Body Surface Area (BSA) Calculator

What Is Body Surface Area (BSA)?

Body surface area (BSA) is an estimate of the total external surface area of the human body. It is usually expressed in square meters (m²) and is calculated primarily from a person's height and weight.

BSA is used in several areas of medicine, including certain medication and chemotherapy dosing protocols and calculations that normalize physiological measurements for body size. For example, cardiac index is calculated by dividing cardiac output by BSA.

BSA is different from body mass index (BMI). BMI compares weight with height to provide a weight-for-height screening measure, while BSA estimates body surface area.

How to Use the Body Surface Area Calculator

Our BSA calculator supports both metric and US measurements and automatically handles the required unit conversions.

Step 1: Choose Metric or US Units

Select the measurement system you want to use.

With Metric, enter:

• Body weight in kilograms (kg)

• Body height in centimeters (cm)

With US, enter:

• Body weight in pounds (lb)

• Height in feet (ft)

• Remaining height in inches (in)

Step 2: Select Gender

Choose your gender from the available option. Gender does not change the standard Mosteller calculation itself because that formula uses only height and weight.

However, gender is relevant to some alternative BSA equations, such as the Schlich formulas, which use different equations for men and women.

Step 3: Enter Your Body Weight

Enter your current weight. Use kilograms when Metric is selected or pounds when US units are selected.

Step 4: Enter Your Height

For Metric calculations, enter your height in centimeters.

For US measurements, enter your height using separate feet and inches fields. For example, a height of 5 feet 8 inches should be entered as 5 ft and 8 in.

Step 5: Click Calculate

Click the Calculate button. The calculator converts your measurements when necessary and performs the body surface area calculation.

Step 6: Review Your Results

Your main result shows your estimated body surface area in m² using the Mosteller equation.

The result area also displays your:

• Weight used

• Height used

• BMI

• Formula used

You can then compare BSA estimates from Du Bois, Mosteller, Haycock, Gehan & George, Boyd, Fujimoto, Takahira, and Schlich formulas. Results are displayed in square meters (m²), square feet (ft²), and square inches (in²).

Body Surface Area Formula

There is no single BSA equation used in every situation. Several formulas have been developed, and they can produce slightly different estimates from the same height and weight.

BSA calculator uses the Mosteller formula for its main BSA result.

Mosteller Formula

The Mosteller body surface area formula is:

$\text{BSA} = \sqrt{\frac{H \times W}{3600}}$

Where:

• BSA = body surface area in m²

• H = height in centimeters

• W = weight in kilograms

The equation was published by R. D. Mosteller in 1987 and provides a relatively simple way to estimate BSA from height and weight.

Example: How to Calculate Body Surface Area

Suppose a person weighs 70 kg and is 175 cm tall.

Using the Mosteller formula:

$\text{BSA} = \sqrt{\frac{175 \times 70}{3600}}$

First, multiply height by weight:

$175 \times 70 = 12{,}250$

Divide by 3,600:

$\frac{12{,}250}{3600} = 3.4028$

Now take the square root:

$\text{BSA} = \sqrt{3.4028} \approx 1.8447\,\text{m}^2$

So, the estimated body surface area is approximately 1.84 m².

When calculating BSA manually, avoid rounding intermediate values too early. Round the final result instead.

Other Formulas Used by the BSA Calculator

The calculator does more than provide a single Mosteller estimate. It calculates BSA using eight formulas, making it easy to see how different equations compare.

For the formulas below, H represents height in centimeters and W represents weight in kilograms unless otherwise stated.

Du Bois Formula

The Du Bois and Du Bois equation is one of the earliest and most established methods of calculating BSA.

$\text{BSA} = 0.007184 \times H^{0.725} \times W^{0.425}$

Haycock Formula

The Haycock equation is:

$\text{BSA} = 0.024265 \times H^{0.3964} \times W^{0.5378}$

The Haycock formula was developed using measurements that included infants, children, and adults.

Gehan & George Formula

The Gehan and George formula estimates BSA as:

$\text{BSA} = 0.0235 \times H^{0.42246} \times W^{0.51456}$

Like other BSA equations, it uses a mathematical relationship between height and body weight.

Boyd Formula

The Boyd equation requires special attention because body weight is expressed in grams, rather than kilograms, in this version of the formula:

$\text{BSA} = 0.0003207 \times H^{0.3} \times W_g^{\left(0.7285-0.0188\log_{10}(W_g)\right)}$

Where:

• H = height in centimeters

• Wg​ = weight in grams

Using kilograms directly in a Boyd formula designed for grams will produce an incorrect result.

