Body surface area (BSA) is calculated using your height and weight plugged into a mathematical formula. The most commonly used formula in clinical settings is the Mosteller formula, which multiplies your height in centimeters by your weight in kilograms, divides by 3,600, and takes the square root of the result. The answer comes out in square meters (m²), and the average adult BSA is roughly 1.7 to 2.0 m².
The Main BSA Formulas
Several formulas exist because researchers over the past century have tried different mathematical approaches to estimate how much skin covers the human body. Each formula uses height and weight but applies different exponents and constants. The five you’ll encounter most often are:
- Mosteller: BSA = √(height in cm × weight in kg / 3,600). This is the simplest and most widely used in everyday clinical practice.
- Du Bois and Du Bois: BSA = 0.007184 × height (cm)^0.725 × weight (kg)^0.425. Published in 1916, this is the oldest formula and remains a standard reference.
- Haycock: BSA = 0.024265 × height (cm)^0.3964 × weight (kg)^0.5378. Developed with newborns, infants, children, and adults, making it especially reliable for younger patients.
- Gehan and George: BSA = 0.0235 × height (cm)^0.42246 × weight (kg)^0.51456.
- Boyd: BSA = 0.017827 × height (cm)^0.3 × weight (kg)^0.4838.
For most adults, the Mosteller and Du Bois formulas produce nearly identical results and are considered interchangeable. The Mosteller formula became popular because it’s easy to calculate by hand or with a basic calculator, while the Du Bois formula requires working with fractional exponents.
Why BSA Matters in Medicine
BSA is not just an academic measurement. It’s built into several areas of routine medical care where your body size affects how a treatment or test result should be interpreted.
The most prominent use is in cancer treatment. Dosing for most chemotherapy drugs is written as milligrams per square meter of BSA. A patient with a BSA of 1.8 m² receiving a drug dosed at 100 mg/m² would get 180 mg. The idea is that people with larger bodies have more blood volume and tissue mass, so they need proportionally more drug to reach the same concentration. BSA-based dosing is an attempt to reduce the variation in how different-sized patients respond to the same drug.
BSA also plays a central role in kidney function testing. When your doctor orders a blood test to check your kidneys, the result (called estimated GFR, or glomerular filtration rate) is automatically adjusted to a standard BSA of 1.73 m². This normalization allows doctors to compare your kidney function against established thresholds for kidney disease, regardless of whether you’re a small or large person. The definitions of chronic kidney disease stages are all based on this BSA-adjusted number.
In cardiology, BSA converts raw cardiac output (how much blood your heart pumps per minute) into the cardiac index, which is cardiac output divided by BSA. This makes it possible to compare heart performance between a 50 kg woman and a 100 kg man on equal footing.
Where the 1.73 m² Standard Comes From
If you’ve seen kidney function results reported “per 1.73 m²,” that number has an interesting origin. It was the average BSA of 25-year-old American men and women in the 1920s, drawn from life insurance data. Researchers in the early 20th century found that adjusting kidney clearance values by surface area produced more consistent normal ranges than adjusting by weight alone, and 1.73 m² became the reference point.
That figure no longer reflects the average adult in Western populations, since people are generally taller and heavier now. But the medical community has chosen to keep it as a permanent constant so that kidney function data can be compared across different countries, time periods, and studies. Changing it would break continuity with decades of clinical research.
Which Formula Works Best for Children
The Du Bois formula was originally derived from measurements of just nine patients, and it performs poorly at the extremes of body size, particularly in infants and small children. For pediatric patients, the Haycock formula is generally recommended. It was developed using a diverse group of 81 individuals ranging from newborns to adults, including children of varying body compositions and ethnic backgrounds. Validation studies in children with blood cancers have confirmed that the Haycock formula produces estimates closest to measured BSA values. The Mosteller formula also performs well in children and is a reasonable alternative.
Accuracy Problems in Obese Patients
None of the classic BSA formulas were validated for people with obesity, and this creates a real clinical problem. The Du Bois formula can underestimate BSA in obese patients by as much as 20%. Gehan and George themselves acknowledged their formula failed for obese individuals. Since more than a third of adults in many countries are now obese, this isn’t a niche concern.
The issue matters most in chemotherapy dosing. If BSA is underestimated, the calculated drug dose will be lower than it should be, potentially reducing treatment effectiveness. Researchers have proposed weight-only scaling formulas that perform better at higher body weights, but these haven’t fully replaced the traditional equations in standard practice. For patients weighing over 80 kg, the standard Du Bois model introduces progressively larger errors compared to models specifically calibrated for larger body sizes.
BSA in Burn Assessment
There’s a completely different version of “body surface area calculation” used in emergency medicine for burns. Rather than calculating your total BSA in square meters, burn assessment estimates what percentage of your skin is affected. The most common method is the Rule of Nines:
- Head: 9%
- Each arm: 9%
- Chest: 9%
- Abdomen: 9%
- Upper back: 9%
- Lower back: 9%
- Each leg: 18% (9% front, 9% back)
- Groin: 1%
This system works well for adults between roughly 10 kg and 80 kg, but it assumes everyone has the same proportions. For obese patients, the trunk contributes a much larger share of total surface area. A modified “Rule of Fives” assigns 50% to the trunk, 5% to each arm, and 20% to each leg. For infants under 10 kg, a “Rule of Eights” gives the head 20% (since babies’ heads are proportionally larger) and reduces the leg percentage to 16% each.
For smaller burns, you can use the palm method: your palm with fingers together represents roughly 1% of your total body surface area, though this slightly overestimates, since the true value is closer to 0.8% in men and 0.7% in women.