BSA Calculator (Mosteller)
Last reviewed: by Jayson Minagawa, BSN, RN
Free BSA (Body Surface Area) calculator using the Mosteller formula. Enter height (cm) and weight (kg) — returns BSA in m². Used for chemotherapy dosing, cardiac index, pediatric maintenance fluids, and many specialty drug calculations. Clinical reference only.
BSA dosing is the load-bearing math in oncology and pediatrics. Most chemotherapy is dosed in mg/m², and pediatric maintenance fluids are calculated per BSA. The Mosteller formula is the most commonly cited because it's simple enough to do in your head; Du Bois (1916) is more historically used but requires logarithms.
— Jayson Minagawa, BSN, RNCalculate
The Mosteller formula
BSA (m²) = √[(height × weight) ÷ 3600] when height is in cm and weight is in kg. Mosteller (1987) is the most commonly used formula in clinical practice because it's simple and accurate within ±2% for adults and pediatric patients across normal body habitus.
Worked example
Adult patient: 170 cm tall, 70 kg. Calculation: √[(170 × 70) ÷ 3600] = √[11,900 ÷ 3600] = √3.31 = 1.82 m² BSA. For a chemotherapy order of paclitaxel 175 mg/m², the actual dose would be 175 × 1.82 = 318.5 mg.
Why BSA and not weight?
Many drugs (especially chemotherapy) have a tighter therapeutic window in mg/kg dosing than in mg/m² because BSA correlates better with metabolic rate, cardiac output, and renal clearance than weight alone. Cardiac output is reported as cardiac index (CO ÷ BSA) for the same reason — it normalizes for body size.
Limitations of BSA dosing
BSA dosing has been called into question for some chemotherapy drugs because the variability in drug clearance is poorly captured by BSA. Some protocols are moving to fixed-dose strategies for specific agents (e.g., flat-dose carboplatin per Calvert formula using GFR). Always follow facility-specific oncology protocols.
How nurses actually use BSA at the bedside
Three places I check BSA most often: chemotherapy dose verification, cardiac index in critical care (CI = cardiac output ÷ BSA, with a normal range of 2.5–4.0 L/min/m²), and pediatric maintenance fluid calculations. Whenever a chemo order arrives, I cross-check BSA against the height and weight in the chart, then verify the dose-per-square-meter against the protocol. A 5% error in height entry can shift BSA by 0.05–0.10 m², which on a high-dose carboplatin or doxorubicin order can be the difference between therapeutic effect and dangerous toxicity. For obese patients, the question of capping BSA comes up constantly. Most institutional protocols cap BSA at 2.0 or 2.2 m² for select chemotherapy agents to avoid overdosing patients whose lean body mass does not scale linearly with total body weight. The capping rule is protocol-driven; never apply your own cap without checking with pharmacy.
Mosteller vs Du Bois vs Haycock
Three BSA formulas show up in clinical literature: Mosteller (1987), Du Bois & Du Bois (1916), and Haycock (1978). Mosteller is the modern default because the math fits on a paper form and the agreement with Du Bois is within 2–3% for adults of normal body habitus. Du Bois is still used in some legacy oncology protocols and dialysis prescriptions; if the order says "Du Bois BSA," do not substitute Mosteller without checking with pharmacy. Haycock is preferred in pediatrics because it was derived from a pediatric reference population, but in practice most pediatric chemotherapy programs validate against Mosteller anyway. Use whichever formula your institution's protocol specifies, and never mix formulas across the same dose calculation.
BSA in critical care: cardiac index and stress-test dosing
Outside oncology, BSA shows up in invasive hemodynamics. Cardiac output divided by BSA gives cardiac index, the size-normalized perfusion metric used in pulmonary artery catheter management, post-cardiac-surgery care, and shock states. A cardiac output of 4 L/min sounds adequate until you realize the patient is 6'4" and 130 kg — at a BSA of 2.5 m², that is a CI of only 1.6 L/min/m², firmly in cardiogenic shock territory. The same 4 L/min in a 5'2" 50 kg patient (BSA 1.5) gives a CI of 2.7 — normal. BSA is also used to dose stress-test isotopes (thallium, gallium) and to scale glomerular filtration rate (eGFR adjusted to 1.73 m² for renal-dosed antibiotics). For vasoactive infusions like nitroprusside and dobutamine where mcg/kg/min dosing dominates, BSA is generally not used. When in doubt, follow the order set, double-check the math, and document your verification.
Frequently asked
What's the difference between Mosteller and Du Bois BSA formulas?
Mosteller (1987) uses a square root: simple, accurate, easy to do mentally. Du Bois (1916) uses logarithms and is the historical gold standard. Both produce nearly identical results (±1%) for most patients. Mosteller is preferred in clinical practice for its simplicity.
Should I use actual or ideal body weight?
Use actual body weight unless the patient is morbidly obese, in which case some protocols call for adjusted body weight. Oncology protocols often specify which to use; check the order set.
Is BSA used in pediatrics?
Yes. Pediatric maintenance fluid is sometimes calculated as 1,500-2,000 mL/m²/day. Many pediatric chemotherapy and immunosuppressant doses are also BSA-based.