Body Surface Area (BSA) Calculator
Calculate Body Surface Area using the Mosteller formula.
Supports metric and imperial units.
Used for medication dosing and clinical assessments.
Body Surface Area (BSA) is the total exposed surface area of the human body measured in square meters (m²). Unlike body weight alone, BSA accounts for both height and weight, which tracks metabolic function more closely and is why it is used so heavily in medical dosing.
Mosteller Formula (most widely used in clinical practice):
Metric:
BSA (m²) = √[(Height cm × Weight kg) / 3600]
Imperial:
BSA (m²) = √[(Height in × Weight lbs) / 3131]
Other validated BSA formulas:
Height in cm and weight in kg throughout, except where noted.
| Formula | Equation | Notes |
|---|---|---|
| Mosteller (1987) | √[(Ht × Wt) / 3600] | Simple; most commonly used |
| Du Bois & Du Bois (1916) | 0.007184 × Ht^0.725 × Wt^0.425 | The original; derived from just nine subjects |
| Haycock (1978) | 0.024265 × Ht^0.3964 × Wt^0.5378 | Best accuracy for children |
| Boyd (1935) | 0.0003207 × Ht^0.3 × Wt(grams)^(0.7285 − 0.0188 × log₁₀ Wt(grams)) | Complex; rarely used clinically |
Watch the Boyd row. Its weight term is in grams, not kilograms, and the exponent contains a base-10 logarithm of that same gram figure. Feed it kilograms and the answer comes out roughly ten times too small. This catches people constantly, because every other formula on the list takes kilograms.
Run all four on an adult of average build and they land within about 1%: for a 180 cm, 80 kg man you get 2.00 m² from Mosteller, 2.00 from Du Bois and 2.01 from Haycock. The agreement falls apart at the edges. In obese patients Du Bois underestimates, sometimes by 5% or more, because its exponents came from nine people in 1916, none of them obese. In small children Haycock is the one to reach for.
Reference BSA values by population:
| Population | Average BSA |
|---|---|
| Newborn (3.5 kg) | 0.23 m² |
| 1-year-old | 0.47 m² |
| Child (10 years) | 1.14 m² |
| Adolescent (16 years) | 1.60 m² |
| Adult female | 1.60–1.70 m² |
| Adult male | 1.85–1.95 m² |
Worked example: Adult male: Height 180 cm, Weight 80 kg BSA (Mosteller) = √[(180 × 80) / 3600] = √[14400 / 3600] = √4.0 = 2.00 m²
Clinical applications of BSA:
- Chemotherapy dosing: Most cytotoxic drugs are dosed in mg/m² to normalize across body sizes and reduce toxicity risk
- Burn assessment: The Rule of Nines estimates percentage of BSA burned to guide fluid resuscitation (e.g., each arm = 9%, each leg = 18%, torso front = 18%)
- Cardiac index: Cardiac output (L/min) ÷ BSA → normalizes heart function across different body sizes
- Drug clearance: Renal function (GFR) is often reported as mL/min/1.73 m² (normalized to a reference BSA of 1.73 m²)
- Pediatric dosing: Children’s doses are often BSA-based rather than weight-based for precision
Important note: BSA is a geometric approximation. Individual anatomy, age, sex, and ethnicity can influence the accuracy of any formula.
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