Body Surface Area Calculator
Estimate body surface area from height and weight using four validated formulas. BSA is used to calculate drug doses, cardiac index, and burn assessment.
🧍 What is Body Surface Area?
Body surface area (BSA) is the total external surface area of the human body, expressed in square metres. Because it is impractical to measure directly, BSA is estimated from height and weight using validated formulas. The most common, the Mosteller formula, is simply the square root of height in centimetres times weight in kilograms divided by 3600. An average adult has a BSA of roughly 1.7 square metres.
BSA matters most in medicine. Many drugs, especially chemotherapy agents, are dosed per square metre of body surface area rather than per kilogram of weight, because BSA tracks blood volume, kidney function, and metabolic rate more closely than weight alone. BSA is also used to calculate cardiac index (cardiac output divided by BSA), to estimate the extent of burns, and to index other physiological measurements so that patients of different sizes can be compared fairly.
A common misconception is that a single formula is definitively correct. Several formulas exist, including Mosteller, Du Bois, Haycock, and Gehan-George, and they were derived from different populations. In practice they agree within a few percent for most people, so the choice rarely changes a clinical decision. Another misconception is that BSA is the same as body mass index; BMI relates weight to height squared as a measure of relative weight, while BSA estimates actual surface area.
This calculator computes BSA from height and weight using four widely used formulas at once, in either metric or imperial units, and shows the converted height and weight so you can see exactly what values were used. The Mosteller result is highlighted because it is the standard clinical choice, with the others provided for comparison.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Average Adult (metric)
A person 170 cm tall weighing 70 kg
Example 2 — Larger Adult (metric)
A person 180 cm tall weighing 85 kg
Example 3 — Imperial Units
A person 67 in tall weighing 154 lb
❓ Frequently Asked Questions
🔗 Related Calculators
What is body surface area (BSA)?
Body surface area (BSA) is the total surface area of the human body, measured in square metres (m²). It is calculated from height and weight and is used in medicine as a more reliable basis than body weight alone for drug dosing, especially chemotherapy, and for indexing physiological measurements such as cardiac output. An average adult BSA is about 1.7 m².
What is the Mosteller formula for BSA?
The Mosteller formula is BSA = √(height × weight / 3600), with height in centimetres and weight in kilograms, giving BSA in square metres. For a person 170 cm tall weighing 70 kg, BSA = √(170 × 70 / 3600) = √3.306 = 1.82 m². It is the most popular formula because it is simple and accurate.
How do you calculate body surface area?
Enter height and weight into a BSA formula. The Mosteller formula, BSA = √(height × weight / 3600), is the easiest. Others such as Du Bois use powers of height and weight. For 180 cm and 85 kg, Mosteller gives √(180 × 85 / 3600) = √4.25 = 2.06 m². This calculator shows four formulas at once for comparison.
Which BSA formula is most accurate?
No single formula is universally most accurate, but Mosteller and Du Bois are the most validated and widely used. Haycock and Gehan-George were derived to work well across children and adults. In practice the formulas agree within a few percent for most people, so the choice rarely changes a clinical decision. Mosteller is preferred for its simplicity.
Why is BSA used for drug dosing?
BSA correlates better than body weight with physiological parameters like blood volume, kidney filtration, and metabolic rate, which govern how drugs are distributed and cleared. Dosing per square metre gives more consistent drug exposure across patients of different sizes. This is why chemotherapy doses are almost always prescribed in milligrams per square metre of BSA.
What is a normal body surface area?
The average adult BSA is about 1.7 m². Adult men average roughly 1.9 m² and women about 1.6 m². Newborns are near 0.25 m², and children rise toward adult values through growth. BSA depends on both height and weight, so a tall, heavy person has a larger BSA than a short, light one.
How do I convert height and weight for BSA?
The formulas use centimetres and kilograms. To convert, multiply inches by 2.54 to get centimetres and divide pounds by 2.2046 to get kilograms. This calculator does the conversion automatically when you choose imperial units, so you can enter height in inches and weight in pounds directly.
What is the difference between the Mosteller and Du Bois formulas?
Mosteller uses a simple square root: BSA = √(height × weight / 3600). Du Bois uses BSA = 0.007184 × weight^0.425 × height^0.725, an older formula from 1916 derived from a small sample. The two usually agree within about 1 to 2 percent. Mosteller is easier to compute by hand and is now the more common clinical choice.
Can BSA be used for children?
Yes. The Mosteller and Haycock formulas were validated across a wide range of body sizes including infants and children, so they work for paediatric dosing. Because children's doses are sensitive to size, accurate height and weight are important. Always follow paediatric dosing protocols, which may specify a particular BSA formula.
What is cardiac index and how does BSA relate to it?
Cardiac index is cardiac output divided by body surface area, expressed in litres per minute per square metre. Dividing by BSA normalises cardiac output for body size so values can be compared between patients. A normal cardiac index is about 2.5 to 4.0 L/min/m². BSA is therefore essential for interpreting many haemodynamic measurements.