How to Calculate Body Surface Area (BSA)

Body surface area is one of the most important measurements in clinical medicine, yet most people have never heard of it. BSA is used to calculate chemotherapy doses, normalize kidney function measurements, assess burn injuries, and estimate cardiac output. Unlike body mass index, which classifies weight categories, BSA estimates the total external area of your skin and serves as a better proxy for metabolic body size than weight alone.

This guide covers the major BSA formulas, explains when each is most appropriate, walks through clinical applications with real-world examples, and helps you understand why your healthcare team may calculate your BSA as part of your care.

What Is Body Surface Area?

Body surface area (BSA) is the measured or calculated total area of the external surface of the human body, expressed in square meters (m²). Because directly measuring the skin surface of a living person is impractical, BSA is estimated from height and weight using mathematical formulas derived from empirical data.

The concept of using BSA in medicine dates back to the early 1900s, when researchers discovered that metabolic rate, blood volume, and drug clearance correlate more closely with body surface area than with body weight. A 100-kilogram person who is 190 cm tall has a very different physiological profile than a 100-kilogram person who is 155 cm tall, even though they weigh the same. BSA captures this difference in a single number.

Why BSA Matters in Medicine

BSA is the standard metric for several critical clinical calculations. In oncology, most chemotherapy drugs are dosed in milligrams per square meter of BSA (mg/m²) because this approach produces more consistent drug exposure across patients of different sizes than weight-based dosing. In nephrology, GFR is normalized to a standard BSA of 1.73 m² so that kidney function can be compared between individuals of different body sizes. In burn medicine, BSA provides the framework for estimating the percentage of body surface affected by burns, which determines fluid resuscitation requirements and treatment protocols.

BSA Formulas Compared

Several formulas have been developed to estimate BSA from height and weight. Each was derived from different study populations and uses slightly different mathematical approaches. The three most commonly used in clinical practice are the Du Bois, Mosteller, and Haycock formulas.

The Du Bois Formula

Published in 1916 by Delafield Du Bois and Eugene Du Bois, this was the first widely adopted BSA formula. It was derived from direct measurements of only nine individuals, yet has proven remarkably durable.

• H = height in centimeters
• W = weight in kilograms
• Result in m²

The Du Bois formula is accurate for adults of average build but tends to overestimate BSA in obese individuals and may be less accurate at the extremes of body size.

The Mosteller Formula

Published in 1987 by R.D. Mosteller, this simplified formula produces nearly identical results to the Du Bois formula while being much easier to calculate by hand or with a basic calculator.

The Mosteller formula has become the most widely used BSA equation in clinical practice due to its simplicity and accuracy. It agrees with the Du Bois formula to within 1 to 2 percent for most adult patients.

The Haycock Formula

Published in 1978 by George Haycock and colleagues, this formula was specifically developed for pediatric populations using data from neonates through adolescents.

The Haycock formula is the preferred choice for pediatric drug dosing because it was validated across a wide range of ages from birth through adolescence. For adult patients, it produces results similar to Du Bois and Mosteller.

Formula Accuracy Comparison Table

For most clinical scenarios, the differences between formulas are less than 5 percent. The choice of formula matters most at the extremes: very low birth weight neonates, morbidly obese adults, and patients receiving drugs with narrow therapeutic windows.

Clinical Uses of BSA

Chemotherapy Drug Dosing

The most common clinical application of BSA is calculating chemotherapy doses. Since the 1950s, oncologists have dosed most cytotoxic drugs in milligrams per square meter of body surface area (mg/m²). This approach aims to deliver a consistent drug exposure across patients of different body sizes, reducing the risk of underdosing (which reduces efficacy) or overdosing (which increases toxicity).

For example, if a chemotherapy protocol prescribes a drug at 75 mg/m² and a patient's BSA is 1.85 m², the calculated dose would be 75 × 1.85 = 138.75 mg, typically rounded to the nearest practical amount. Oncologists recalculate BSA before each treatment cycle because weight changes during therapy can alter the appropriate dose.

GFR Normalization

Glomerular filtration rate (GFR) is reported in mL/min/1.73 m², meaning it is normalized to a standard body surface area of 1.73 m². This normalization allows doctors to compare kidney function between individuals of different body sizes. Without this adjustment, a large person with more kidney tissue would appear to have better kidney function than a smaller person, even if their kidneys are functioning at the same capacity per unit of tissue. The standard value of 1.73 m² was chosen based on the average BSA of 25-year-old Americans measured in the 1920s.

Burn Assessment

In burn medicine, the percentage of total body surface area (TBSA) affected by burns is the primary metric for determining treatment severity and fluid resuscitation requirements. The Rule of Nines is the most common method: each arm represents 9 percent, each leg represents 18 percent, the front and back torso each represent 18 percent, the head represents 9 percent, and the perineum represents 1 percent. For more precise assessment, the Lund-Browder chart adjusts these percentages for patient age, which is especially important in children where the head is proportionally larger.

