Body Surface Area (BSA) Calculator

BSA cannot be measured directly without coating the patient in something — Du Bois used melted paraffin in 1916, Boyd used wax paper in 1935. So clinical practice uses formulas that estimate BSA from height and weight, validated against direct-measurement studies. The five formulas in widespread use are: Mosteller (1987): BSA = √(H × W / 3600), simple and the modern default. Du Bois (1916): BSA = 0.007184 × W^0.425 × H^0.725, the original equation, validated against 9 subjects, surprisingly accurate. Haycock (1978): BSA = 0.024265 × W^0.5378 × H^0.3964, optimized for infants and children. Gehan and George (1970): BSA = 0.0235 × W^0.51456 × H^0.42246, derived from a 401-subject pediatric-to-adult sample. Boyd (1935): a logarithmic formula traditional in pediatrics. All five typically agree within 5% for adults; differences are largest in infants and very small or very obese patients. Our BSA calculator runs all five so you can see the spread and choose the formula your institution's protocol specifies.

Mosteller's 1987 letter in the New England Journal of Medicine reduced BSA calculation to a single square root: BSA (m²) = √(height_cm × weight_kg / 3600). In imperial units: BSA = √(height_in × weight_lb / 3131). The formula was derived to approximate Du Bois with mental-math simplicity, and it has since displaced Du Bois as the most widely used BSA equation in modern clinical oncology. NCCN guidelines, Cancer Care Ontario, and most US oncology pharmacies use Mosteller as default. Validation studies show Mosteller agrees with Du Bois to within 1-2% for adults of normal body size, slightly overestimates BSA in very small children, and slightly underestimates BSA in extreme obesity. For most clinical purposes, Mosteller is the right first choice. Our Mosteller BSA calculator implements the exact formula from the 1987 publication.

Du Bois & Du Bois published the original BSA equation in 1916 after directly measuring skin area on 9 subjects coated in melted paraffin. Their equation: BSA (m²) = 0.007184 × weight_kg^0.425 × height_cm^0.725. Despite the small original sample, it remains validated by modern studies — direct measurement using 3D body scanning confirms Du Bois is accurate within 5% across most adults. Du Bois is still the dosing default at some institutions in Europe and Australia (eviQ Australia uses it as default). The Mosteller approximation matches Du Bois to within 1-2% in adults but is simpler to compute. Our Du Bois BSA calculator runs the original 1916 exponents — useful for cross-referencing with Mosteller and for institutions that specify Du Bois in protocol.

Pediatric BSA dosing requires more care than adults because the formulas diverge at small body sizes. Haycock, Schwartz, and Wisotsky (1978) derived a formula specifically tuned for infants and children: BSA = 0.024265 × W^0.5378 × H^0.3964. Gehan and George (1970) sampled 401 subjects spanning newborns to adults: BSA = 0.0235 × W^0.51456 × H^0.42246. Both outperform Mosteller and Du Bois below 10 kg body weight. Haycock is the recommended formula in pediatric oncology dosing at many children's hospitals worldwide; Gehan-George is used by the European Organisation for Research and Treatment of Cancer (EORTC). Our pediatric BSA calculator displays both Haycock and Gehan-George values so you can see the spread. For chemotherapy doses in patients under 10 kg, always cross-check with a pediatric clinical pharmacist.

Most cytotoxic chemotherapy regimens specify dose in mg/m² of body surface area, dating to Pinkel's 1958 observation that drug clearance correlates with BSA rather than weight alone. An adult with BSA 1.85 m² on a 200 mg/m² regimen receives 370 mg. Standard practice: recompute BSA when weight changes by 10% or more. Most institutions use Mosteller as default; some specify Du Bois. ASCO 2012 (updated 2019) guidance argues against capping BSA in obese patients for most curative-intent regimens — historical capping at 2.0-2.2 m² resulted in underdosing and worse outcomes. Our BSA calculator for chemotherapy provides all five formulas so the prescribing physician can match institutional protocol. Always have chemotherapy doses verified by a clinical pharmacist; BSA-calculation errors account for a measurable share of pediatric and adult chemotherapy dose errors.

Healthy adult BSA spans roughly 1.6-2.0 m². Reference points by sex: average adult women BSA ≈ 1.6 m² (US population, NHANES data). Average adult men BSA ≈ 1.9 m². 1.73 m² is the GFR normalization reference (average adult body, used for kidney function reporting like 'eGFR 65 mL/min/1.73 m²'). Pediatric reference points: newborn ~0.25 m², 1-year-old ~0.5 m², 5-year-old ~0.75 m², 10-year-old ~1.0 m², adolescent (13-15) ~1.4-1.6 m², adult values by 16-18. Adult height doesn't change meaningfully until age 60+, so adult BSA tracks weight: a 10 kg weight change produces ~5% BSA change. There is no 'too low' or 'too high' BSA in the way there is for BMI — BSA is a scaled body-size measure used for dosing and physiology, not for health categorization.

