Body Surface Area (BSA) Calculator

Many physiological quantities — metabolic rate, cardiac output, kidney filtration — scale more closely with body surface area than with weight. Indexing to BSA lets clinicians compare people of different sizes on one scale, and size a dose to the patient.

• Chemotherapy and some other drug doses are prescribed in mg per m².
• Cardiac index is cardiac output divided by BSA, so a small and a large heart can be compared fairly.
• Estimated GFR is reported per 1.73 m², a standard adult body surface area, rather than for the individual's own size.
• Some paediatric drug doses and fluid calculations are scaled to BSA.

Burn assessment is a separate question. The percentage of the body surface burned is estimated with charts such as the rule of nines or the Lund–Browder chart, not with a height-and-weight formula.

Skin area cannot be measured directly in practice, so every BSA formula is an equation fitted to a set of people whose surface area was measured or modelled. All of them take height and weight; they differ in the exponents and in the population used to derive them.

DuBois & DuBois is the oldest and remains widely cited. Haycock and colleagues calculated surface area geometrically from 34 body measurements in 81 people ranging from premature infants to adults, and found that the DuBois formula increasingly underestimated surface area below 0.7 m² — by almost 8% in newborns. Their own equation fitted the whole range closely, which is why it is often preferred for children.

For an average-sized adult the choice barely matters: at 170 lb and 5'10" the four formulas agree to within 0.01 m². The differences open up at the extremes of body size, particularly with high body weight and in small children.

There is no single correct formula, because none of them measures skin area directly. In practice the right one is whichever the protocol or institution specifies, applied consistently so that doses are comparable over a course of treatment.

Many cytotoxic drugs are prescribed in mg/m², so the dose is the protocol dose multiplied by the patient's BSA. Height and weight are usually measured at the start of treatment and rechecked when weight changes meaningfully.

A long-standing question is whether to cap doses in people with obesity. The American Society of Clinical Oncology's 2021 guideline update continues to recommend full, weight-based cytotoxic chemotherapy doses for obese adults with cancer, and recommends that full approved doses of immunotherapy and targeted therapy also be offered. If toxicity occurs, the panel advises handling dose changes the same way as for patients without obesity.

Estimated GFR is reported in mL/min per 1.73 m² so that results from people of different sizes can be compared against one set of thresholds. The figure of 1.73 m² is a conventional standard adult body surface area.

Because of that indexing, an eGFR describes kidney function for a standard-sized adult, not for the person tested. For a very large or very small patient, multiplying by BSA ÷ 1.73 gives the un-indexed filtration rate, which is sometimes needed when dosing drugs cleared by the kidney.

• Recorded rather than measured height and weight — self-reported figures are a common source of error.
• Large fluid shifts such as oedema or ascites, which add weight without adding metabolically active tissue.
• Amputation, which changes the relationship between height, weight and surface area.
• Extremes of body size, where the formulas disagree most.

Where accuracy matters, measure height and weight on the day rather than relying on the record, and use the formula the protocol names.