Body Surface Area Calculator

Body surface area (BSA) is the engineering unit of modern oncology. When a clinician writes "5-FU 425 mg/m²," they are saying that the cytotoxic dose should scale with the patient's external surface area rather than weight alone, because surface area correlates more tightly with metabolic rate, cardiac output, and renal clearance than mass does. That single insight has driven chemotherapy prescribing since the 1950s and is the reason that every infusion bag in a cancer center is calculated to two or three decimal places of m². It is also why pediatric intensivists report cardiac index in L/min/m² instead of L/min, why nephrologists normalize glomerular filtration rate to 1.73 m², and why pharmacology textbooks devote entire chapters to BSA-based dosing.

The challenge is that BSA cannot be measured directly without an awkward and impractical procedure. Every BSA value you see in a chart is an estimate from a regression formula fitted to a finite cohort of subjects. The most famous of these — Du Bois & Du Bois (1916) — was fitted using just nine subjects. Mosteller (1987) simplified the math down to a square root that you can do mentally. Haycock (1978) was specifically calibrated for pediatrics. Fujimoto (1968) was developed for Japanese adults. Schlich (2010) added separate male and female coefficients. This calculator implements all seven of the most widely used formulas, lets you compare them side by side, and shows you where they agree (almost always) and where they disagree (typically at the extremes of body size).

In daily practice, the choice of formula matters less than people imagine. For an average 70 kg / 175 cm adult, Mosteller, Du Bois, Haycock, Gehan & George, and Boyd all return BSA values within about 0.02 m² of each other — well under the dose-banding tolerance most oncology pharmacies use. For pediatric and underweight patients the spread widens to 5-10%, and for severely obese adults it can exceed 10%, which is why institutional protocols often specify both the formula and a BSA cap (commonly 2.0 m² or 2.2 m²). Whether you are an oncology pharmacist verifying a dose at 6 a.m., an ICU nurse sanity-checking cardiac index during an overnight shift, or a medical student learning the math for the first time, this tool gives you the answer in seconds.

The Formulas Explained

All formulas return BSA in m² when weight is in kg and height is in cm. Internally, this calculator converts pounds to kilograms, inches to centimeters, and feet-and-inches to centimeters before applying the math, so you can mix units freely.

Formula History & Accuracy Comparison

How to Use This Calculator

1. 1. Pick a preset (optional): Choose Adult Male, Adult Female, Pediatric, or Infant to auto-fill realistic weight and height — useful for quick sanity checks or teaching.
2. 2. Enter weight and height: Toggle between kg/lbs and cm/in/ft-in. The calculator normalizes everything to kg and cm internally.
3. 3. Enter age and sex: Age drives the pediatric recommendation (Haycock under 12). Sex is used by the Schlich formula and for reporting.
4. 4. Pick a primary formula: Mosteller for adults, Haycock for pediatrics. Toggle "Show all 7 formulas" to compare every estimate at once.
5. 5. Add clinical inputs: Enter a mg/m² chemo dose (or pick a preset like 5-FU, paclitaxel, doxorubicin, methotrexate) and an optional cardiac output to see the indexed values.

Common Use Cases

Pro Tips for Accurate BSA Dosing

• • Use Mosteller for adults: Simple, validated, and well within institutional dose-banding tolerances. It is the de facto modern standard in US, UK, EU, and Australian oncology.
• • Use Haycock for pediatrics: Specifically derived using neonates and children. Most pediatric oncology protocols cite Haycock or Mosteller.
• • Du Bois is the historical standard: Many older oncology papers quote Du Bois. If you are reproducing a 20th-century protocol, use Du Bois to match the original math.
• • Formula choice affects dose by 1-3%: Across the validated formulas, the spread is small for average adults but can exceed 10% at the extremes of body size. Always check the spread before approving an outlier.
• • Cap BSA per institutional policy: Many cancer centers cap BSA at 2.0 m² or 2.2 m² for cytotoxic dosing to avoid extrapolating beyond trial data. This calculator shows uncapped values; clinical judgment decides what to prescribe.
• • Confirm units before signing off: mg/m² and mg/kg are not interchangeable. A common error is multiplying a mg/kg dose by BSA — always read the protocol carefully.

International & Regulatory Notes

BSA-based dosing is recognized worldwide. The UK British National Formulary (BNF) and BNFc, US FDA product labels, European Medicines Agency (EMA) SmPC documents, Indian CDSCO references, and the Japanese PMDA all express cytotoxic doses in mg/m². Some regions favor specific formulas — Mosteller in most English-speaking countries, Du Bois in older European and US literature, Fujimoto in Japan, and Schlich in some German-speaking centers. Always follow your local institutional protocol and drug product information.

Whether you are preparing a chemotherapy infusion, reviewing pediatric dosing, or checking cardiac index at the bedside, BSA is one of the small handful of numbers that gets used dozens of times a day in clinical pharmacy and acute care. Bookmark this calculator, share it with your team, and use it whenever the question "what is this patient's m²?" comes up.