Body Surface Area (BSA) Calculator — 4 Formulas

About

Body Surface Area (BSA) is a calculated measurement of the external surface area of the human body, expressed in square meters (m²). The concept was first formalized by Du Bois and Du Bois in 1916, who developed the first widely accepted formula using nine anatomical measurements. BSA is commonly used in clinical practice for normalizing physiological parameters — including cardiac output indexed as cardiac index (CI = CO/BSA), calculating chemotherapy doses (most cytotoxic agents are dosed per m²), assessing burn severity using the Rule of Nines and Lund-Browder chart, and determining glucocorticoid replacement doses. The reference standard value of 1.73 m² is used for indexing renal function (eGFR is normalized to 1.73 m²). Multiple formulas have been developed over the past century, with Mosteller (1987) being the most widely used today due to its simplicity and ease of bedside calculation. The choice of formula may affect dosing by 10–20% at extremes of body size, so consistency within institutions is important. The evidence level for BSA estimation is Grade B, supported by extensive physiological validation studies but limited by the absence of a true gold standard for surface measurement.

Formula

Mosteller: √(Height(cm) × Weight(kg) / 3600) | DuBois: 0.007184 × W^0.425 × H^0.725 | Boyd: 0.0003207 × H^0.3 × (W×1000)^(0.7285-0.0188×log(W×1000)) | Haycock: 0.024265 × W^0.5378 × H^0.3964

The Mosteller formula is calculated as the square root of (height in cm × weight in kg) / 3600. For example, a patient weighing 70 kg and measuring 175 cm has BSA = √(175 × 70 / 3600) = √(12250 / 3600) = √3.403 = 1.84 m². The DuBois-DuBois formula (1916) is BSA = 0.007184 × W^0.425 × H^0.725, which was derived from direct anatomical measurements of only 9 subjects. The Boyd formula (1935) uses BSA = 0.0003207 × H^0.3 × (W × 1000)^(0.7285 − 0.0188 × log(W × 1000)) and was derived from a larger sample. The Haycock formula (1978) uses BSA = 0.024265 × W^0.5378 × H^0.3964 and was developed specifically for pediatric populations. Each formula yields slightly different results, with discrepancies of up to 10–15% at extremes of body weight. In clinical practice, the Mosteller formula is preferred for its simplicity and sufficient accuracy. To interpret BSA, values are compared against known population norms: the average adult BSA is approximately 1.73 m². Chemotherapy doses are calculated as Dose = Published Dose × (Patient BSA / 1.73). Burn severity uses BSA percent involvement, where >20% BSA in adults constitutes major burns requiring referral to a burn center.

Score Interpretation

BSA is one of the most important anthropometric measurements in clinical medicine with applications across multiple specialties. In oncology, the American Society of Clinical Oncology (ASCO) and the National Comprehensive Cancer Network (NCCN) recommend BSA-based dosing for the majority of cytotoxic chemotherapy agents, as this approach reduces inter-individual variability in drug exposure. For carboplatin, the Calvert formula uses BSA along with renal function to calculate the optimal dose. In cardiology, cardiac output is indexed to BSA as cardiac index (normal: 2.5–4.0 L/min/m²), which is essential for classifying heart failure severity and guiding therapy. In nephrology, the estimated glomerular filtration rate (eGFR) reported by all laboratories is normalized to a standard BSA of 1.73 m², allowing comparison across individuals of different body sizes. In burn management, the total body surface area (TBSA) of burn involvement, estimated using the Rule of Nines or Lund-Browder chart, determines fluid resuscitation requirements via the Parkland formula (4 mL × TBSA% × weight) and guides transfer decisions to burn centers. The American Burn Association recommends that burns involving >10% TBSA in patients under 50 or >5% in patients over 50 be referred to specialized centers. BSA also guides dosing of certain medications including acyclovir, aminoglycosides, and antifungals in some protocols. Despite its broad utility, BSA-based dosing has limitations — it does not account for differences in drug metabolism, organ function, or body composition, which has led to a shift toward therapeutic drug monitoring where feasible.

Reference Ranges