Body Surface Area Calculator

What Is Body Surface Area (BSA)?

Body Surface Area (BSA) is a measurement of the total surface area of the human body, expressed in square meters (m²). Unlike weight alone, BSA accounts for both height and weight, providing a more accurate reflection of body size for clinical calculations. The average adult male has a BSA of approximately 1.9 m² and the average adult female approximately 1.6 m². BSA is used extensively in medicine for calculating drug doses (especially chemotherapy), cardiac output indexing, burn assessment, and renal function adjustment.

The Mosteller Formula

The Mosteller formula, published in 1987, is the simplest and most widely used BSA calculation: BSA (m²) = √(Height (cm) × Weight (kg) / 3600). Its simplicity makes it easy to calculate at the bedside and has been validated against more complex formulas. It produces results within 1–2% of the original DuBois formula for most adults. The original DuBois and DuBois formula (1916) was the first BSA estimation, derived from direct measurements of only 9 subjects — remarkably, it has held up well over a century of clinical use.

BSA in Chemotherapy Dosing

Most chemotherapy drugs are dosed in mg/m² rather than mg/kg because BSA correlates better with physiological processes like drug metabolism and clearance. For example, a chemotherapy drug prescribed at 75 mg/m² for a patient with a BSA of 1.8 m² would receive a dose of 135 mg. BSA-based dosing helps reduce the risk of under-dosing (which reduces efficacy) or over-dosing (which increases toxicity). However, BSA-based dosing is not perfect — pharmacogenomics and therapeutic drug monitoring are increasingly used to personalize doses further.

Pediatric BSA Considerations

The Haycock formula is generally preferred for pediatric BSA calculations because it was validated across a wider range of body sizes including neonates and children. Children have a higher BSA-to-weight ratio than adults, which affects drug distribution, metabolism, and clearance. A 20 kg child has relatively more surface area per kilogram than a 70 kg adult, which is one reason why weight-based dosing alone can be inaccurate for certain drugs in pediatrics.

The Rule of Nines

BSA is also used in burn assessment through the "Rule of Nines" — a quick method for estimating the percentage of body surface area affected by burns. In adults: each arm is 9%, each leg is 18%, the front torso is 18%, the back torso is 18%, the head is 9%, and the perineum is 1%. Pediatric proportions differ: a child's head accounts for a larger percentage. Accurate burn area estimation is critical for fluid resuscitation calculations (using the Parkland formula), which is directly based on BSA percentage burned.