- Enter the patient's height (centimeters or inches — use the unit selector).
- Enter the patient's weight (kilograms or pounds — use the unit selector). The three BSA values update automatically.
- For most adult bedside and chemotherapy dosing, use the Mosteller value. For infants and small children, prefer the Haycock value.
- Round to two decimals (m²) for documentation. Re-check inputs at extremes of body habitus — methods can disagree by >5% in infants <10 kg and in severe obesity.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use a body surface area (BSA) calculator whenever a dose, an organ-function metric, or a fluid prescription needs to be scaled to the size of the patient rather than to weight alone. Common nephrology and internal-medicine uses include normalizing measured GFR (or eGFR) to the conventional 1.73 m², dosing chemotherapy agents (e.g. cyclophosphamide, doxorubicin), dosing some immunosuppressants and corticosteroid courses used in glomerular disease (e.g. high-dose prednisone in MCD/FSGS protocols), prescribing fluid resuscitation after burns (Parkland and modified formulas), and calculating cardiac index (cardiac output ÷ BSA).
Appropriate population
Children and adults at the bedside or in the clinic when accurate height and weight are available. Mosteller is the default in most adult and oncology settings; Haycock was specifically validated in infants and small children and is preferred in pediatrics; Du Bois is the historic standard often retained in older protocols and physiology references.
When NOT to rely on it
BSA-scaled dosing is a convention, not a physiologic law. In severe obesity Du Bois underestimates BSA and may push doses higher than is safe; many protocols cap or adjust BSA above a threshold (commonly 2.0–2.2 m²) — follow the specific drug or institutional protocol. BSA-normalized eGFR (per 1.73 m²) can mislead drug dosing in very small or very large patients; for renally cleared drugs with a narrow therapeutic index, dose using the de-indexed (absolute) GFR. Garbage in, garbage out — verify the measured height and current weight before relying on the number.
Pearls & Pitfalls
Mosteller is the bedside default
The Mosteller formula (BSA = √(height_cm × weight_kg / 3600)) is fast enough to do mentally with a calculator and agrees with Du Bois and Haycock within ~5% across the typical adult and pediatric range. It is the preferred bedside method in adult medicine and oncology and is the one most chemotherapy protocols now specify.
Method choice
Mosteller (1987) — simplest; preferred adult and oncology bedside formula. Du Bois & Du Bois (1916) — the classical formula behind most published normal-value tables and many old physiology references; slightly underestimates in obese patients. Haycock (1978) — validated across the full pediatric range; preferred for infants and small children, where the older Du Bois equation underestimates BSA.
Pitfalls
(1) Unit errors dominate real-world mistakes — confirm the unit selector matches what was charted (cm vs. in, kg vs. lb). (2) BSA-scaled dosing assumes a body habitus close to the reference population; in severe obesity, edema, ascites, or amputation, BSA may overestimate the size of metabolically active tissue. (3) Some chemotherapy protocols cap BSA (e.g. at 2.0 or 2.2 m²) for obese adults — check the protocol. (4) When dosing renally cleared drugs, do not assume "eGFR per 1.73 m²" gives the right dose in a non-average-sized patient — use absolute GFR (mL/min) or Cockcroft–Gault as the protocol specifies. (5) For infants <10 kg, prefer Haycock over Du Bois.
Why Use It
BSA is a single, reproducible number that scales doses and physiologic measures across the enormous size range of human patients — from a 3 kg neonate to a 150 kg adult — far better than weight alone. Many cytotoxic and immunosuppressive drugs were originally developed and dose-finding studied per m², and dosing in m² is what their evidence base assumes; using mg/kg instead, or skipping the calculation, breaks the link to the original studies. Likewise, the convention of expressing GFR per 1.73 m² exists so that a 50 kg patient's kidney function can be compared to a 90 kg patient's on the same scale. BSA gives clinicians a common, transparent way to individualize doses and to compare physiology across patients, and the three formulas in this tool are the ones routinely cited in clinical guidelines and protocols.
Body Surface Area — Mosteller, Du Bois & Haycock
Enter the patient's height and weight (any common units). All three BSA formulas are computed simultaneously in m² to two decimals, with a short interpretation suggesting which value to use.
⚕ Mosteller RD. N Engl J Med. 1987;317(17):1098. · Du Bois D, Du Bois EF. Arch Intern Med. 1916;17:863–871. · Haycock GB et al. J Pediatr. 1978;93(1):62–66. BSA-scaled dosing is a convention; in severe obesity or extreme body habitus check the relevant drug protocol (many cap BSA). For licensed clinicians; not a substitute for individualized assessment.
Next Steps
Once BSA is known, apply it to the dose, the metric, or the prescription that needs it.
- Drug dosing: most cytotoxic chemotherapy, several biologics, some pediatric corticosteroid regimens (e.g. high-dose prednisone in MCD/FSGS), and selected immunosuppressants are dosed as mg/m². Multiply the protocol dose by BSA; check whether the protocol caps BSA (commonly 2.0–2.2 m²) for obese adults.
- GFR normalization: convert measured GFR or absolute eGFR (mL/min) to mL/min/1.73 m² for staging — divide by patient BSA, multiply by 1.73. For dosing renally cleared narrow-therapeutic-index drugs, use the de-indexed (absolute) GFR rather than the per-1.73 m² value.
- Fluid prescriptions: pediatric maintenance fluids and burn resuscitation formulas (Parkland, modified Brooke) frequently use BSA — pick Haycock in small children for accuracy.
- Pair this with the Cockcroft–Gault calculator for absolute creatinine clearance and the combined BMI / BSA / IBW tool when you need ideal body weight alongside BSA.
Evidence & References
Formulas
| Method | Formula (BSA in m²; height in cm; weight in kg) |
|---|---|
| Mosteller (1987) | BSA = √( height_cm × weight_kg / 3600 ) |
| Du Bois & Du Bois (1916) | BSA = 0.007184 × height_cm0.725 × weight_kg0.425 |
| Haycock (1978) | BSA = 0.024265 × height_cm0.3964 × weight_kg0.5378 |
Method Selection
| Setting | Preferred formula |
|---|---|
| Adult bedside & most oncology protocols | Mosteller — simplest; widely adopted |
| Historic standard / older protocols & physiology references | Du Bois & Du Bois — slightly underestimates in obese patients |
| Infants and small children | Haycock — validated across the full pediatric range |
All three formulas agree to within ~5% across the typical adult and child range; differences become clinically meaningful at extremes of body size (infants <10 kg, severe obesity), where Haycock or Mosteller are generally preferred over Du Bois.
References
- Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317(17):1098. doi:10.1056/NEJM198710223171717.
- Du Bois D, Du Bois EF. A formula to estimate the approximate surface area if height and weight be known. Arch Intern Med. 1916;17:863–871.
- Haycock GB, Schwartz GJ, Wisotsky DH. Geometric method for measuring body surface area: a height–weight formula validated in infants, children, and adults. J Pediatr. 1978;93(1):62–66. doi:10.1016/s0022-3476(78)80601-5.
