Body Surface Area (BSA) Calculator

What Is Body Surface Area?

Body surface area (BSA) is the calculated or measured surface area of the human body. The average adult has a BSA of roughly 1.7 m² for women and 1.9 m² for men, with a typical range of about 1.5 to 2.2 m².

BSA is considered a better basis than body weight alone for scaling many physiological quantities, because metabolic rate, blood volume, cardiac output, and kidney filtration all track more closely with surface area than with weight. This is why clinicians rely on it whenever a measurement or dose needs to be "normalized" to body size.

How Does the BSA Calculator Work?

1. Enter your height. In centimeters or feet and inches.

2. Enter your weight. In kilograms or pounds.

3. The formulas run. The calculator applies the Mosteller, Du Bois, Haycock, and Gehan-George equations, using metric units internally.

4. You get your result. You see BSA in m² for each formula, plus an average and, for reference, a note on how your value compares with typical adult ranges.

BSA Formulas

Mosteller Formula (1987)

This is the most widely used formula today because it is simple enough to do on a basic calculator and performs well across body sizes.

Du Bois and Du Bois Formula (1916)

The original and most historically important formula, derived from measurements of just nine people. It remains widely used in research and cardiology.

Haycock Formula (1978)

Developed using data that included infants and children, so it is often preferred in pediatrics.

Gehan and George Formula (1970)

Derived from a larger sample of children and commonly used in pediatric oncology.

Example Calculation

A person who is 175 cm tall and weighs 75 kg:

• Mosteller: √(175 × 75 ÷ 3,600) = √3.646 = 1.91 m²
• Du Bois: 0.007184 × 75^0.425 × 175^0.725 = 0.007184 × 6.27 × 44.4 ≈ 2.00 m²
• Haycock: 0.024265 × 75^0.5378 × 175^0.3964 ≈ 1.93 m²
• Gehan-George: 0.0235 × 75^0.51456 × 175^0.42246 ≈ 1.92 m²

The formulas agree within about 0.1 m², which is typical for adults of average build.

Which BSA Formula Should You Use?

For adults, Mosteller and Du Bois usually differ by 1–5%. Whichever you choose, use the same formula consistently, especially when comparing values over time or against a protocol that specifies one.

Average BSA by Age and Sex

The traditional "standard" adult BSA used in many reference ranges is 1.73 m², a figure established in the 1920s and still used to normalize kidney function results.

BSA Chart by Height and Weight (Mosteller)

Why BSA Matters in Medicine

Chemotherapy dosing. Many cancer drugs are dosed in milligrams per square meter (mg/m²). A person with a BSA of 1.9 m² receiving a drug dosed at 100 mg/m² would get 190 mg. This is the most common clinical use of BSA.

Cardiac index. Cardiac output (liters of blood pumped per minute) is divided by BSA to get the cardiac index, allowing fair comparison between small and large people. A normal cardiac index is about 2.5–4.0 L/min/m².

Kidney function. The glomerular filtration rate (GFR) is often normalized to a standard 1.73 m² BSA, so results can be compared across body sizes.

Burn assessment. Burn severity and fluid resuscitation needs are calculated as a percentage of total body surface area (TBSA).

Other uses: Dosing certain medications in children, calculating cardiac stress test values, some immunosuppressant regimens, and estimating heat loss and metabolic rate in physiology research.

Example: Chemotherapy Dose

A drug protocol calls for 75 mg/m². For a patient with a BSA of 1.85 m²:

Dose = 75 × 1.85 = 139 mg

For a patient with a BSA of 2.10 m², the dose would be 158 mg. Even a small difference in BSA changes the dose, which is why oncology teams often verify height and weight measurements carefully.

BSA and Body Weight

BSA rises with both height and weight, but not in proportion. The square root in the Mosteller formula means that doubling your weight does not double your surface area. It increases it by about 41%. That's why BSA-based dosing gives smaller relative doses to very large patients than weight-based dosing would.

Should BSA Be Capped for Obese Patients?

Whether to cap BSA in patients with obesity, commonly at 2.0 m², has been debated in oncology. Older practice sometimes capped doses out of concern for toxicity, but current guidelines from the American Society of Clinical Oncology (ASCO) recommend using full weight-based dosing in adults with cancer, including those with obesity, when the goal is cure or improved survival, because under-dosing can reduce effectiveness.

This is a clinical decision made by the treating team, not something to decide from a calculator.

