Body Surface Area —
four clinical formulas.

Field guide

What is Body Surface Area?

Body Surface Area (BSA) is the measured or estimated total surface area of the human body, expressed in square meters (m²). Unlike body weight, which varies by hydration and tissue composition, BSA provides a more stable reference that correlates well with cardiac output, metabolic rate, renal function, and the volume of distribution of many drugs, making it the standard normalization factor in clinical medicine.

The average adult female has a BSA of approximately 1.6–1.7 m²; the average adult male, 1.9–2.0 m². Children and adolescents have significantly smaller values; a newborn’s BSA is roughly 0.25 m², and a 10-year-old child is approximately 1.1 m². This size-dependence is precisely why BSA is used for drug dosing: a dose expressed as “mg per m²” automatically scales to the patient’s size.

BSA cannot be measured directly without complex imaging (e.g., 3D scanning) or laborious physical measurements of body segments. In clinical practice, it is always estimated from height and weight using one of several validated regression formulas. The four formulas this calculator implements are the most widely cited.

The four BSA formulas explained.

Mosteller (1987): the clinical standard

Proposed by Richard Mosteller in a 1987 NEJM letter, this formula is notable for its extreme simplicity; it requires only a square root of a fraction. Despite (or because of) that simplicity, it is the formula recommended by most contemporary oncology dosing guidelines, the National Cancer Institute, and numerous pharmacy references. The denominator 3600 = 60² is the only constant you need to remember.

For clinical use, Mosteller is the default. Use it unless a specific protocol explicitly requires another formula.

Du Bois & Du Bois (1916): the original

Published by Delafield Du Bois and Eugene Du Bois in 1916, this was the first quantitative BSA formula and remained the global standard for over 70 years. It was derived from direct measurements of nine subjects, a remarkably small sample that has nonetheless proven remarkably durable. Du Bois is still explicitly required by some older drug package inserts and institutional protocols, so it remains clinically relevant.

For a typical adult, Du Bois produces results within 1–2% of Mosteller. Discrepancies grow at extremes of height or weight.

Haycock et al. (1978): preferred for children

Published in 1978, the Haycock formula was derived specifically to improve accuracy in pediatric patients. It tends to give slightly higher BSA estimates than Du Bois for children and slightly lower for very large adults. Many pediatric oncology and nephrology protocols specify Haycock by name, and it is the default formula in several pediatric dosing references.

If you are calculating BSA for a child or adolescent, or if the protocol specifies Haycock, use this result rather than Mosteller.

Gehan & George (1970)

Derived by Gehan and George in 1970 from a re-analysis of the Du Bois dataset plus additional subjects, this formula was intended to improve accuracy over the original Du Bois equation. It is used in some European and Australian dosing guidelines and appears in certain drug monographs. For most practical purposes, its results are very close to Mosteller and Du Bois.

Formula agreement: why all four give similar results

For adults of typical stature, all four formulas typically agree within 2–3%. The bar chart in the calculator above illustrates this graphically; the bars will be nearly equal for measurements in the normal adult range. Disagreements widen at the extremes of the height and weight spectrum (very short and very heavy, or very tall and very light), which is precisely why pediatric protocols prefer Haycock and why formula choice matters most in those edge cases.

Clinical uses of BSA.

Chemotherapy dosing

The most common reason a patient or clinician looks up BSA is for chemotherapy dosing. Many cytotoxic agents, including doxorubicin, fluorouracil, paclitaxel, cisplatin, and cyclophosphamide, are dosed as “X mg per m²”. The oncologist multiplies the protocol’s dose per m² by the patient’s BSA to get the actual milligram dose to administer.

Example: A protocol calls for doxorubicin 60 mg/m². A patient with a BSA of 1.87 m² (Mosteller) receives:

The premise of BSA-based dosing is that drug clearance, toxicity thresholds, and therapeutic windows scale with body size in a way that weight alone does not capture. Whether this premise holds for all agents is still debated; some pharmacokinetic evidence suggests that flat dosing or lean body mass dosing may be more accurate for specific drugs, but BSA-based dosing remains the dominant paradigm in oncology.

Carboplatin (Calvert formula)

Carboplatin uses a unique dosing method called the Calvert formula that incorporates both BSA and measured kidney function:

Where GFR (glomerular filtration rate) is itself often normalized to 1.73 m² of BSA using the CKD-EPI or MDRD equation. This is why the 1.73 m² reference appears in kidney-function contexts — it is the “standard” adult BSA used to anchor GFR comparisons.

GFR normalization

Kidney function (GFR) is routinely reported as mL/min/1.73 m², meaning “adjusted to a reference adult of 1.73 m² BSA.” This normalization lets clinicians compare kidney function across patients of different sizes. When your BSA differs significantly from 1.73 m², as it does in very large adults or children, the reported GFR can require de-normalization (multiply by your BSA, divide by 1.73) to understand your actual absolute clearance.

Cardiac output indexing

Cardiac output (CO) is the volume of blood the heart pumps per minute. To compare across body sizes, clinicians use the Cardiac Index (CI):

Normal CI is 2.5–4.0 L/min/m². Using absolute CO without BSA normalization would make a large athlete’s normal value look like elevated output and a small patient’s normal value look like low output.

Burns and fluid resuscitation

The extent of burns is measured as a percentage of total body surface area (%TBSA), and the Parkland formula for burn resuscitation is BSA-based. Fluid requirements in the first 24 hours are calculated as:

(This uses weight, not BSA directly, but BSA remains the reference for estimating burn extent.)

Other applications

• Basal metabolic rate (BMR): several equations use BSA as an input.
• Drug dosing in obesity: BSA-based dosing may over- or under-dose obese patients; some protocols adjust using ideal body weight or lean BSA.
• Organ size scaling: normal organ sizes (liver volume, spleen weight, lung capacity) are often expressed per m² of BSA.
• Echocardiography: left ventricular dimensions and aortic root diameter are indexed to BSA.

Limitations and important caveats.

BSA estimation from height and weight is an approximation. All four formulas were derived from regression analysis on relatively small samples of subjects who may not represent your demographic, body composition, or medical condition. Known limitations include:

• Obesity: BSA-based dosing may overestimate the appropriate dose in obese patients because adipose tissue contributes disproportionately to body weight without proportionally increasing the volume of distribution for many drugs. Some protocols cap BSA at 2.0–2.2 m² for dose calculation.
• Extreme heights or weights: All formulas were derived from datasets that did not include patients at the extremes of the height/weight distribution. Accuracy degrades outside the normal adult range.
• Pediatric patients: The Du Bois formula and Gehan-George may underestimate BSA in infants; Haycock is generally preferred. Below age 2, even Haycock carries more uncertainty.
• The 1987 vs. 1916 debate: Despite its simplicity, Mosteller was validated against measured BSA and performs comparably or better than Du Bois in most studies. The historic dominance of Du Bois is partly inertia.

Medical disclaimer

This calculator is for educational and informational purposes only. Drug dosing decisions must always be made by a licensed healthcare professional who can account for the full clinical picture: renal function, hepatic function, prior treatment history, body composition, and the specific protocol being followed. Never use this tool to determine, adjust, or verify medication doses without professional supervision.