Burns covering more than 20 percent TBSA in adults (or 10 percent in children and the elderly) typically require aggressive intravenous fluid resuscitation using the Parkland formula, which calculates fluid needs based on burn percentage and body weight.

Cardiac Index

Cardiac output (the volume of blood the heart pumps per minute) is normalized to BSA to produce the cardiac index (CI = CO / BSA). A normal cardiac output varies with body size, but the cardiac index has a consistent normal range of 2.5 to 4.0 L/min/m² regardless of patient size. This makes the cardiac index a more useful clinical measurement than raw cardiac output for comparing heart function between patients or tracking changes in an individual patient.

Average BSA by Age and Gender

These values are population averages and vary considerably between individuals. BSA is determined entirely by height and weight, so two adults of the same sex and age but different builds will have different BSA values. There is no single normal BSA; the number is used as a scaling factor for clinical calculations, not as a health metric to be compared against a target.

Real-World BSA Examples

Example 1: Adult Chemotherapy Dosing

Linda, age 52, weighs 68 kg and is 165 cm tall. She has been prescribed carboplatin-based chemotherapy, and her oncologist needs to calculate the drug dose based on her BSA. Using the Mosteller formula:

1. Multiply height by weight: 165 × 68 = 11,220
2. Divide by 3,600: 11,220 / 3,600 = 3.117
3. Take the square root: √3.117 = 1.765 m²

Linda's BSA is approximately 1.77 m². If her chemotherapy is dosed at 175 mg/m², her calculated dose would be 175 × 1.77 = 309.75 mg, which her oncologist rounds to 310 mg. Before her next cycle, her weight will be rechecked and BSA recalculated to ensure the dose remains appropriate.

Example 2: Pediatric Drug Dosing

Kevin, age 7, weighs 25 kg and is 122 cm tall. His pediatric oncologist uses the Haycock formula for more accurate pediatric BSA estimation:

1. Apply the Haycock formula: BSA = 0.024265 × 122^0.3964 × 25^0.5378
2. Calculate 122^0.3964 = 7.527
3. Calculate 25^0.5378 = 5.492
4. Multiply: 0.024265 × 7.527 × 5.492 = 1.003 m²

Kevin's BSA is approximately 0.93 m² (note: Mosteller would give approximately 0.92 m² for comparison). At this age, the difference between formulas is small, but the Haycock formula is preferred because it was validated specifically in the pediatric population. His medication doses are calculated based on this BSA value, and his treatment team monitors his growth closely since children's BSA changes rapidly.

Example 3: Burn Assessment

Martin, age 40, weighing 85 kg, suffers burns to his entire right arm (9 percent TBSA) and the front of his torso (18 percent TBSA) in a workplace accident. The total burn area is estimated at 27 percent TBSA using the Rule of Nines.

• Burns exceed 20% TBSA, triggering the Parkland formula for fluid resuscitation
• Parkland formula: Fluid (mL) = 4 × body weight (kg) × %TBSA burned
• Calculation: 4 × 85 × 27 = 9,180 mL of lactated Ringer's in the first 24 hours
• Half is given in the first 8 hours (4,590 mL), the remaining half over the next 16 hours

While the Parkland formula itself uses weight rather than calculated BSA, the TBSA percentage is an assessment of how much of the patient's total body surface area has been affected by the burn. This demonstrates how the concept of body surface area underpins critical emergency medical decisions.

Tips for Understanding BSA in Your Care

• Ask your healthcare team about BSA-based dosing. If you are receiving chemotherapy or other BSA-dosed medications, understanding how your dose is calculated empowers you to be an informed participant in your care.
• Report weight changes promptly. Significant weight gain or loss affects your BSA and may require dose adjustments. Alert your treatment team to any weight changes of more than 5 percent between visits.
• Understand the formula your team uses. Different institutions may default to different BSA formulas. The differences are usually small, but knowing which formula is being used helps you compare results across providers.
• Know that BSA has limitations. BSA does not account for body composition (muscle vs. fat), hydration status, or extreme body habitus. In patients with morbid obesity or cachexia, adjusted dosing strategies may be more appropriate than standard BSA-based calculations.

Common Mistakes to Avoid

• Confusing BSA with BMI. These are fundamentally different measurements. BSA is a surface area in square meters used for clinical calculations. BMI is a weight-to-height ratio used for weight classification. They serve different purposes and are not interchangeable.
• Assuming one formula fits all patients. While formulas agree closely for average-sized adults, they can diverge at the extremes. Pediatric patients, morbidly obese patients, and very small adults may get more accurate results from formulas validated in their specific population.
• Using outdated weight measurements for dosing calculations. If your weight has changed since your last treatment, make sure your healthcare provider recalculates BSA with your current weight. Using an old weight can lead to over- or under-dosing.
• Treating BSA as a health target. Unlike blood pressure or BMI, BSA is not a number you aim to change. It is a descriptive measurement that clinicians use for calculations. There is no healthy or unhealthy BSA value.
• Mixing up units. BSA formulas require height in centimeters and weight in kilograms. Using inches and pounds without converting will produce meaningless results. Always double-check units before calculating.

Last updated: February 23, 2026