BSA Calculator

About This Tool

Body Surface Area (BSA) is a measurement of the total surface area of the human body, expressed in square meters (m²). Unlike body weight alone, BSA provides a more accurate representation of metabolic mass because it correlates more closely with many physiological parameters including cardiac output, basal metabolic rate, blood volume, and renal function. This is why BSA is the preferred metric for dosing many medications, particularly chemotherapy drugs where precise dosing is critical to balance efficacy against toxicity.

The Du Bois Formula

Published in 1916 by Delafield Du Bois and Eugene Du Bois, this is the oldest and most widely used BSA formula. The equation is: BSA (m²) = 0.007184 × Height(cm)^0.725 × Weight(kg)^0.425. Despite being over a century old, the Du Bois formula remains the standard in most clinical settings. It was derived from measurements of only nine subjects, which has led some researchers to question its accuracy, but subsequent validation studies across larger populations have confirmed its reasonable accuracy for most adults. The formula tends to underestimate BSA slightly in obese patients and overestimate it in very thin individuals.

The Mosteller Formula

R.D. Mosteller proposed a simplified formula in 1987: BSA (m²) = square root of (Height(cm) × Weight(kg) / 3600). This formula is particularly popular because it is easy to calculate mentally or with a basic calculator, making it convenient in clinical settings where quick estimates are needed. The Mosteller formula produces results very close to the Du Bois formula for most adult patients, with differences typically less than 2%. Its simplicity has made it a favorite in pediatric oncology and emergency medicine.

The Boyd Formula

Edith Boyd published her BSA formula in 1935, based on measurements from a larger and more diverse set of subjects than the Du Bois study. The Boyd formula accounts for the logarithmic relationship between weight and body surface area and is sometimes preferred for extreme body weights. While less commonly used today than the Du Bois or Mosteller formulas, it provides a useful third data point for comparison. Some researchers argue it is more accurate for children and infants.

Clinical Applications of BSA

The primary clinical use of BSA is in drug dosing, particularly for chemotherapy agents. Most cytotoxic drugs are dosed in mg/m² because BSA correlates better with drug clearance than body weight alone. This is critical for chemotherapy where the therapeutic window is narrow and both underdosing (reduced efficacy) and overdosing (increased toxicity) can have serious consequences. BSA is also used in burn medicine to estimate the extent of burn injuries (the rule of nines), in cardiology to index cardiac output and valve areas, and in nephrology where GFR is normalized to 1.73 m² of BSA. Additionally, BSA-based dosing is used for many pediatric medications, antibiotics, and immunosuppressive drugs used in transplant medicine.

Limitations and Considerations

BSA formulas are estimates derived from population-level data and may not be perfectly accurate for every individual. Accuracy decreases at extremes of body size, including very obese patients, very muscular individuals, and cachectic patients. For obese patients receiving chemotherapy, there is ongoing debate about whether to use actual body weight, ideal body weight, or adjusted body weight in BSA calculations. Some chemotherapy protocols have adopted flat-dosing (fixed doses regardless of BSA) for certain drugs, particularly those with wide therapeutic indices or those that show no correlation between BSA-based dosing and outcomes. Always defer to institutional protocols and physician judgment for clinical dosing decisions.