Body Surface Area Calculator
Calculate body surface area (BSA) in m² using the Mosteller, Du Bois or Haycock formula from your weight and height. Clinicians use BSA to dose chemotherapy, normalise cardiac index and estimate burn coverage.
Last updated: May 2026
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About this calculator
Body Surface Area (BSA) is the total skin-covered area of the body expressed in square metres. Many physiological variables — cardiac output, glomerular filtration, blood volume and the clearance of cytotoxic drugs — scale more closely with BSA than with weight alone, which is why oncologists, cardiologists and intensivists use BSA-normalised dosing and indices. This calculator supports three validated formulae: Mosteller, BSA = √(weight(kg) × height(cm) / 3600), the simplest and the default; Du Bois, BSA = 0.007184 × weight^0.425 × height^0.725, the historical reference since 1916; and Haycock, BSA = 0.024265 × weight^0.5378 × height^0.3964, originally derived in paediatric patients and often preferred under 10 kg. Inputs assume both weight and height are positive; entering 0 returns 0 or NaN depending on the formula. Average adult BSA is roughly 1.7–1.9 m², newborns about 0.25 m². Edge cases: in patients with significant oedema, ascites or amputation, measured weight does not reflect actual body envelope and BSA-based doses can over- or under-dose; for very obese patients, capped or adjusted body weight is sometimes used in chemotherapy dosing (consult dosing guidelines). All three formulae agree to within 2–5% for adults of average build but can diverge by 10% at the extremes of size.
How to use
Example 1 — average adult, Mosteller. Weight 70 kg, height 170 cm. Step 1: 70 × 170 = 11,900. Step 2: 11,900 ÷ 3,600 ≈ 3.306. Step 3: √3.306 ≈ 1.818 m². Verify: average adult BSA is 1.7–1.9 m², so 1.82 m² is plausible. Cross-check with Du Bois: 0.007184 × 70^0.425 × 170^0.725 ≈ 0.007184 × 6.21 × 41.0 ≈ 1.83 m² — within 1% of Mosteller ✓. Example 2 — paediatric patient, Haycock formula for chemotherapy dosing. Weight 18 kg, height 110 cm. Step 1: 0.024265 × 18^0.5378 ≈ 0.024265 × 4.83 ≈ 0.1172. Step 2: 110^0.3964 ≈ 6.515. Step 3: 0.1172 × 6.515 ≈ 0.764 m². Verify: a roughly 6-year-old child typically has a BSA around 0.7–0.8 m², matching the result. If a chemotherapy agent is dosed at 100 mg/m², this child's dose would be 100 × 0.764 ≈ 76 mg — illustrating why BSA precision matters in paediatric oncology, where a 5% error in BSA shifts dose by several milligrams. ✓
Frequently asked questions
Why is body surface area used for chemotherapy dosing instead of body weight?
BSA correlates more closely with blood volume, organ size and the renal and hepatic clearance pathways most cytotoxic drugs use than simple body weight does, which helps maintain consistent drug plasma concentrations across patients of very different sizes. Using BSA reduces the risk of toxic overdose in smaller patients and sub-therapeutic dosing in larger ones, both of which can be life-threatening with narrow-therapeutic-index agents like methotrexate or cisplatin. The approach dates from work by Pinkel in the late 1950s and remains standard for most traditional chemotherapeutics, although some modern targeted therapies and monoclonal antibodies use fixed flat dosing because their pharmacokinetics are less BSA-dependent. Clinicians combine BSA-based dosing with renal function (Cockcroft-Gault or measured GFR), hepatic function, prior toxicity and the specific protocol when finalising a regimen. For very obese patients many guidelines now cap or adjust body weight to avoid over-dosing, again reflecting that BSA is a useful approximation but not a perfect predictor of drug handling.
What is a normal body surface area for adults and children?
Adult men average around 1.9 m² and adult women around 1.6–1.7 m². Newborns have a BSA of approximately 0.25 m², which rises steeply through infancy and early childhood — a 1-year-old is around 0.45 m², a 6-year-old around 0.7–0.8 m², a 12-year-old around 1.3 m² — before reaching adult values in the mid-teens. Children have a substantially higher BSA-to-weight ratio than adults, meaning they lose heat and fluids faster relative to body mass; this is why paediatric burn fluid resuscitation and drug dosing rely so heavily on BSA. Very tall or very heavy individuals can exceed 2.5 m². For an individual patient the relevant question is rarely 'is BSA normal' but rather 'does the BSA-normalised dose, cardiac index or fluid requirement fall within the expected range', because BSA is used as a denominator rather than as a target.
How accurate are the Du Bois, Mosteller and Haycock formulae compared with each other?
For adults of average build the three formulae typically agree within 2–5%, which is well inside the precision needed for clinical decisions. Du Bois, derived from just nine subjects in 1916, tends to under-estimate BSA in obese individuals by 5–8% compared with direct measurement methods like 3D body scanning. Mosteller (1987) is mathematically simple, easy to calculate at the bedside, and performs comparably to Du Bois for most adults — it is the formula most widely used in oncology today. Haycock (1978) was derived in a paediatric population and is preferred under about 10 kg, where Du Bois and Mosteller can diverge more noticeably. Direct measurement using 3D scanning is the most accurate method but is impractical at the bedside. When dosing a drug with a narrow therapeutic index, use whichever formula your local protocol specifies and stick to it for consistency rather than switching between them between cycles.
What are the common mistakes when calculating or using BSA?
The biggest mistake is unit mix-ups — entering weight in pounds or height in inches into a metric formula gives nonsense; both Du Bois and Mosteller assume kg and cm. Using actual weight in patients with significant oedema, ascites or massive obesity inflates BSA artificially and can produce drug doses that are unsafe; many oncology protocols cap BSA at 2.0 m² or use adjusted/ideal body weight for very obese patients. Round-trip errors are another pitfall: computing BSA in one formula and re-using it after weight changes without re-calculating means doses lag behind reality, which matters over multi-cycle chemotherapy regimens. People also occasionally apply adult formulae to neonates, where Haycock is more accurate. Finally, do not chain BSA into derived quantities (cardiac index, GFR/1.73 m²) without remembering which BSA reference value the index uses — 1.73 m² is the convention for GFR, but cardiac index is divided by the patient's actual BSA.
When should I not use this BSA calculator?
Do not use it in patients with significant amputation, massive oedema or ascites, or severe burns, where the measured body envelope no longer corresponds to weight and height in the usual way — clinical adjustments or direct measurement are needed. For neonates and very low birth-weight infants, use neonatal-specific formulae or local protocols rather than the adult-derived equations here. In morbidly obese patients (BMI > 40), the calculator will compute a BSA but most oncology dosing guidelines recommend capping or adjusting body weight before applying the formula to avoid dangerous overdoses; defer to your institution's protocol. Do not use BSA-normalised values to compare across age groups directly: paediatric cardiac index, GFR and many physiological measures have their own age-adjusted reference ranges. Finally, for modern targeted therapies and monoclonal antibodies that are licensed at fixed flat doses, BSA is irrelevant — always check the drug's specific dosing strategy before using BSA-based maths.