Body Surface Area (BSA) Calculator

Body Surface Area (BSA) is the estimated total outer surface of the human body, expressed in square meters (m²). It is used clinically because some physiological measures — cardiac output is the classic example — scale with BSA better than with weight alone, and it is the standard basis for dosing most cytotoxic chemotherapy (mg/m²), for calculating the cardiac index (CI = CO/BSA) and for some pediatric drug doses. In 3,613 UK adults receiving chemotherapy, mean BSA was 1.91 m² in men and 1.71 m² in women (Sacco et al., PLoS One 2010).

The Du Bois formula (Du Bois D & Du Bois EF, Arch Intern Med 1916) is: BSA = 0.007184 × H^0.725 × W^0.425, where H is height in cm and W is weight in kg. Although it was derived from measurements of only nine people, it became the reference formula and is still the one used in adult medical oncology (Sawyer & Ratain, Invest New Drugs 2001). Its weak point is small bodies: Haycock et al. found that it increasingly underestimates BSA below 0.7 m², by up to 8% in newborns (J Pediatr 1978).

The Mosteller formula (N Engl J Med 1987) simplifies the calculation to BSA = √(H × W / 3600), with H in cm and W in kg. Its appeal is arithmetic simplicity: it can be worked out with a basic calculator. In adults of average build it gives values very close to Du Bois — for 170 cm and 70 kg, 1.82 m² versus 1.81 m². Whichever formula you use, stick to the one your institution's protocol specifies.

The Haycock formula (Haycock GB et al., J Pediatr 1978) is BSA = 0.024265 × H^0.3964 × W^0.5378. It was derived from geometric measurements of 81 subjects ranging from premature infants to adults and fitted well across the whole range, from under 0.2 m² to over 2.0 m² (r = 0.998), whereas Du Bois underestimates BSA in infants. For clinical pediatric dosing, always use the formula your institutional protocol specifies, as switching formulas changes the calculated dose.

Most cytotoxic chemotherapy agents (including platinum compounds, taxanes, anthracyclines, and fluorouracil) are prescribed in mg/m²: the dose per m² is multiplied by the patient's BSA. BSA dosing was introduced to derive safe starting doses for phase I trials from animal toxicology data; its relationship with renal and hepatic drug clearance is weak (Sawyer & Ratain, Invest New Drugs 2001). Capping BSA at 2.0 m² in large patients used to be common, but the ASCO guideline recommends full weight-based doses in obese adults, because they do not increase toxicity (Griggs et al., J Clin Oncol 2012). Never use a BSA calculator as the sole basis for clinical dosing without physician verification.

Cardiac Index (CI) = Cardiac Output (CO) / BSA. It normalizes cardiac output to body size, making it comparable across patients. Normal CI is 2.5–4.0 L/min/m². A CI below 2.2 L/min/m², together with low blood pressure and signs of poor perfusion, supports a diagnosis of cardiogenic shock; values above 4.0 may indicate high-output states (sepsis, hyperthyroidism, arteriovenous fistula). BSA is also used to express GFR per 1.73 m² (the standard eGFR unit) and for indexed hemodynamic values such as the oxygen consumption index.