Body Surface Area: The Medical Metric That Goes Beyond BMI
Body Surface Area (BSA) estimates the total surface of the human body in square metres. It is used clinically to calculate drug doses (especially chemotherapy), fluid requirements for burns patients, and cardiac output measurements.
Mosteller Formula (Most Widely Used Clinically)
BSA (m²) = √(height(cm) × weight(kg) ÷ 3600)Du Bois Formula (Original, 1916)
BSA (m²) = 0.007184 × height(cm)⁰·⁷²⁵ × weight(kg)⁰·⁴²⁵Haycock Formula (Preferred for Children)
BSA (m²) = 0.024265 × height(cm)⁰·³⁹⁶⁴ × weight(kg)⁰·⁵³⁷⁸Typical BSA Values
- Average adult man: ~1.9 m²
- Average adult woman: ~1.6 m²
- Newborn: ~0.25 m²
- 10-year-old child: ~1.14 m²
Clinical Use
Chemotherapy agents are typically dosed in mg/m² to account for the wide variation in body size between patients. A standard dose might be 100 mg/m² — for a patient with BSA 1.8 m² that is 180 mg, while for BSA 2.1 m² it is 210 mg. Getting this right reduces both toxicity and under-dosing.
Calculate your body surface area: Free BSA Calculator
BSA Formulas: Which to Use
Several validated formulas exist for BSA. The Mosteller formula (1987) is the simplest and most widely adopted in oncology: BSA (m²) = √[(height cm × weight kg) ÷ 3600]. The Du Bois formula (1916) was historically the standard and is still used in physiology research: BSA = 0.007184 × height⁰·⁷²⁵ × weight⁰·⁴²⁵. The Haycock formula is preferred for paediatrics. Most clinical calculators allow you to choose the formula; the differences between them are usually less than 5% for adults of normal proportions.
Clinical Applications
- Chemotherapy dosing: Most cytotoxic drugs are dosed in mg/m² of BSA to normalise for individual differences in body size. A drug dose of 75 mg/m² for a patient with BSA 1.8 m² = 135 mg total.
- Burns treatment: The extent of burn injury is expressed as a percentage of total BSA using the Rule of Nines or Lund-Browder chart. BSA helps calculate fluid resuscitation volumes.
- Cardiac output normalisation: Cardiac index (CI) = cardiac output ÷ BSA, allowing heart function to be compared between individuals of different sizes.
- Renal function: GFR measurements are normalised to 1.73 m² (average adult BSA) for comparison purposes.
Frequently Asked Questions
What is the average BSA for an adult?
The average BSA for adult men is approximately 1.9 m² and for adult women approximately 1.6 m². The commonly referenced "standard" value of 1.73 m² is an average across both sexes and is used as a normalisation denominator in many physiological measurements.
How is BSA different from BMI?
BMI (weight ÷ height²) is a simple ratio used as a population-level obesity screening tool. BSA is an estimate of the actual surface area of the body and scales differently with height and weight. BSA is used clinically for dosing and physiological normalisation; BMI is used for health risk stratification. BSA is not affected by obesity classification thresholds.
Does BSA change with age?
BSA changes as height and weight change. In children, BSA increases rapidly with growth. In adults, BSA is relatively stable unless significant weight change occurs. The formulas used for adults are not validated for neonates and young children, where Haycock or Gehan-George formulas are preferred.