What is Body Surface Area?
Body Surface Area (BSA) is a clinical measurement that estimates the total area of the outer surface of the human body, expressed in square metres. First described mathematically by Du Bois and Du Bois in 1916, BSA has become one of the most important anthropometric measurements in medicine, pharmacology, and physiology.
BSA matters because many physiological processes scale more closely with surface area than with body weight. Metabolic rate, heat loss, fluid requirements, and organ function all correlate with body surface area. This makes BSA a better normalising factor than weight for comparing physiological measurements across patients of different sizes.
The most critical clinical application of BSA is drug dosing. Many medications -- especially chemotherapy agents -- have narrow therapeutic windows where the difference between an effective dose and a toxic dose is small. Dosing by BSA rather than body weight reduces the variability in drug exposure between patients, leading to safer and more effective treatment.
The Mosteller Formula
This calculator uses the Mosteller formula, published by R.D. Mosteller in the New England Journal of Medicine in 1987. It is the most widely used BSA formula in clinical practice:
[\text{BSA} = \sqrt{\frac{H \times W}{3{,}600}}]
Where:
- BSA is the Body Surface Area in square metres (m²).
- H is the height in centimetres.
- W is the weight in kilograms.
- 3,600 is a constant that scales the product of height and weight to produce a result in m².
If your measurements are in Imperial units, the calculator converts them before applying the formula: inches to centimetres (multiply by 2.54) and pounds to kilograms (multiply by 0.453592).
Calculation Example
Calculate the BSA for a person who is 170 cm tall and weighs 70 kg.
Step 1: Multiply height by weight.
[\text{170} \times 70 = 11{,}900]
Step 2: Divide by 3,600.
[\frac{11{,}900}{3{,}600} = 3.3056]
Step 3: Take the square root.
[\sqrt{3.3056} \approx 1.8182]
The Body Surface Area is approximately 1.82 m².
BSA Reference Values
| Category | Typical BSA (m²) |
|---|---|
| Newborn (3.5 kg) | 0.20 - 0.25 |
| Child, age 5 (18 kg) | 0.70 - 0.80 |
| Child, age 10 (30 kg) | 1.00 - 1.20 |
| Adult woman (average) | 1.60 - 1.80 |
| Adult man (average) | 1.80 - 2.00 |
| Large adult (>100 kg) | 2.10 - 2.40 |
Clinical Applications of BSA
Drug Dosing
BSA-based dosing is standard for chemotherapy agents such as carboplatin, doxorubicin, and paclitaxel. The dose is expressed in milligrams per square metre (mg/m²), and the patient's BSA is multiplied by this value to determine the actual dose administered. For example, if a drug is prescribed at 75 mg/m² and the patient's BSA is 1.82 m², the dose would be 75 × 1.82 = 136.5 mg.
Cardiac Index
The cardiac index normalises cardiac output by BSA, allowing comparison between patients of different sizes. It is calculated as cardiac output (litres per minute) divided by BSA (m²). A normal cardiac index is 2.5 to 4.0 L/min/m². Values below 2.2 L/min/m² indicate cardiogenic shock.
Burn Assessment
BSA is used to estimate the extent of burn injuries. The "Rule of Nines" divides the adult body into regions each representing approximately 9 percent of BSA. The total BSA affected by burns guides fluid resuscitation calculations using formulas such as the Parkland formula, which requires 4 mL of crystalloid per kilogram of body weight per percent of BSA burned during the first 24 hours.
Renal Function
The glomerular filtration rate (GFR) is often normalised to a standard BSA of 1.73 m² -- a value chosen in the 1920s as the average BSA of young adults at that time. Reporting GFR per 1.73 m² allows comparison of kidney function across patients regardless of body size.
Comparing BSA Formulas
Several BSA formulas have been published over the past century:
| Formula | Year | Equation |
|---|---|---|
| Du Bois | 1916 | 0.007184 × H⁰·⁷²⁵ × W⁰·⁴²⁵ |
| Boyd | 1935 | Complex logarithmic formula |
| Gehan-George | 1970 | 0.0235 × H⁰·⁴²²⁴⁶ × W⁰·⁵¹⁴⁵⁶ |
| Haycock | 1978 | 0.024265 × H⁰·³⁹⁶⁴ × W⁰·⁵³⁷⁸ |
| Mosteller | 1987 | √(H × W / 3600) |
The Mosteller formula produces values within 2 percent of the Du Bois formula across the normal adult range and is preferred because it can be calculated quickly with a basic calculator or even mental arithmetic. For the average adult, the difference between formulas is clinically insignificant.
BSA in Paediatric Medicine
Body Surface Area has an especially important role in paediatric medicine, where patients range from premature neonates weighing less than 1 kilogram to adolescents approaching adult size. Drug dosing in children cannot rely on simple weight-based calculations because children are not just small adults -- their organ function, body water distribution, and metabolic rates differ from adults in ways that affect drug absorption, distribution, and clearance.
BSA-based dosing is the standard for most paediatric chemotherapy protocols. Because BSA accounts for both height and weight, it correlates better with metabolic rate and organ size than weight alone. A 30-kilogram child who is tall for their age has a different BSA (and therefore a different drug dose) than a 30-kilogram child who is short and heavy. This distinction is clinically important for drugs with narrow therapeutic margins.
For very small children and neonates, some clinicians argue that BSA-based dosing is less reliable because the Mosteller formula was validated primarily on adults and older children. The Haycock formula, which uses different exponents for height and weight, is sometimes preferred for patients under 10 kilograms. In practice, the difference between formulas is small and most institutions use the Mosteller formula across all age groups for consistency.
Beyond drug dosing, paediatric cardiologists use BSA to normalise echocardiographic measurements. Heart chamber sizes, valve areas, and aortic dimensions are all reported as z-scores relative to BSA-predicted normal values. A left ventricular dimension that would be normal for an adolescent would indicate severe dilation in a toddler. BSA normalisation allows the same reference tables to be used across the entire paediatric age range.
Paediatric nephrologists similarly normalise the glomerular filtration rate to 1.73 m² BSA when assessing kidney function in children. This standardisation is essential because a raw GFR of 60 mL/min might be normal for a small child but would indicate significant impairment in an adult.
Practical Considerations
- Measure accurately. A 2 cm error in height or a 2 kg error in weight can shift the BSA by 1 to 2 percent, which translates directly to a 1 to 2 percent change in drug dose. Use calibrated scales and a stadiometer for the most reliable inputs.
- Obesity considerations. In obese patients, BSA-based dosing can lead to excessively high drug doses because BSA increases with body size. Some oncology protocols cap the BSA at 2.0 m² or use adjusted body weight for dose calculation in patients above a certain BMI.
- Paediatric use. The Mosteller formula is validated for children as well as adults and is the standard BSA formula in paediatric oncology. However, neonates and infants have different body proportions than adults, so the Haycock formula is sometimes preferred for very young patients.