
- Mosteller: the easiest to calculate; the method most used in emergency and clinical settings.
- Du Bois: the oldest reference; the basis of many sources and nomograms.
- Others: formulas such as Haycock, Gehan-George, and Boyd can be used especially in children.
Frequently asked questions
No. BMI (kg/m²) gives the ratio of your weight to your height and is used to screen for overweight and obesity. BSA (m²) estimates the body's outer surface area and is mainly a clinical measure used for drug dosing and for normalizing cardiac and renal values.
In chemotherapy especially, how drugs are distributed and eliminated is related to body size. Adjusting the dose to body surface area as mg/m² aims to give more consistent drug exposure in both very small and very large patients.
Each formula uses different coefficients derived from a different population, so the results usually differ by a few percent. In healthy adults the difference is small and rarely affects the clinical decision.
Mosteller and Du Bois are widely used in adults and can also be applied to children. In pediatrics, though, formulas validated for children such as Haycock or Gehan-George may be preferred. For precise dosing, always consult a doctor.
Because for some drugs, how they're distributed and eliminated in the body relates more to the amount of metabolically active tissue than to total weight; body surface area (BSA) reflects that amount better than weight does. The best-known example is chemotherapy drugs: the dose is usually given as 'mg/m²' (milligrams per square meter), because too much can be toxic and too little ineffective, and BSA strikes that balance more safely. BSA is also used in the cardiac index, which assesses heart performance, and in burn-area calculations.
Both are clinically accepted, and the difference between them is very small in daily practice (usually a few percent). <b>Mosteller</b> (1987) can be worked out even by hand because it uses a single square root, and it's the method most used in hospitals today; it's the primary result in this tool. <b>Du Bois</b> (1916) is the oldest and still a reference formula, the basis of many tables and nomograms (a chart that lets you solve a calculation graphically). In practice you can rely on Mosteller; the small difference between the two isn't enough to change a treatment decision.