Body surface area (BSA) is calculated using your height and weight, plugged into a mathematical formula that estimates the total area of your skin in square meters. The most widely used formula is the Mosteller formula: BSA equals the square root of (height in centimeters × weight in kilograms ÷ 3600). For an average adult man, the result is about 1.91 m², and for an average adult woman, about 1.71 m².
The Two Main Formulas
Two formulas dominate clinical practice. The older one, published by Du Bois and Du Bois in 1916, multiplies your weight (in kg) raised to the power of 0.425 by your height (in cm) raised to the power of 0.725, then multiplies by the constant 0.007184. It’s accurate but clunky to compute by hand.
The Mosteller formula, developed later, simplified things dramatically. You multiply your height in centimeters by your weight in kilograms, divide by 3600, and take the square root. The result is your BSA in square meters. Both formulas produce nearly identical numbers for most people, with correlations of 0.97 or higher. Because the Mosteller version is so much easier to use and just as accurate, it’s now the recommended first choice in both clinical practice and research.
A Quick Example
Say you’re 170 cm tall (about 5’7″) and weigh 70 kg (about 154 lbs). Using the Mosteller formula: 170 × 70 = 11,900. Divide by 3600 to get 3.31. The square root of 3.31 is roughly 1.82 m². That’s your estimated body surface area. Most online BSA calculators use this exact method behind the scenes.
Accuracy Differences in Larger Bodies
If you have a higher body weight, the formula you use starts to matter more. The classic Du Bois formula underestimates BSA in people with obesity by 3% in men and up to 5% in women. That might sound small, but when BSA is used to calculate drug doses, even a few percentage points can shift how much medication you receive. The Mosteller formula holds up better across normal weight, overweight, and obese adults, which is one reason it’s become the preferred option.
Why BSA Matters in Medicine
BSA isn’t just an academic number. It plays a direct role in several areas of medical care, and understanding why it exists helps explain why your doctor or pharmacist might calculate it.
Drug Dosing
Most cancer treatment drugs are dosed in milligrams per square meter of body surface area rather than by body weight alone. The logic is straightforward: larger people generally have a bigger volume of distribution (more space for the drug to spread through) and greater metabolizing capacity (they break down the drug faster). Dosing by BSA attempts to account for these differences so that a 50 kg person and a 90 kg person both get an effective amount. This is especially important for drugs with a narrow margin between a therapeutic dose and a toxic one.
Kidney Function Tests
When your doctor checks how well your kidneys are filtering blood, the result (called GFR, or glomerular filtration rate) is typically adjusted to a standard BSA of 1.73 m². This normalization lets doctors compare your kidney function against a reference range, regardless of your body size. Without it, a large person and a small person with identical kidney health would produce different raw numbers, making interpretation harder.
Burn Assessment
BSA shows up in a completely different way for burn injuries. Rather than calculating your total skin area with a formula, emergency teams use the “Rule of Nines” to quickly estimate what percentage of your total body surface area has been burned. The body is divided into regions, each representing roughly 9% (or a multiple of 9%) of total BSA. Each arm is 9%, each leg is 18%, the front torso is 18%, the back is 18%, the head is 9%, and the perineum is 1%. For smaller burns, the patient’s own palm (including fingers) represents approximately 1% of their total body surface. These percentages guide decisions about fluid replacement and treatment urgency.
BSA vs. BMI
People sometimes confuse BSA with BMI, but they measure different things. BMI (body mass index) is a ratio of weight to height squared, used as a rough screening tool for weight categories. BSA estimates actual skin surface area and is used for medical calculations that depend on body size, like drug dosing. You can have two people with the same BMI but different BSAs if their height and weight combinations differ. BMI tells you something about relative weight; BSA tells you something about physical size.