About the Body Surface Area Calculator
A body surface area (BSA) calculator estimates the total external surface area of a person's body, used in clinical medicine for drug dosing (especially chemotherapy), burn assessment, fluid management, and certain physiological calculations. The most widely used formula is DuBois & DuBois (1916), which remains the reference for many clinical applications despite its age. Other formulas (Mosteller, Haycock) give similar results within a few percent.
Why BSA matters in medicine
Many clinical parameters scale with BSA rather than weight or height alone. Cardiac output, glomerular filtration rate, certain hormone secretion rates, and many drug dosages are conventionally normalized to BSA, allowing meaningful comparison between adults and children, large and small patients.
The most consequential clinical use is chemotherapy dosing, where the right drug dose for a 60-kg patient and a 100-kg patient is rarely 'mg/kg × weight' but 'mg/m² × BSA'. The BSA-based dose better predicts pharmacokinetic exposure for many drugs, particularly cytotoxic agents where over-dosing risks severe toxicity and under-dosing risks treatment failure.
The DuBois formula and its alternatives
DuBois & DuBois (1916): BSA = 0.007184 × W^0.425 × H^0.725, where W is weight in kg and H is height in cm. Originally developed by measuring 9 subjects with strips of paper — surprisingly accurate considering the small sample, and still in widespread use over a century later.
Mosteller (1987): BSA = √(W × H / 3600). Popular because it's easier to compute mentally and gives results very close to DuBois for typical adult populations. Slightly more accurate for obese patients in some studies.
Haycock (1978): BSA = 0.024265 × W^0.5378 × H^0.3964. Often used for pediatrics; produces similar results to DuBois for normally-proportioned adults.
Differences between formulas are typically within ±3–5% for adults and slightly larger for very obese, very lean, or pediatric patients. For most clinical purposes, the choice of formula matters less than consistent use of one approach.
Clinical applications
Chemotherapy dosing: most cytotoxic drug doses are expressed as mg/m². Patient BSA × dose-per-m² = patient dose. Some institutions cap BSA at 2.0 or 2.2 m² for very obese patients to avoid potentially toxic doses; others use "adjusted body weight" to compute BSA. Practice varies; institutional protocols apply.
Burn assessment: the "rule of nines" provides quick body-area estimates for burn coverage, but precise burn percentages can use BSA-based calculations (Lund-Browder chart). Total burn area drives fluid resuscitation rates, particularly the Parkland formula's calculation of crystalloid requirement.
Cardiac index: cardiac output (L/min) is normalized to BSA to give cardiac index (L/min/m²), allowing comparison across patient sizes. A normal cardiac index of 2.5–4.0 L/min/m² applies regardless of patient size.
Limitations and the future of BSA dosing
BSA assumes a relationship between body surface and drug pharmacokinetics that's at best approximate. For some drugs, BSA-based dosing has been shown to be no better — sometimes worse — than fixed dosing. Modern pharmacology has moved selectively away from BSA-based dosing for newer agents, particularly biologics and immunotherapies, where flat dosing or weight-based dosing performs better.
BSA also doesn't capture body composition. Two patients of the same BSA may have very different lean body mass and very different drug distribution and clearance. For some drug classes, lean body mass or adjusted body weight gives more accurate dosing than BSA. Clinical practice varies by drug and institution.
Formula
- BSA = Body surface area in m²
- W = Weight in kg
- H = Height in cm
Worked examples
Average adult man, 80 kg, 180 cm
DuBois: 0.007184 × 80^0.425 × 180^0.725 ≈ 1.99 m². Mosteller: √(80 × 180 / 3600) = √4 = 2.00 m². Both formulas agree closely for typical adults.
Average adult woman, 65 kg, 165 cm
DuBois: ~1.71 m². Mosteller: √(65 × 165 / 3600) = √2.979 ≈ 1.73 m². The two formulas typically agree within ~1–2% for normal-proportioned adults.
Chemotherapy dose calculation
Patient BSA: 1.85 m². Drug dose: 75 mg/m² IV. Patient dose: 75 × 1.85 = 138.75 mg, rounded per institutional rounding rules to 139 mg or 140 mg. The same drug at the same dose-per-m² gives very different total doses for patients of different sizes — preventing under-dosing of large patients and over-dosing of small ones.
Frequently asked questions
Why is BSA used instead of weight for some drug doses?
Many drugs — especially cytotoxic chemotherapy — have pharmacokinetics that scale better with BSA than with weight alone. BSA-based dosing tends to predict exposure more accurately across the range of adult sizes. Newer biologic drugs are sometimes dosed differently (flat doses, weight-based) when evidence supports those approaches.
Which BSA formula is best?
DuBois remains the reference standard. Mosteller is easier to compute and gives nearly identical results for most adults. Haycock is often used for pediatrics. Differences between formulas are typically under 5%; the choice usually doesn't change clinical decisions, and consistent institutional use of one formula matters more than the choice itself.
How accurate is BSA from a calculator?
BSA formulas are estimates, not measurements. Direct measurement (3D body scanning, mathematical integration of body shape) is more accurate but rarely used clinically. For typical adults, formula-based BSA is accurate within a few percent — sufficient for most clinical purposes.
Does BSA change with weight loss?
Yes — BSA depends on weight, so weight loss reduces BSA. A patient losing 20 kg might see BSA drop by 0.2–0.3 m². For drugs dosed by BSA, dose adjustments may be needed during meaningful weight changes. Clinicians often recalculate BSA at each chemotherapy cycle.
Is there an upper limit on BSA used for dosing?
Some institutions cap calculated BSA at 2.0 m² (or 2.2 m²) for very obese patients to avoid potentially toxic chemotherapy doses, on the theory that body surface relationships break down at extreme body sizes. Others use 'adjusted body weight' formulas. Practice varies and is institution-specific; literature on optimal practice is still evolving.
Is BSA used for anything besides drug dosing?
Yes — burn extent assessment, fluid resuscitation calculations, cardiac index, glomerular filtration rate (mL/min/1.73 m²), and various pediatric clinical calculations all use BSA. It's a common normalization across body sizes for many physiological measurements.