BSA and BMI serve different clinical purposes. BMI (weight ÷ height²) is a screening tool for under/overweight categorization, designed for population-level health risk assessment — 'are you in a healthy weight range?' BSA is a clinical dosing and physiology tool used for chemotherapy mg/m², pediatric fluid maintenance, cardiac index, and GFR normalization — it doesn't classify you as healthy or unhealthy. BMI uses simple division: BMI = kg / m². BSA uses validated empirical formulas with non-linear height/weight exponents. A 100-kg adult and a 50-kg child have very different per-kg dosing needs but BSA captures the difference cleanly; BMI does not. Use BMI for personal weight-status conversations and population screening; use BSA for clinical drug dosing and physiology normalization.

All five clinical BSA formulas are most cleanly expressed in metric units — height in cm, weight in kg, BSA output in m². For imperial inputs, our BSA calculator converts internally: cm = inches × 2.54; kg = pounds × 0.4536. The Mosteller formula has a direct imperial form: BSA (m²) = √(height_in × weight_lb / 3131) — useful for mental math in countries that use imperial units clinically (notably the US). Du Bois, Haycock, Gehan-George, and Boyd are all defined in metric and require conversion. The BSA value is always reported in m² regardless of input units, because all clinical dosing protocols use m². Our BSA calculator metric/imperial toggle handles both ends of the conversion.

BSA is used across multiple medical specialties. Cardiology: cardiac index = cardiac output (L/min) ÷ BSA (m²), used to compare cardiac function across body sizes; normal cardiac index is 2.5-4.0 L/min/m². Nephrology: glomerular filtration rate is reported normalized to 1.73 m² of BSA so 'eGFR 65 mL/min/1.73 m²' is comparable across all body sizes; some specialists argue for un-normalized GFR for drug dosing decisions. Pediatrics: fluid maintenance is sometimes calculated per m² (e.g., 1500 mL/m²/day instead of the weight-based 4-2-1 rule), particularly in oncology and critical care. Burn medicine: 'total body surface area burned' uses the Rule of Nines, a different concept that estimates burned fraction. Anesthesia: drug clearance and ventilation targets occasionally use BSA. Our BSA calculator is suitable for all these uses — the underlying m² value is identical regardless of downstream application.

Historical practice capped BSA at 2.0 m² or 2.2 m² for chemotherapy in obese patients, on the theory that linear BSA-dose scaling overestimates required dose at large body sizes. ASCO 2012 guidance (Griggs et al., updated 2019) reversed this: in curative-intent regimens, obese patients should receive full-BSA dosing, and historical capping resulted in measurable under-treatment and worse outcomes. ASCO recommends actual BSA dosing for most cytotoxic agents in obese adults, with specific exceptions (carboplatin uses AUC dosing instead; some agents have published pharmacokinetic data supporting modified dosing in obesity). Pediatric obesity dosing is less studied and institutional protocols vary. If you're a patient: ask your oncologist whether your dose was calculated on full or capped BSA, and what the institutional protocol is for your specific regimen.

Pediatric BSA values across age. Newborn (3.5 kg, 50 cm): Mosteller 0.221 m², Haycock 0.219 m², Gehan-George 0.220 m² — formulas agree to ~1%. 6-month-old (7 kg, 65 cm): Mosteller 0.355 m², Haycock 0.353 m². 2-year-old (12 kg, 86 cm): Mosteller 0.535 m², Haycock 0.534 m². 5-year-old (18 kg, 110 cm): Mosteller 0.742 m², Haycock 0.751 m² (formulas start to diverge ~1%). 10-year-old (32 kg, 138 cm): Mosteller 1.107 m², Haycock 1.115 m². Adolescent (50 kg, 165 cm): Mosteller 1.514 m², Haycock 1.519 m². For dosing under 10 kg, Haycock or Gehan-George is preferred; above 10 kg, Mosteller is acceptable and matches institutional defaults at most pediatric oncology centers. Always cross-check pediatric chemotherapy doses with a clinical pharmacist.

BSA dosing standards are similar but not identical across English-speaking healthcare systems. United States: Mosteller is the most common institutional default; Du Bois is acceptable. NCCN and ASCO guidelines reference both. United Kingdom: NICE oncology guidance specifies BSA-based dosing with Mosteller or Du Bois acceptable; institutional pharmacy services typically default to Mosteller. Canada: Cancer Care Ontario and provincial cancer agencies use Mosteller or Du Bois with institutional preference; Canadian Pediatric Society pediatric oncology protocols default to Haycock for under-10-kg patients. Australia: eviQ (Cancer Institute NSW) specifies Du Bois as default with Mosteller acceptable; pediatric protocols use Haycock. Our BSA calculator works for all four jurisdictions — choose the formula your institutional protocol specifies as default. For chemotherapy dosing, always have the calculation verified by a clinical pharmacist before administration.