Body surface area (BSA) is calculated using your height and weight plugged into a mathematical formula, with the result expressed in square meters (m²). The simplest and most widely used method is the Mosteller formula: take your height in centimeters, multiply it by your weight in kilograms, divide by 3,600, then take the square root. For an average adult male, the result lands around 1.91 m²; for an average adult female, about 1.71 m².
The Mosteller Formula
If you only learn one BSA formula, make it this one. The Mosteller formula is the go-to in most clinical settings because it’s fast and accurate enough for the vast majority of patients:
- BSA (m²) = √(height in cm × weight in kg ÷ 3,600)
Here’s a worked example. Say you’re 170 cm tall and weigh 70 kg. Multiply 170 × 70 to get 11,900. Divide by 3,600 to get 3.306. Take the square root: 1.818 m². That’s your body surface area.
If your measurements are in pounds and inches (common in the U.S.), convert first. Multiply inches by 2.54 to get centimeters, and multiply pounds by 0.4536 to get kilograms. So a person who is 5’7″ (67 inches = 170.18 cm) and 154 lbs (69.85 kg) would get nearly the same result as the example above.
The Du Bois Formula
The Du Bois and Du Bois equation dates back over a century and remains one of the most referenced BSA formulas in medical literature:
- BSA (m²) = 0.007184 × height (cm)0.725 × weight (kg)0.425
Those fractional exponents make it harder to calculate by hand, which is why the Mosteller formula largely replaced it for everyday use. The two formulas produce very similar results for adults of typical body size. Where they diverge is at the extremes: the Du Bois formula can underestimate BSA in people with obesity by as much as 20%, because weight increases without a proportional increase in height, and the formula’s height coefficient doesn’t account for that shift.
BSA Formulas for Infants and Children
Children, especially infants, have different body proportions than adults. Their heads are proportionally larger and their limbs shorter, which means adult-derived formulas can miss the mark. The Haycock formula was validated across a range from premature infants to adults and handles small body sizes more reliably:
- BSA (m²) = 0.024265 × weight (kg)0.5378 × height (cm)0.3964
This formula was derived from direct measurements of 81 subjects and produces accurate estimates for BSA values as low as 0.2 m² (a small infant) up through 2.0 m² and above. For very small patients under 10 kg, some researchers have proposed a weight-only formula: BSA = 0.1037 × weight (kg)0.6724. This avoids relying on length measurements, which can be difficult to obtain accurately in squirming newborns.
Why BSA Matters in Medicine
BSA is not just an academic number. It drives real medical decisions in two major areas: drug dosing and burn assessment.
Chemotherapy Dosing
Most chemotherapy drugs are dosed based on BSA rather than body weight alone. The logic is that BSA correlates more closely with metabolic rate, blood volume, and kidney function, all of which influence how quickly the body processes a drug. Fixed-dose chemotherapy is rarely used for cytotoxic agents. Instead, a dose might be written as milligrams per square meter, and your BSA determines the actual amount you receive. This is why even small differences in BSA calculations can have meaningful clinical consequences.
Burn Assessment
When someone suffers a burn, clinicians estimate what percentage of total body surface area is affected. That percentage determines how much IV fluid the patient needs, whether they should be transferred to a burn center, and what their likely prognosis looks like. Several estimation methods exist. The “rule of nines” divides the adult body into sections each representing roughly 9% of total BSA: each arm is 9%, each leg is 18%, the torso front and back are each 18%, and the head is 9%. A common shortcut uses the patient’s own palm (not including fingers), which represents about 0.78% of their BSA.
For children, these proportions shift. A toddler’s head can account for a much larger percentage of total BSA than an adult’s, while the legs represent less. The Lund and Browder chart adjusts these percentages by age, making it the most accurate of the manual estimation methods, with a resolution as fine as 0.25% of total BSA. The rule of nines tends to overestimate burn size, especially in people with high BMI, which has led to modified versions: a “rule of five” for patients over 80 kg and a “rule of eight” for infants under 10 kg.
Accuracy Problems in Obesity
None of the commonly used BSA formulas were originally validated for people with obesity. Research published in the American Journal of Physiology found that the Du Bois equation underestimated BSA in obese patients by up to 20%. The core problem is straightforward: as someone gains weight, their surface area increases, but not in the way these height-and-weight formulas predict. The extra tissue distributes differently than the equations assume.
For patients over 80 kg, a simpler weight-only formula has been shown to outperform the traditional equations: BSA (m²) = 0.1173 × weight (kg)0.6466. By dropping height from the equation entirely, it avoids the distortion that obesity introduces into height-weight models. This matters most when BSA is being used to calculate drug doses, where a 20% underestimate could mean a meaningfully lower dose than intended.
How to Calculate BSA Without the Math
In practice, most people (including many clinicians) use an online calculator or a built-in function in electronic health records rather than computing BSA by hand. If you want to check the math yourself, the Mosteller formula is the easiest to do on any phone calculator: multiply height in centimeters by weight in kilograms, divide by 3,600, and hit the square root button. The whole calculation takes about 10 seconds.
If you’re working in imperial units, the conversion step adds a moment: multiply your height in inches by 2.54, and your weight in pounds by 0.4536. From there, the formula works identically. For a quick sanity check, most adults fall between 1.5 and 2.2 m². If your result is outside that range and you’re an average-sized adult, double-check your unit conversions.