BSA Calculator

How BSA Calculator Works

Mosteller: BSA = √(Height(cm) × Weight(kg) ÷ 3600) | Du Bois: BSA = 0.007184 × W^0.425 × H^0.725

BSA

Body Surface Area — Total surface area of the body in square meters. Used clinically for medication dosing and burn assessment.

Mosteller

Mosteller Formula — BSA = √(Height × Weight ÷ 3600). The most commonly used formula — simple, accurate, and recommended by the FDA for oncology dosing.

Du Bois

Du Bois Formula — BSA = 0.007184 × W^0.425 × H^0.725. The original BSA formula developed in 1916 from direct measurement of 9 subjects.

How to Use

1. Select your unit system (Imperial or Metric).
2. Enter your weight and height.
3. Review your BSA from all three formulas — Mosteller is the clinical standard.
4. Note the average BSA across all formulas for comparison.
5. Read the clinical warning: this is for educational purposes, not medical decision-making.

Quick Reference

Common Uses

• •Chemotherapy dosing — oncologists use BSA to calculate chemotherapy drug doses, as drug distribution correlates better with surface area than body weight
• •Burn severity assessment — the "rule of nines" estimates burn surface area as a percentage of total BSA, guiding fluid resuscitation and treatment decisions
• •Clinical medication dosing — certain medications (e.g., immunosuppressants, some anesthetics) use BSA-based dosing where weight-based dosing is insufficient
• •Cardiac output indexing — cardiac index (cardiac output divided by BSA) normalizes heart function measurements across patients of different sizes

Understanding the Result

Worked Examples

Frequently Asked Questions

Why is BSA used for chemotherapy dosing?

Chemotherapy drugs have a narrow therapeutic index — too little is ineffective, too much is toxic. BSA correlates better with drug distribution than body weight because many chemo drugs distribute throughout total body water and extracellular space, which scales with surface area. The FDA and most oncology protocols (including the National Comprehensive Cancer Network guidelines) require BSA-based dosing for drugs like doxorubicin, cyclophosphamide, and carboplatin. The Calvert formula for carboplatin specifically uses BSA combined with renal function (GFR) to calculate the precise dose in milligrams. BSA-based dosing reduces the risk of both under-dosing (treatment failure) and over-dosing (life-threatening toxicity).

What is a normal BSA?

The average adult BSA is ~1.7 m² (ranging from ~1.5–2.0 m² for most adults). Children have lower BSA. There is no "normal" BSA per se — it is simply a measured value used for clinical calculations, not a health marker like BMI. A 6'5" athlete and a 5'2" person will have different BSAs simply due to their different heights, which is expected and not meaningful as a standalone number. BSA becomes relevant only when used as a denominator for clinical calculations (e.g., cardiac index = cardiac output / BSA, or eGFR normalized to 1.73 m²).

Which BSA formula should I use?

Mosteller is the current clinical standard recommended by the FDA and most oncology protocols. It gives nearly identical results to Du Bois (within 3%) but is much simpler to calculate — it requires only multiplication, division, and a square root. Haycock is more accurate for infants and children because it was validated on a population that included pediatric subjects. Du Bois, despite being the oldest formula (1916) and based on only 9 subjects, has proven remarkably accurate across diverse populations over the past century. For practical purposes, any of the three formulas is acceptable — they differ by less than 3% for typical adult measurements.

Why does BSA matter for burn assessment?

Burn severity and fluid resuscitation are calculated as a percentage of total BSA affected. The "Rule of Nines" divides the body into regions each representing ~9% of total BSA: head and neck (9%), each arm (9%), anterior trunk (18%), posterior trunk (18%), each leg (18%), and perineum (1%). The percentage of BSA burned determines the fluid resuscitation volume using the Parkland formula: 4 mL × weight(kg) × %BSA burned of lactated Ringer's solution, with half given in the first 8 hours. For example, a 70 kg patient with 30% BSA burns needs 4 × 70 × 30 = 8,400 mL in the first 24 hours, with 4,200 mL in the first 8 hours. Accurate BSA estimation is literally life-saving in burn care — underestimating burn size leads to inadequate fluid resuscitation and organ failure.

How does BSA relate to kidney function (eGFR)?

Estimated glomerular filtration rate (eGFR) is normalized to a standard BSA of 1.73 m² — this was the average BSA of the 25-year-old men in the original kidney function studies. This normalization allows comparison of kidney function between people of different sizes. A very large person (BSA 2.2 m²) naturally has a higher absolute GFR because they have more kidney tissue and more blood to filter — but their eGFR per 1.73 m² should be similar to a smaller person's if kidney health is comparable. The raw (unadjusted) GFR is calculated first, then multiplied by (1.73 / patient's BSA) to normalize. This means that if your BSA is larger than 1.73 m², your raw GFR is divided by a number greater than 1, and your reported eGFR is lower than your raw GFR — which can sometimes make large muscular individuals appear to have lower kidney function when their kidneys are actually healthy.

What is the relationship between BSA and cardiac output?

