Drug Dosage by BSA Calculator

Body surface area (BSA) is a measurement of the total external surface of the human body, expressed in square meters (m²). In clinical pharmacology, BSA has long been used as a basis for calculating drug doses, particularly for chemotherapy agents and some other medications where the therapeutic window is narrow and dose-related toxicity is a significant concern.

This calculator uses the Mosteller formula—one of the most widely adopted BSA formulas in oncology practice—to estimate BSA from height and weight, then multiplies that value by a dose-per-m² figure to produce an estimated total dose. It also shows the BSA calculated by the Du Bois formula for comparison. This tool is intended strictly for educational reference; all actual drug dosing decisions must be made by qualified healthcare professionals.

The Mosteller Formula

The Mosteller formula, published by R.D. Mosteller in the New England Journal of Medicine in 1987, calculates BSA as: BSA (m²) = the square root of (weight in kg times height in cm divided by 3600). It was designed to be computationally simple while producing results consistent with earlier, more complex formulas.

The Mosteller formula is widely used in oncology because its simplicity makes it practical at the bedside and it has been specifically adopted in many chemotherapy dosing protocols. Many oncology treatment guidelines and drug labels specify that BSA should be calculated using the Mosteller formula, so clinicians often need to verify which formula a particular protocol requires.

For adults in the normal and moderate weight range, the Mosteller formula produces results very close to those of the Du Bois formula. Differences between formulas are typically less than 2 to 3 percent for average-sized adults, though they can be larger at the extremes of body weight.

The Du Bois Formula

The Du Bois formula, published by Delafield Du Bois and Eugene Du Bois in 1916, was the first widely used mathematical model for estimating BSA: BSA (m²) = 0.007184 times weight in kg to the power of 0.425 times height in cm to the power of 0.725. It was derived by measuring the surface area of nine subjects using a paper pattern technique.

Despite being based on a very small sample and being over a century old, the Du Bois formula became the historical standard in medical literature and is still frequently cited. It remains a useful reference point, particularly for comparing results against older published data or protocols that specified it.

This calculator shows both the Mosteller and Du Bois results side by side, allowing you to see how closely the two formulas agree for the entered height and weight values.

Why BSA-Based Dosing Is Used

The rationale for using BSA in drug dosing is that BSA correlates more closely than body weight with several physiological parameters that influence how a drug is distributed and eliminated, including kidney filtration rate, liver blood flow, and cardiac output. Research has shown that for certain drugs, especially cytotoxic chemotherapy agents, dosing per square meter of BSA produces more consistent drug plasma concentrations across patients of varying sizes than dosing per kilogram of body weight.

Chemotherapy is the most prominent clinical application because many cytotoxic agents have a narrow therapeutic window: doses too low may be less effective, while doses too high can cause serious toxicity. BSA-based dosing attempts to account for individual differences in body size to reduce this variability.

However, BSA-based dosing is not universally superior to weight-based dosing. For some drugs, weight-based or flat dosing may achieve equally consistent outcomes. The appropriate dosing approach depends on the pharmacokinetic properties of each specific drug, as established through clinical trials and pharmacological research.

How to Interpret the Calculated Dose

This calculator multiplies the BSA calculated by the Mosteller formula by the dose per m² figure you enter to produce an estimated total dose. It also expresses this as a dose per kilogram (mg/kg) for reference. These figures represent a mathematical estimate based on the inputs provided.

Actual drug dosing in clinical practice involves many additional factors beyond BSA and the nominal dose per m²: the patient's organ function (kidney and liver), concurrent medications, prior treatment history, body composition, and specific clinical protocol requirements. Dosing protocols may also specify maximum doses, rounding rules, or capping of BSA to limit total dose in certain patients.

The dose per kg reference value shown is intended to provide a comparative figure; it does not imply that weight-based dosing would produce the same result or is interchangeable with the BSA-based dose.

BSA Reference Values

For context, the average BSA for an adult is approximately 1.7 m², with a typical range of about 1.5 to 2.2 m² across the normal adult population. Men tend to have slightly higher BSA values than women on average, reflecting differences in average height and weight. Pediatric BSA values are lower, rising from approximately 0.2 m² for a newborn to adult values by late adolescence.

BSA formulas were primarily validated in adult populations. The Mosteller formula, like most BSA formulas, was not originally derived from pediatric data, which is why pediatric clinical pharmacology often uses formulas specifically validated for children, such as the Haycock formula.

Body composition, specifically the relative proportion of lean mass and adipose tissue, can also affect the accuracy of BSA estimates. In patients with extreme obesity, actual body surface area may differ from formula estimates, which has implications for dosing decisions that are beyond the scope of a calculator.

Limitations and Important Disclaimer

BSA formulas are mathematical models that estimate surface area from only two variables: weight and height. They do not account for body shape, limb proportions, or composition. The same height and weight can correspond to very different actual body configurations, and the estimated BSA may not fully capture physiologically relevant differences between individuals.

All BSA formulas carry inherent estimation error. For the purposes of clinical dosing, differences between Mosteller and Du Bois results are typically small enough to be clinically negligible for average-sized adults, but the choice of formula can matter when following a protocol that specifies a particular formula.

This calculator is provided for educational and informational purposes only. It is not a substitute for professional medical advice, clinical judgment, or the expertise of qualified healthcare professionals. Drug dosing must always be calculated, verified, and supervised by licensed healthcare providers following current clinical guidelines, institutional protocols, and individual patient assessment.