Fujimoto Formula

The Fujimoto formula is:

$\text{BSA} = 0.008883 \times H^{0.663} \times W^{0.444}$

Takahira Formula

The Takahira equation uses exponents similar to the Du Bois equation but applies a different coefficient:

$\text{BSA} = 0.007241 \times H^{0.725} \times W^{0.425}$

Schlich Formula

Unlike Mosteller and several other equations, the Schlich method uses different formulas for men and women.

For men:

$\text{BSA} = 0.000579479 \times W^{0.38} \times H^{1.24}$

For women:

$\text{BSA} = 0.000975482 \times W^{0.46} \times H^{1.08}$

Here, weight is measured in kilograms and height in centimeters.

Why Do BSA Formulas Give Different Results?

You may notice that the BSA values in the comparison table are not identical. This is expected.

BSA cannot usually be determined simply by measuring a person's height and weight directly against their skin surface. Instead, equations estimate surface area using mathematical models developed from different groups of people and different datasets.

The formulas also use different coefficients and exponents. For example, Mosteller uses the square root of a height-and-weight calculation, while Du Bois applies separate exponents to height and weight.

As a result, the same person can receive slightly different BSA estimates depending on the equation used. Medical references recognize formula-related variation as an important consideration when BSA is used clinically.

This is why the Body surface area calculator shows multiple formulas side by side rather than suggesting that every BSA equation should return exactly the same value.

BSA vs. BMI: What's the Difference?

BSA and BMI both use height and weight, but they measure different things.

For example, a person can have a BSA of 1.80 m² and a BMI of 23 kg/m². These numbers should not be compared directly because they represent different measurements.

The BMI shown by the BSA calculator is provided as an additional body-size reference; it does not replace or modify the main BSA result.

When Is Body Surface Area Used?

BSA has several applications in medicine and clinical research. The appropriate use depends on the specific calculation, treatment protocol, and patient.

Medication and Chemotherapy Dosing

Some medications, particularly certain chemotherapy drugs, use dosing expressed as an amount per square meter of body surface area, such as mg/m². However, actual dosing decisions can involve many factors beyond BSA, including the specific drug, treatment protocol, laboratory results, organ function, and clinical judgment.

A BSA calculator therefore should not be used to independently determine or change a prescription medication dose.

Cardiac Index

BSA is used to account for differences in body size when calculating cardiac index.

The basic relationship is:

$\text{Cardiac Index} = \frac{\text{Cardiac Output}}{\text{BSA}}$

Cardiac index is typically expressed in liters per minute per square meter.

Pediatric Clinical Calculations

BSA may be used in certain pediatric medication and physiological calculations. The formula and dosing method used should follow the applicable clinical protocol rather than being selected solely from a calculator result.

Indexed Physiological Measurements

Some physiological measurements are normalized to BSA so values can be interpreted in relation to a person's body size. BSA is used this way in areas including cardiovascular and renal assessment.

Clinical Research

Researchers may use BSA to normalize measurements or compare data across people with different body sizes. This can help standardize certain pharmacological and physiological measurements.

Common Body Surface Area Calculation Mistakes

• Mixing measurement units: Do not enter pounds into a formula requiring kilograms or inches into an equation requiring centimeters.

• Using kilograms instead of grams in the Boyd formula: Check the required weight unit before performing the equation manually.

• Forgetting the square root: The Mosteller equation requires taking the square root after multiplying height and weight and dividing by 3,600.

• Rounding too early: Keep additional decimal places during intermediate calculations and round the final BSA instead.

• Expecting every formula to agree: Different BSA equations can produce different estimates from identical measurements.

• Confusing BSA with BMI: BSA estimates surface area, while BMI measures weight relative to height.

• Using BSA as a medication prescription: A calculated BSA alone does not determine a safe medication dose.

Tips for a More Accurate BSA Estimate

Use your current body weight rather than an old or estimated value whenever possible. Measure height accurately and make sure you have selected the correct unit system before calculating.

When performing a body surface area calculation manually, use the exact units required by the chosen equation and avoid rounding values until the final step.

If BSA is being tracked over time, using the same formula consistently makes comparisons easier. When BSA is required for a medical purpose, use the equation and calculation method specified by the relevant healthcare professional or clinical protocol.

Helpful Resources

• Clinical Use of Body Surface Area

• Medication Dosing Principles

• Understanding Medical Calculations

Pro Tips

• Normal adult BSA typically ranges from 1.5 to 2.0 m²

• BSA is more accurate than weight alone for dosing medications

• Used in calculating cardiac output and metabolic rate

• Important in determining chemotherapy dosages