BSA Is Not About Health or Fitness

It's worth being clear: BSA does not measure whether your weight is healthy. A higher BSA doesn't mean anything is wrong. It just reflects a larger body. For weight-related health screening, use tools like the BMI, waist to height ratio, and body fat percentage calculators instead.

Limitations of BSA Formulas

• They're estimates. Actual measured surface area can differ from calculated values.
• Limited original data. The Du Bois formula came from only nine subjects. Later formulas used more people but still have limits.
• Less accurate at the extremes. Very small children, very tall or very short adults, and people with severe obesity can be misestimated by any formula.
• They ignore body composition. Two people with the same height and weight, but different muscle and fat, have the same calculated BSA.
• Different formulas give different answers. In extreme body sizes, the gap between formulas can reach 5–10%.
• Not validated for all populations. Some formulas were built from specific groups and may not fit every ethnicity equally well.

Common Mistakes

Using inaccurate height or weight. A 2 cm height error or a 2 kg weight error changes BSA by only about 1%, but larger mistakes matter, especially for dosing.

Switching formulas mid-treatment. When BSA is used for dosing, the protocol may specify a formula. Stick with it.

Treating BSA as a health score. It isn't. It's a size measurement.

Using an adult formula for a newborn. Infants and small children need pediatric-appropriate formulas such as Haycock.

Self-dosing medication. BSA calculations for medical doses belong with a doctor or pharmacist.

Myths vs. Facts

Myth: A bigger BSA means you're less healthy.
Fact: BSA reflects body size only. Tall and muscular people naturally have larger values.

Myth: BSA can be calculated from weight alone.
Fact: It requires both height and weight, because two people of the same weight but different heights have different surface areas.

Myth: All BSA formulas give the same answer.
Fact: They usually agree within a few percent for adults but can differ more at extreme sizes.

Myth: BSA determines how many calories you burn.
Fact: BSA correlates with resting metabolic rate, but calorie needs are better estimated using formulas such as Mifflin-St Jeor or Katch-McArdle, which also account for age, sex, and activity.

Myth: Chemotherapy is always dosed by BSA.
Fact: Many drugs are, but some use flat doses, weight-based doses, or other calculations. The prescribing team decides.

Tips for Getting an Accurate BSA

• Measure your height without shoes, standing straight against a wall.
• Weigh yourself on a calibrated scale, ideally in the morning.
• Use current values, since weight changes affect BSA.
• Pick one formula and keep using it for comparison.
• Double-check units before you calculate.
• For medical dosing, let your healthcare team perform and verify the calculation.
• Recalculate after significant weight changes.

How Accurate Is a BSA Calculator?

For most adults of average build, BSA formulas are accurate to within about 5% of measured surface area, and the Mosteller and Du Bois results are usually very close to each other. Accuracy drops for young children, people with extreme body proportions, and severe obesity.

For everyday curiosity and general reference, a calculator is perfectly adequate. For clinical dosing, follow your healthcare provider's protocol, which may specify the formula and rounding rules to use.

Frequently Asked Questions

What is body surface area?
 Body surface area is the total area of your skin in square meters. The average adult BSA is about 1.7 m² for women and 1.9 m² for men.

How do I calculate my BSA?
 Using the Mosteller formula, multiply your height in centimeters by your weight in kilograms, divide by 3,600, and take the square root.

Which BSA formula is most accurate?
 No single formula is best for everyone. Mosteller is the most widely used for adults because it's simple and accurate, while Haycock is often preferred for children.

What is a normal BSA?
 Most adults fall between 1.5 and 2.2 m². The conventional standard adult value is 1.73 m².

Why do doctors use BSA for chemotherapy?
 Many physiological processes, including drug distribution and clearance, scale with body size, and BSA has been the standard basis for dosing for decades. It helps match the dose to the person's size.

Is BSA the same as BMI?
 No. BMI compares weight to height squared to screen for weight category. BSA estimates skin surface area and is used for medical dosing and physiology.

Does BSA change with weight loss?
 Yes. As weight drops, BSA falls, though less than proportionally. Losing 20% of body weight lowers BSA by roughly 10%.

Can I use this calculator for my child?
 You can get an estimate, but pediatric doses should always be calculated and verified by a doctor or pharmacist, ideally with a pediatric formula such as Haycock.

Does BSA tell me my calorie needs?
 Not directly. It correlates with resting metabolism, but calorie needs are better estimated with a BMR formula that includes age and sex, such as Mifflin-St Jeor.