Body Surface Area Calculator

Many physiological processes — blood volume, cardiac output, kidney filtration rate, metabolic rate — correlate more closely with the surface area of the body than with its mass. Two people of very different weight but similar height and build can have surprisingly similar surface areas, and drugs that are cleared or distributed in proportion to that area are dosed more safely by BSA than by weight alone, particularly for medications with a narrow margin between an effective and a harmful dose.

The best-known application is chemotherapy dosing, where BSA has been the conventional basis for decades because many cytotoxic drugs have a narrow therapeutic window and their toxicity tracks metabolic rate more closely than raw body weight does. Beyond oncology, BSA is used to normalise cardiac output into cardiac index for comparing heart function across patients of different sizes, and clinicians estimating the extent of a burn injury describe it as a percentage of total body surface area.

Du Bois, published in 1916, was the original — derived from direct surface measurements of just a handful of people, one of them a child, using a method involving paper moulded to the body. Despite the tiny sample, later and much larger studies never improved on it enough to displace it entirely. Mosteller’s 1987 formula reduces the calculation to a single square root and agrees closely with Du Bois while being simple enough to compute at the bedside without a calculator, which is why it became the modern clinical default. Haycock (1978) and Gehan–George (1970) were both developed with a particular focus on accuracy in infants and children, where the adult-derived formulas are known to be less reliable.

BSA-based dosing is not universally accepted as the best approach. For obese patients in particular, BSA calculated from actual body weight can substantially overestimate the appropriate drug dose relative to what body composition would suggest, and oncology practice increasingly caps or adjusts BSA-based doses for these patients rather than applying the formula unmodified. Which formula a given protocol specifies, and whether it is adjusted for obesity, varies by institution and by drug.