Body surface area (BSA) is the measured or calculated surface of the human body, expressed in square metres (m²). In clinical medicine, BSA is a critical variable for drug dosing, burn assessment, and cardiac output indexing — making accurate body surface area calculation a daily necessity for oncologists, pharmacists, and emergency physicians.
What Is Body Surface Area and Why Doctors Use It
Unlike body weight alone, body surface area accounts for the geometric relationship between height and weight. Two patients who both weigh 90 kg can have very different BSA values if one is 160 cm tall and the other is 190 cm. Because metabolic rate, blood volume, and drug clearance all correlate more closely with surface area than with weight, BSA gives clinicians a more physiologically accurate basis for medical calculations.
The Mosteller Formula — Clinical Gold Standard
BSA (m²) = √(height [cm] × weight [kg] / 3600)
Example: A patient 175 cm tall weighing 70 kg → BSA = √(175 × 70 / 3600) = √(3.403) = 1.84 m²
Published in 1987 in the New England Journal of Medicine, the Mosteller formula became the gold standard because it requires only height in centimetres and weight in kilograms. Its simplicity made bedside BSA estimation practical without a nomogram.
Mosteller vs DuBois Formula — When to Use Each
The older DuBois & DuBois formula (1916) uses BSA = 0.007184 × H0.725 × W0.425. It remains widely cited in research, but the Mosteller formula is preferred in most oncology and paediatric settings because the two agree within 1–2 % for typical adult body sizes and Mosteller is far easier to compute.
| Formula | Year | Equation | Best For |
|---|---|---|---|
| Mosteller | 1987 | √(H × W / 3600) | Bedside, oncology, paediatrics |
| DuBois & DuBois | 1916 | 0.007184 × H⁰·⁷²⁵ × W⁰·⁴²⁵ | Research, historical comparison |
| Haycock | 1978 | 0.024265 × H⁰·³⁹⁶⁴ × W⁰·⁵³⁷⁸ | Neonates, infants |
| Boyd | 1935 | Complex (weight + log H) | Underweight patients |
BSA-Based Drug Dosing — Why mg/m² Instead of mg/kg
Many chemotherapy drugs are dosed in milligrams per square metre (mg/m²) rather than per kilogram. BSA-based dosing better predicts how the body will metabolise and clear drugs because metabolic rate correlates more closely with body surface area than with weight alone. A 90 kg muscular patient and a 90 kg obese patient have very different BSA values, and thus different safe dosages.
Burns Assessment — The Rule of Nines
Emergency physicians use the Rule of Nines to estimate the percentage of total body surface area affected by burns. This rapid assessment guides fluid resuscitation using the Parkland formula: 4 mL × body weight (kg) × %BSA burned over 24 hours.
| Body Region | Adult %BSA | Child %BSA |
|---|---|---|
| Head & Neck | 9% | 18% |
| Each Arm | 9% | 9% |
| Anterior Trunk | 18% | 18% |
| Posterior Trunk | 18% | 18% |
| Each Leg | 18% | 14% |
| Perineum | 1% | 1% |
Cardiac Index — Normalising Heart Performance
Cardiac index normalises cardiac output by dividing it by BSA: CI = CO / BSA. This allows clinicians to compare heart performance across patients of different sizes. A normal cardiac index is approximately 2.5–4.0 L/min/m².
Calculate Your Body Surface Area
Use our Body Surface Area Calculator to compute BSA instantly using both the Mosteller and DuBois formulas — enter height and weight, and the calculator shows the step-by-step breakdown for both methods.
Related Reading
For the geometric foundations behind surface area measurement, read why a sphere has surface area 4πr². To understand how the Ellipsoid Calculator approximates irregular body shapes, see our article on ellipsoid approximation. For other real-world surface area applications, explore estimating paint from surface area.