Body Surface Area Calculator

What Is Body Surface Area?

Body Surface Area (BSA) estimates the total surface of the human body in square meters. Unlike BMI, which relates only weight and height, BSA is critical in clinical medicine for dosing chemotherapy drugs, calculating burn percentages, determining cardiac output index, and adjusting renal function measurements. The most commonly used formula is the Du Bois formula (1916): BSA = 0.007184 × Height(cm)⁰·⁷²⁵ × Weight(kg)⁰·⁴²⁵. The average adult BSA is approximately 1.7 m² for women and 1.9 m² for men.

BSA Formulas Compared

Several formulas exist because BSA cannot be measured directly on living patients. The Mosteller formula (√(height(cm) × weight(kg) / 3600)) is simpler and produces values within 1–2% of Du Bois for most adults. The Haycock formula is preferred for pediatric patients. The Boyd formula accounts for extreme body sizes. For chemotherapy dosing, the choice of formula can affect the dose by 5–10%, which matters for drugs with narrow therapeutic windows. While BMI is used for general health screening, BSA is the medically preferred metric for drug dosing because it correlates better with metabolic rate than weight alone.

Average Body Surface Area by Age and Sex

What Is Body Surface Area (BSA)?

Body surface area measures the total external area of the human body in square meters. Unlike body weight alone, BSA accounts for both height and weight to produce a value that more accurately reflects physiological scaling — how the body interacts with its environment for heat exchange, drug metabolism, and fluid balance. BSA is the standard metric for calculating chemotherapy dosages, burn severity assessment, cardiac output indexing, and renal function normalization. This calculator uses the Du Bois formula (BSA = 0.007184 × height^0.725 × weight^0.425), which has been validated across diverse populations since 1916 and remains the most widely referenced BSA equation in clinical practice. Alternative formulas (Mosteller, Haycock, Gehan-George) produce slightly different results but are clinically interchangeable for most applications.

BSA Reference Values

BSA in Drug Dosing

Chemotherapy drugs are among the most toxicity-sensitive medications in clinical use — the therapeutic window between effective and dangerous dosage is extremely narrow. BSA-based dosing adjusts the amount of drug administered to each patient's body size, producing more consistent plasma drug concentrations than weight-based dosing alone. The standard dosing formula multiplies the recommended dose per square meter by the patient's BSA: a chemotherapy drug prescribed at 75 mg/m² for a patient with BSA of 1.8 m² yields a dose of 135 mg. This approach accounts for the physiological reality that drug metabolism, distribution volume, and clearance scale more closely with surface area than with weight. However, BSA-based dosing is not perfect — factors including organ function, genetics, body composition, and concurrent medications also affect drug handling, which is why oncologists frequently adjust doses based on toxicity observations during treatment.

BSA and Cardiac Output

The cardiac index — cardiac output divided by BSA — normalizes heart function measurements across body sizes, allowing clinicians to compare cardiac performance between a 100-pound woman and a 250-pound man on equal footing. Normal cardiac index ranges from 2.5 to 4.0 liters per minute per square meter. Values below 2.2 L/min/m² indicate cardiogenic shock or severe heart failure. BSA normalization reveals that what appears to be adequate cardiac output for a small patient might actually represent heart failure for a larger patient whose tissues demand proportionally greater blood flow. Echocardiography reports routinely index cardiac measurements to BSA, including left ventricular mass index, stroke volume index, and valve area index. These indexed values are essential for diagnosing conditions like left ventricular hypertrophy, aortic stenosis severity, and valvular insufficiency.

BSA in Burn Assessment

Burn severity is classified by the percentage of total body surface area (TBSA) affected. The "rule of nines" provides rapid estimation: each arm represents 9% TBSA, each leg 18%, the anterior trunk 18%, the posterior trunk 18%, the head 9%, and the perineum 1%. Burns covering more than 15–20% TBSA in adults (or 10% in children and elderly) require intravenous fluid resuscitation calculated by the Parkland formula: 4 mL × patient weight (kg) × % TBSA burned, delivered over 24 hours with half in the first 8 hours. Accurate BSA calculation is therefore critical for burn management — underestimating BSA involvement leads to inadequate fluid resuscitation (risking organ failure from hypovolemic shock), while overestimating leads to fluid overload with its own serious complications. Specialized burn charts (Lund-Browder charts) provide more accurate TBSA percentages than the rule of nines by adjusting for age-related differences in body proportions, particularly important in pediatric burn patients where the head represents a larger proportion of total surface area.

BSA and Metabolic Scaling

The relationship between body surface area and metabolic rate has been recognized since the early 1900s — Kleiber's law demonstrates that metabolic rate scales with body surface area rather than body mass across species. In humans, BSA-normalized values provide clinically meaningful comparisons for metabolic measurements including oxygen consumption (VO2), carbon dioxide production, and resting energy expenditure. Normal basal metabolic rate expressed per unit BSA falls between 35–40 kcal/m²/hour in healthy adults. Deviations from this range may indicate thyroid dysfunction, fever, metabolic syndrome, or other conditions affecting cellular metabolism. In critical care settings, indirect calorimetry measures actual metabolic rate and compares it against BSA-predicted values to guide nutrition support — a patient producing significantly less CO2 than predicted for their BSA may be underfed, while excess production may indicate overfeeding or metabolic stress from infection or trauma.

BSA and Renal Function Assessment

Glomerular filtration rate (GFR) — the primary measure of kidney function — is standardized to BSA (reported as mL/min/1.73 m², where 1.73 m² represents an average adult BSA). This normalization allows direct comparison of renal function between patients of different body sizes. A raw GFR of 90 mL/min might represent normal function in a small-framed woman (BSA 1.5 m²) but could indicate early decline in a large-framed man (BSA 2.2 m²). When BSA-adjusted GFR falls below 60 mL/min/1.73 m² for three or more months, chronic kidney disease is diagnosed. Drug dosing adjustments for renal impairment use BSA-normalized GFR to determine appropriate dose reductions, particularly for medications cleared primarily by the kidneys. The CKD-EPI equation, the current standard for estimating GFR, incorporates serum creatinine, age, sex, and race — the result is then normalized to 1.73 m² BSA for standardized reporting. Use our Creatinine Calculator for related kidney function estimates.

Comparing BSA Formulas

For most adults, formula selection makes minimal clinical difference — values typically agree within 2–3%. The Du Bois and Mosteller formulas are used most frequently. The Haycock formula is preferred for pediatric dosing due to its validation in infant and child populations. In clinical practice, consistency matters more than formula choice — the same formula should be used throughout a patient's treatment course to ensure dosing comparability across visits.

See also: BMI Calculator · Ideal Weight Calculator · Body Fat Calculator