Body Surface Area (BSA) Calculator

Body Surface Area (BSA) is the total surface area of the human body, expressed in square metres. It is a fundamental parameter in clinical medicine, especially for dosing chemotherapy drugs, calculating cardiac index, and determining fluid resuscitation in burn patients. Unlike weight or BMI, BSA scales approximately with metabolic rate, which is why it is preferred over weight-based dosing for drugs with narrow therapeutic windows.

BSA cannot be measured directly except by elaborate coating-and-measurement procedures used in early 20th-century research. In clinical practice, BSA is estimated from height and weight using validated formulas — the most common being Du Bois (1916) and Mosteller (1987).

Multiple validated formulas exist for BSA estimation, all producing similar results at typical adult body sizes. The most-used formulas:

Most cytotoxic chemotherapy agents are dosed per m² of body surface area (e.g. "50 mg/m²"), not per kg of body weight. The historical rationale is that BSA correlates better with organ function (liver metabolism, renal clearance) than raw body weight — an obese patient does not need proportionally more chemotherapy than a lean patient of the same height because the extra body mass is largely adipose tissue that does not metabolise the drug.

BSA-based dosing has come under increasing scrutiny for its imprecision — inter-patient variability in drug metabolism is far larger than BSA would predict, and pharmacogenomic testing offers better dose personalisation for some drugs. Nonetheless, BSA remains the standard for most cytotoxic dosing regimens because of decades of accumulated clinical evidence at BSA-based doses.

For obese patients (BMI ≥ 30), several institutions cap the BSA calculation at 2.0-2.2 m² to prevent dosing toxic doses driven by the weight component. Institutional protocols vary, and the American Society of Clinical Oncology (ASCO) has published guidelines on chemotherapy dosing in obese patients that address this specifically.

Cardiac Index (CI) = Cardiac Output / BSA. Normal cardiac index is 2.5-4.0 L/min/m². Indexing to BSA lets clinicians compare cardiac function across patients of different body sizes without confounding by patient bulk.

In burn medicine, BSA is central to fluid resuscitation calculations. The Parkland formula for burn fluid resuscitation is 4 mL × %burn × weight(kg), where "%burn" is percentage of body surface area burned. Estimating %burn requires knowing regional BSA percentages — the "rule of nines" (head 9%, each arm 9%, each leg 18%, chest 9%, abdomen 9%, back 18%, perineum 1%) is the standard first-approximation tool.

For pediatric burn resuscitation the rule of nines is adjusted because children have proportionally larger heads and smaller legs than adults; the Lund-Browder chart accounts for this age-adjustment.

For adult subjects with typical body proportions, all standard BSA formulas produce results within 3-5% of each other. Choosing between them rarely affects clinical decisions. The Mosteller formula is preferred in most modern clinical settings because of its computational simplicity — it can be done on a calculator or in the head with a mobile phone in seconds.

For extreme body sizes (infants under 5 kg, adults over 150 kg), formula divergence widens significantly. Du Bois underestimates BSA in the very obese; Boyd was specifically developed for these extremes and is more accurate there. When BSA precision matters clinically at the extremes, the ASCO guidelines and individual institutional protocols specify which formula to use.