Pediatric Body Surface Area Calculator
Estimates a child's body surface area (BSA) in m² using the Mosteller formula. Used by clinicians to calculate pediatric drug doses, IV fluid rates, and chemotherapy regimens.
About this calculator
Body surface area (BSA) is a critical clinical measurement used to scale medication doses for children, where weight alone can lead to under- or overdosing. The Mosteller formula calculates BSA as: BSA (m²) = √[(weight (kg) × height (cm)) / 3600]. This is preferred in pediatrics for its simplicity and accuracy across a wide range of ages and body sizes. For example, a child weighing 20 kg and 110 cm tall has a BSA of √[(20 × 110) / 3600] = √0.611 ≈ 0.78 m². Oncologists, nephrologists, and intensivists rely on BSA to determine safe, effective doses of cytotoxic drugs and other weight-sensitive medications.
How to use
Suppose a 6-year-old child weighs 22 kg and is 116 cm tall. Step 1: Multiply weight by height: 22 × 116 = 2,552. Step 2: Divide by 3,600: 2,552 / 3,600 = 0.7089. Step 3: Take the square root: √0.7089 ≈ 0.842 m². This child's BSA is approximately 0.84 m². A physician prescribing a drug at 50 mg/m² would give 0.84 × 50 = 42 mg. Always confirm dosing with clinical guidelines and a pharmacist.
Frequently asked questions
What is the Mosteller formula for pediatric body surface area?
The Mosteller formula calculates BSA as the square root of (weight in kg multiplied by height in cm, divided by 3600). It is written as BSA = √[(weight × height) / 3600]. It was introduced in 1987 and is now the most widely adopted BSA formula in clinical practice due to its ease of use and strong agreement with more complex methods. Most pediatric drug references specify doses in mg/m², making this formula essential for safe prescribing.
Why is body surface area used instead of weight for pediatric drug dosing?
Weight-based dosing can be inaccurate for drugs that distribute primarily through lean body mass or that require precise targeting of physiological processes scaled to body size. BSA correlates more closely with cardiac output, glomerular filtration rate, and metabolic rate — all of which influence how a drug behaves in the body. This is particularly important in chemotherapy, where even small dosing errors can cause serious toxicity or treatment failure. BSA-based dosing helps standardize drug exposure across patients of different sizes and ages.
How accurate is the Mosteller formula compared to other BSA formulas for children?
The Mosteller formula is generally considered highly accurate for children and shows strong agreement with the DuBois & DuBois and Haycock formulas across most pediatric weight and height ranges. Studies have found differences of less than 2–3% in most clinical scenarios. The Haycock formula is sometimes preferred for neonates and very small infants because it was validated specifically in that population. For routine clinical and pharmacy use in children older than one month, Mosteller remains the standard of care in most institutions.