Body Surface Area (BSA) Calculator

Body Surface Area (BSA) is the total surface area of the human body, measured in square meters (m²). Unlike body weight or height alone, BSA provides a combined measure of body size that correlates more closely with several physiological parameters—including cardiac output, drug metabolism, and kidney function—than weight alone. This makes it a widely used variable in clinical medicine, particularly for dosing medications that require precise calibration to body size.

This calculator provides BSA estimates using three well-established formulas: the Du Bois formula (1916), the Mosteller formula (1987), and the Haycock formula (1978). Each formula was derived from different study populations and uses slightly different mathematical approaches, which is why they may produce results that differ by a small margin. The calculator accepts metric inputs (kilograms and centimeters) or imperial inputs (pounds, feet, and inches).

The Du Bois Formula

The Du Bois formula, published by Delafield Du Bois and Eugene Du Bois in 1916, was the first widely adopted mathematical model for estimating BSA. The formula is: BSA = 0.007184 × W^0.425 × H^0.725, where W is body weight in kilograms and H is height in centimeters. The result is expressed in square meters.

Du Bois derived the formula by directly measuring the surface area of nine subjects using a paper pattern method, then fitting a power-law equation to the data. Despite being derived from a very small sample, the Du Bois formula became the standard reference in medical literature for decades and is still widely cited in pharmacology and physiology textbooks.

A notable limitation of the Du Bois formula is that it was derived primarily from normal-weight adults, and its accuracy may be reduced for individuals who are significantly underweight, obese, or at the extremes of height. Nevertheless, it remains one of the most commonly referenced formulas in clinical settings.

The Mosteller Formula

The Mosteller formula, published by R.D. Mosteller in the New England Journal of Medicine in 1987, offers a computationally simpler approach: BSA = √(W × H / 3600), where W is weight in kilograms and H is height in centimeters. The square root is taken of the product of weight and height divided by 3600.

The Mosteller formula was designed to be easy to calculate mentally or on a simple calculator, making it practical for clinical use at the bedside. It produces results very close to those of the Du Bois formula for adults in the normal and moderately overweight range. Its simplicity and reliability have made it popular in oncology, where BSA-based chemotherapy dosing is common.

Many oncology treatment protocols and drug labeling documents now specify BSA calculated by the Mosteller formula, so it is important to know which formula is expected when following a specific dosing protocol.

The Haycock Formula

The Haycock formula, developed by G.B. Haycock and colleagues in 1978, was derived specifically using pediatric data as well as adult data: BSA = 0.024265 × W^0.5378 × H^0.3964, where W is weight in kilograms and H is height in centimeters. This formula is particularly valued in neonatal and pediatric medicine because it was validated across a wide range of ages and body sizes, including infants and young children.

For adults, the Haycock formula typically produces results close to those of the Du Bois and Mosteller formulas. For pediatric patients, especially infants and toddlers, the Haycock formula is often preferred as it was validated on that population. Some pediatric drug dosing protocols and clinical guidelines specifically recommend the Haycock formula.

Clinical Applications of BSA

The most prominent clinical use of BSA is in oncology for calculating chemotherapy doses. Many cytotoxic agents have a narrow therapeutic window—doses that are too low may be ineffective, while doses that are too high can cause serious toxicity. Since drug clearance is correlated with body size, dosing per square meter of BSA helps achieve more consistent plasma drug concentrations across patients of different sizes.

BSA is also used to calculate normal ranges for physiological measurements. Cardiac output, glomerular filtration rate (GFR), and other parameters are often indexed to BSA (expressed as, for example, cardiac index in L/min/m²) to allow comparison across individuals of different body sizes. Reference ranges for these indexed values can then be applied universally without needing size-specific normal ranges.

In pediatrics, BSA-based dosing is particularly important because the ratio of body surface area to body mass changes dramatically from infancy to adulthood. Infants have a proportionally larger BSA relative to their weight than adults, which affects how they absorb, distribute, and eliminate drugs. Careful dosing based on validated pediatric BSA formulas is essential in this population.

BSA vs. Body Weight for Drug Dosing

The rationale for using BSA rather than body weight in drug dosing is that BSA correlates more closely with many physiological functions that determine drug distribution and clearance, such as kidney filtration, liver blood flow, and cardiac output. Research has shown that for many drugs, doses expressed per m² of BSA result in more consistent plasma concentrations across patients than doses expressed per kilogram of body weight.

However, BSA-based dosing is not universally superior. For some drugs, weight-based dosing performs equally well or better. The choice of dosing metric depends on the pharmacokinetic properties of the specific drug and the clinical evidence from dosing studies. In obese patients, BSA-based dosing can sometimes result in very high doses relative to lean body mass, which raises questions about whether actual or ideal BSA should be used.

These nuances highlight why drug dosing decisions should always be made by qualified healthcare professionals following current clinical guidelines, rather than solely based on calculator outputs.

Differences Between Formulas and When to Use Each

For average-sized adults within the normal weight range, the Du Bois, Mosteller, and Haycock formulas typically produce results that differ by less than 5%, and often less than 2–3%. The choice of formula matters most at the extremes of body size: in morbidly obese patients or very small children.

When following a specific clinical protocol, always use the formula that the protocol specifies. Many chemotherapy regimens, particularly those published in major oncology journals or included in standard treatment guidelines, specify which BSA formula was used in the original trials. Using a different formula may introduce systematic differences in dose that were not part of the original study design.

For general reference and educational purposes, this calculator shows the results of all three formulas simultaneously so you can compare them. The average adult BSA is approximately 1.7 m², with typical ranges from about 1.5 m² to 2.2 m² across the normal adult population.

Limitations and Important Notes

All three BSA formulas are mathematical models that estimate surface area from just two inputs: weight and height. They do not directly account for body shape, proportions, or composition. Two people with the same weight and height—but different limb lengths, trunk proportions, or muscle-to-fat ratios—may have different actual surface areas, but the same formula-derived BSA.

For very obese patients, all three formulas tend to underestimate BSA compared to direct measurement, because the models were derived primarily from normal-weight populations. Some researchers have proposed adjusted formulas for obese patients, though no single approach has been universally adopted.

The results from this calculator are estimates intended for educational reference only. All clinical decisions involving BSA—including medication dosing, physiological assessments, and medical procedures—should be made by qualified healthcare professionals in consultation with current clinical guidelines and the specific patient's individual circumstances. These results are not a substitute for professional medical advice.