BSA Calculator helps you estimate body surface area (BSA) from height and weight so clinicians can dose certain medications and assess burn or fluid needs. This guide explains the formulas, what inputs mean, and how to convert results between m² and ft².
What Is Body Surface Area (BSA)?
Body surface area is a standardized estimate of the total surface area of the human body. It is used in medicine because many dosing rules scale better with BSA than with weight alone, especially in pediatrics and certain chemotherapy regimens.
BSA is not a direct measurement. It is calculated using an equation based on height and weight. The most common equations are Mosteller and Du Bois.
BSA Calculator Formulas (Mosteller and Du Bois)
Both formulas estimate the same concept but can give slightly different results. Choose the method your program, clinic, or guideline specifies.
Mosteller (Most common for quick calculations)
Formula: BSA (m²) = √((height(cm) × weight(kg)) / 3600)
Variables: height in centimeters, weight in kilograms.
Du Bois (Classic equation)
Formula: BSA (m²) = 0.007184 × height(cm)^0.725 × weight(kg)^0.425
Variables: height in centimeters, weight in kilograms.
How Unit Conversions Work
Your inputs may be in metric or US customary units. The calculator converts everything to the units required by the formulas, then converts the final BSA to your chosen output unit.
- Length: inches → centimeters (1 in = 2.54 cm)
- Weight: pounds → kilograms (1 lb = 0.45359237 kg)
- Area: m² → ft² (1 m² = 10.7639104167 ft²)
Because of rounding, your final value may differ slightly from another tool. Use the same equation and unit system for consistent results.
How to Use the BSA Calculator (Step-by-Step)
- Select your height unit (cm or in) and weight unit (kg or lb).
- Enter your height and weight as numbers.
- Choose the formula (Mosteller or Du Bois).
- Pick the output unit for BSA (m² or ft²).
- Click Calculate to get BSA and the converted value.
If you enter invalid values (like negative numbers or non-numeric text), the calculator flags the field and asks you to correct it.
Practical Examples
Example 1: Adult dosing support
Suppose a patient is 170 cm tall and weighs 70 kg. Using Mosteller:
BSA (m²) = √((170 × 70) / 3600) = √(11900 / 3600) ≈ √3.3056 ≈ 1.82 m².
Clinicians can then apply BSA-based dosing rules that require m².
Example 2: Pediatrics and unit flexibility
A child measures 60 in and weighs 50 lb. Convert to metric for the formula:
- 60 in × 2.54 = 152.4 cm
- 50 lb × 0.45359237 = 22.68 kg
With Du Bois, the calculator estimates BSA and can display it in ft² if your workflow uses US customary units.
When BSA Is Used in Medicine
BSA is commonly referenced in healthcare settings for calculations that scale with body size. Typical uses include:
- Medication dosing for certain therapies where BSA is the scaling factor.
- Burn assessment and fluid planning in some protocols.
- Cardiac output indexing and other body-size normalization tasks.
Always follow the exact equation and rounding rules used by your clinical guideline or institution.
Limitations and Good Practice
BSA is an estimate, not a direct measurement. Small differences in height/weight, equation choice, and rounding can change the final value.
- Equation choice matters: Mosteller vs Du Bois can yield different BSA values.
- Accuracy depends on inputs: use reliable measurements and correct units.
- Clinical context matters: some dosing protocols use additional adjustments beyond BSA.
Important: This calculator provides BSA estimates for general use and planning. For medical dosing, always follow your clinician’s instructions and the relevant guideline.
Frequently Asked Questions
What is BSA used for?
BSA (body surface area) is used to standardize calculations that scale with body size. In medicine, it can support dosing for certain drugs and help normalize measurements like cardiac output. Many protocols use BSA because it correlates better with metabolic and physiological factors than weight alone.
Which BSA formula should I use: Mosteller or Du Bois?
Use the formula your guideline or institution specifies. Mosteller is widely used for quick estimates and is common in many clinical calculators. Du Bois is the classic equation and may appear in older protocols. The equations can produce slightly different BSA values.
How do I convert BSA from m² to ft²?
To convert, multiply by 10.7639104167. For example, 1.80 m² equals about 1.80 × 10.7639 = 19.38 ft². Your calculator can output either unit directly, but the equation still uses metric inputs for height and weight.
Do I need height in cm and weight in kg?
Formulas typically require height in centimeters and weight in kilograms. If you enter inches or pounds, the calculator converts them automatically before computing BSA. That keeps the math consistent and prevents unit mistakes that can cause large errors.
What if my height or weight input is missing or zero?
A valid BSA requires both height and weight. If either value is missing, zero, or negative, the calculation cannot be performed. The calculator will show an error message and ask you to enter positive numbers. Use accurate measurements to avoid incorrect BSA.