Body Surface Area Calculator — Free Online BSA Tool

How to Use the Body Surface Area Calculator

1. Select your unit system: Choose between Metric (kilograms and centimeters) or Imperial (pounds, feet, and inches) using the toggle at the top. The calculator converts between systems automatically.
2. Enter your weight: Input your body weight. For clinical applications, use your most recent measured weight rather than an estimate. The calculator accepts a wide range from 1 kg for pediatric use up to 300 kg.
3. Enter your height: Input your height measurement. Stand barefoot against a flat surface for the most accurate reading.
4. Review your results: The calculator displays BSA values from both the Du Bois formula and the Mosteller formula, along with their average and the difference between them. For most adults, the two formulas agree within 0.01-0.03 m². The average adult BSA is approximately 1.7 m².

This calculator provides BSA estimates for informational purposes. For clinical drug dosing or medical decisions, always work with your healthcare provider who will consider additional factors beyond BSA alone.

BSA Formulas and Calculation Methods

Variables Explained

• Height (cm): Total standing height in centimeters. The exponent 0.725 in the Du Bois formula reflects that surface area increases less than proportionally with height.
• Weight (kg): Total body weight in kilograms. The exponent 0.425 reflects that heavier individuals have proportionally less surface area per kilogram.
• 0.007184: The Du Bois constant, empirically derived from direct surface area measurements of human subjects.
• 3600: The Mosteller constant, chosen to make the simplified formula closely approximate the Du Bois result.

Step-by-Step Calculation Example

For a person weighing 75 kg and 172 cm tall:

1. Du Bois: 0.007184 x 172^0.725 x 75^0.425
2. 172^0.725 = 48.67
3. 75^0.425 = 7.43
4. 0.007184 x 48.67 x 7.43 = 1.88 m²
5. Mosteller: √(172 x 75 / 3600) = √(12900 / 3600) = √3.583 = 1.89 m²
6. Average: (1.88 + 1.89) / 2 = 1.89 m²

The two formulas agree within 0.01 m², confirming reliability. This BSA of 1.89 m² is above the average adult value of 1.7 m², consistent with this person's above-average height and weight.

Practical Examples

Example 1: Chemotherapy Dosing Reference

Dr. Patel needs to determine the dose for a chemotherapy drug prescribed at 100 mg/m² for a patient weighing 68 kg at 165 cm:

• Du Bois BSA: 0.007184 x 165^0.725 x 68^0.425 = 1.76 m²
• Mosteller BSA: √(165 x 68 / 3600) = 1.77 m²
• Average BSA: 1.77 m²
• Drug dose: 100 mg/m² x 1.77 m² = 177 mg

The oncologist uses the calculated BSA to determine that this patient should receive 177 mg of the chemotherapy agent. The pharmacist verifies this calculation independently before preparing the dose.

Example 2: Pediatric BSA Assessment

A pediatric nurse calculates BSA for a 7-year-old child weighing 23 kg and 122 cm tall:

• Du Bois BSA: 0.007184 x 122^0.725 x 23^0.425 = 0.88 m²
• Mosteller BSA: √(122 x 23 / 3600) = 0.88 m²
• Average BSA: 0.88 m²

At 0.88 m², this child has roughly half the BSA of an average adult. This difference is essential for appropriate medication dosing in pediatric patients, where adult doses must be carefully adjusted based on BSA to maintain therapeutic levels while avoiding toxicity.

Example 3: Cardiac Index Calculation

A cardiologist measures a cardiac output of 5.2 L/min for a patient weighing 82 kg at 176 cm and needs the cardiac index:

• BSA: 1.98 m² (average of Du Bois and Mosteller)
• Cardiac Index: 5.2 L/min / 1.98 m² = 2.63 L/min/m²
• Normal range: 2.5-4.0 L/min/m²

The cardiac index of 2.63 L/min/m² falls within the normal range, indicating adequate cardiac function relative to body size. Without BSA normalization, comparing cardiac output between patients of different sizes would be misleading.

BSA Reference Table by Height and Weight

Tips and Complete Guide to Body Surface Area

Clinical Applications of BSA

BSA is used across multiple medical specialties. In oncology, it determines chemotherapy dosing. In cardiology, it normalizes cardiac output to create the cardiac index, enabling meaningful comparisons between patients of different sizes. In nephrology, glomerular filtration rate (GFR) is often indexed to a standard BSA of 1.73 m², the average adult BSA when the reference values were established in the 1920s. In burn medicine, BSA provides the framework for the Rule of Nines and Lund-Browder charts used to estimate burn extent. In pediatric medicine, BSA-based dosing is the standard for most medications because children have a higher surface-area-to-weight ratio than adults.

Why BSA Rather Than Body Weight for Drug Dosing

Drug dosing based on BSA rather than weight alone provides more consistent drug exposure across patients of different sizes. This is because many pharmacokinetic processes, including drug distribution volume, hepatic blood flow, and glomerular filtration rate, correlate more closely with body surface area than with body weight. A large person and a small person with the same weight-based dose may experience very different drug concentrations in their blood. BSA-based dosing reduces this variability, though it is not a perfect solution. Modern pharmacology is increasingly using pharmacokinetic modeling and therapeutic drug monitoring to further individualize dosing beyond BSA calculations.

Comparing BSA Formulas

Over a dozen BSA formulas have been published since the original Du Bois formula in 1916. Besides Du Bois and Mosteller, others include the Haycock formula (commonly used in pediatrics), the Gehan and George formula, and the Boyd formula. Most produce results within 5% of each other for average-sized adults. The choice of formula can matter more for extreme body sizes. In practice, most clinical institutions standardize on one formula for consistency. The Mosteller formula has gained popularity due to its simplicity, while the Du Bois formula remains the most widely cited in pharmacological research and drug labeling.

Common Mistakes to Avoid

• Using BSA calculations for clinical decisions without professional guidance: While this calculator provides accurate BSA estimates, drug dosing and medical decisions should always involve a healthcare professional who considers the complete clinical picture.
• Assuming BSA formulas are equally accurate for all populations: Most BSA formulas were developed from limited, non-diverse samples. Accuracy may vary across different ethnic groups, extreme body sizes, and special populations such as amputees or pregnant women.
• Confusing BSA with BMI: BSA measures the total outer surface of the body in square meters, while BMI is a weight-to-height ratio used for weight classification. They measure entirely different things and serve different clinical purposes.
• Neglecting to update BSA for weight changes: Patients undergoing treatment that affects body weight (such as chemotherapy, critical illness, or surgical recovery) may need BSA recalculated periodically for accurate ongoing dosing.
• Using estimated rather than measured values: For clinical applications, always use measured height and weight rather than patient-reported values, as self-reported values are frequently inaccurate.

Frequently Asked Questions