Cardiac output (the volume of blood the heart pumps per minute) varies with body size — larger people need more blood flow. To compare cardiac function across different-sized patients, cardiac output is divided by BSA to produce the cardiac index (CI). Normal CI is 2.5–4.0 L/min/m². A cardiac output of 5.0 L/min might be normal for a large person (BSA 2.0 m² → CI = 2.5) but could indicate low output for a small person (BSA 1.4 m² → CI = 3.6). This BSA-indexed approach is also used for other cardiovascular measurements including stroke volume index, systemic vascular resistance index, and aortic valve area index.

Why do BSA formulas give slightly different results?

Each formula was derived from different study populations and uses different mathematical models. Du Bois (1916) measured only 9 subjects using paper molds and planimetry — a labor-intensive process involving tracing body outlines. The formula fits a power-law model (BSA ∝ W^0.425 × H^0.725) because surface area scales with the 2/3 power of mass in geometrically similar objects. Mosteller (1987) simplified this to a single square root for ease of bedside calculation. Haycock (1978) used 81 subjects including infants and found slightly different exponents (W^0.5378, H^0.3964) that better fit the pediatric population. The differences between formulas are typically less than 0.05 m² (about 3% of total BSA) for adults — clinically negligible for most purposes. The largest discrepancies appear at the extremes: very tall/short or very heavy/light individuals where the formulas diverge by up to 5–8%.

Pro Tips

• →For clinical calculations (chemotherapy dosing, burn assessment, cardiac index), always use Mosteller — it is the FDA-recommended standard and is simplest. Save the formula in your phone: sqrt(height_cm × weight_kg / 3600). You can calculate it with any basic calculator that has a square root button. The other formulas provide cross-validation — if Mosteller and Du Bois differ by more than 5%, double-check your height and weight inputs, as this usually indicates a measurement error rather than a real divergence between the formulas.
• →BSA is not a health metric like BMI or body fat percentage. A larger BSA does not mean you are unhealthy, and a smaller BSA does not mean you are healthier. BSA is simply a physical measurement used to scale clinical calculations. Do not track your BSA over time as a health goal — it will change if your weight changes, which is already tracked more meaningfully through BMI, waist circumference, or body fat percentage. BSA is a tool for clinicians, not a personal health target.
• →When using BSA for medication dosing, understand that BSA-based dosing can overestimate the dose for obese patients. This is known as the "obesity dosing paradox" — BSA scales with weight, but chemotherapy drug distribution in obese patients may not scale proportionally with BSA due to lower blood flow to adipose tissue. Many oncology protocols cap BSA at 2.0 m² or use adjusted body weight for patients with BMI > 30. If you are calculating BSA for personal education about a medication you are receiving, always follow your doctor's prescribed dose — never adjust your own medication based on BSA calculations.
• →BSA formulas assume a typical body shape and proportion. For people with amputations, severe scoliosis, dwarfism, or other conditions that significantly alter body proportions, BSA formulas may be inaccurate. In burn care, the Rule of Nines is adjusted for children (the head represents a larger percentage in infants: 18% vs. 9% in adults) and for obesity. For these special populations, direct measurement methods (3D body scanning) or condition-specific adjustment factors are used in clinical settings.
• →If you are using BSA for fitness or sports physiology purposes (e.g., calculating heat dissipation capacity or VO2max normalization), note that BSA-to-mass ratio matters more than absolute BSA. A smaller athlete (e.g., a marathon runner at 55 kg, 170 cm) has a higher BSA-to-mass ratio than a larger athlete (e.g., a rugby player at 110 kg, 185 cm). Higher BSA-to-mass ratio = more surface area per unit of body mass = better heat dissipation capacity. This is one reason smaller athletes tend to perform better in hot-weather endurance events. The calculation: BSA (m²) / weight (kg). A typical marathoner might have 1.65 m² / 55 kg = 0.030 m²/kg, while a rugby player might have 2.30 m² / 110 kg = 0.021 m²/kg — the marathoner has ~43% more surface area per kilogram for cooling.

Limitations to Know

• •Body composition and proportion assumptions — BSA formulas assume average body proportions. Individuals with significantly different muscle mass, body fat, or conditions like amputations, severe scoliosis, kyphosis, dwarfism, or gigantism may have actual BSA that differs from formula estimates.
• •Clinical factors not modeled — edema or ascites (fluid retention) can add kilograms to weight without increasing surface area. The obesity dosing paradox means BSA-based chemotherapy may overestimate doses for patients with BMI > 30.
• •Formula derivation limitations — these formulas were derived primarily from Caucasian populations with small sample sizes (Du Bois: 9 subjects; Mosteller: derived mathematically; Haycock: 81 subjects). Ethnic body proportion differences may introduce systematic biases.
• •For clinical medication dosing, carboplatin and chemotherapeutic agents should ONLY be dosed by a licensed physician — never self-calculate or self-adjust doses. The Rule of Nines is taught for concept understanding only; actual burn assessment must be performed by trained medical personnel.

Sources

• NIH - Mosteller BSA Formula