What BSA Measures
Body Surface Area (BSA) is the total area of the exterior surface of the human body, expressed in square meters. For an average adult, BSA is approximately 1.9 m² — about the size of a small bathroom floor. Children have smaller BSAs (a 10-year-old is around 1.1 m²), and neonates are around 0.25 m².
BSA correlates better than body weight with metabolic mass — the part of the body that interacts with drugs, performs work, and consumes oxygen. This is why clinical pharmacology has used BSA as a dosing reference for nearly a century.
The Mosteller Formula (1987)
The most widely used BSA formula today is the Mosteller equation, published by RD Mosteller in the New England Journal of Medicine in 1987:
BSA (m²) = √(height_cm × weight_kg ÷ 3600)
For an adult at 175 cm and 70 kg:
BSA = √(175 × 70 ÷ 3600) = √3.403 = 1.84 m²
Mosteller's formula replaced the earlier DuBois formula (1916) for many clinical uses because the arithmetic is far simpler and accuracy is comparable. Both remain in use; Mosteller is more common in modern oncology and critical care.
Clinical Applications
BSA is the standard metric in several clinical contexts:
- Chemotherapy dosing: Most cytotoxic drugs are dosed in mg per m². This narrows the variability in drug exposure across patients of different body sizes compared with flat dosing.
- Cardiac index: Cardiac output (L/min) divided by BSA gives cardiac index (L/min/m²), which is more comparable across patients than raw cardiac output.
- Burn assessment: The percentage of body surface area burned (using the Rule of Nines) determines fluid resuscitation protocols and prognosis.
- Pediatric drug dosing: Many pediatric medications are dosed per m² rather than per kg because of the better correlation with metabolic mass.
Reference Values
| Population | Typical BSA |
|---|---|
| Adult men (average) | ~2.0 m² |
| Adult women (average) | ~1.7 m² |
| Children (10 years) | ~1.1 m² |
| Neonates | ~0.25 m² |
| Obese adults | 2.5–3.0 m² |
Limitations of BSA
BSA is a geometric construct — it does not directly measure metabolic mass, body composition, or organ function. For obesity pharmacology in particular, BSA-based dosing is increasingly questioned because adipose tissue is less metabolically active than lean mass. Drugs dosed per m² may be overdosed in patients with very high BMI.
For most clinical purposes, however, Mosteller's formula remains the standard. Its simplicity and reproducibility make it preferable to alternatives, and its limitations are well understood.
Resources and references
- Mosteller RD. Simplified calculation of body surface area. N Engl J Med. 1987;317(17):1098.
- DuBois D, DuBois EF. A formula to estimate the approximate surface area if height and weight be known. Arch Intern Med. 1916;17:863–871.
- Verbraecken J, Van de Heyning P, De Backer W, Van Gaal L. Body surface area in normal-weight, overweight, and obese adults. A comparison study. Metabolism. 2006;55(4):515–524.