Free Body Surface Area (BSA) Calculator – Du Bois, Mosteller & Haycock

What Body Surface Area Is and How This Calculator Finds It

Body surface area, or BSA, is the total external surface of the human body expressed in square metres. It matters far more in medicine than body weight alone because many physiological measures scale with surface area rather than mass. Clinicians use BSA to dose chemotherapy drugs, to index cardiac output as cardiac index, to size some fluid and drug regimens, and to estimate the extent of burns. This calculator turns two everyday numbers, your height and your weight, into a BSA figure. It is built for medical students checking a dosing problem, nurses double-checking a chart, and curious patients who have seen the term on a treatment plan and want to understand it. It presents three accepted formulas at once rather than hiding the choice behind a single answer. The reason it shows three results is that the published formulas genuinely disagree, and different specialties prefer different ones. The Du Bois and Du Bois equation from 1916, BSA = 0.007184 x height^0.725 x weight^0.425, is the long-standing clinical standard. The Mosteller equation from 1987, BSA = the square root of (height x weight / 3600), is prized for being simple enough to work out mentally and is widely preferred in paediatrics. The Haycock equation from 1978, BSA = 0.024265 x height^0.3964 x weight^0.5378, was validated across a wide age range including neonates. In every formula height is in centimetres and weight is in kilograms, and the result is in square metres. If you enter imperial units the tool converts inches to centimetres and pounds to kilograms before applying the formula. Work through a standard adult of 170 cm and 70 kg. The Du Bois equation returns 1.81 m squared, which is the textbook anchor value quoted in clinical references. Mosteller gives the square root of (170 x 70 / 3600), which is the square root of 3.306, or 1.82 m squared. Haycock returns 1.83 m squared. The three agree to within about one percent here, which is typical. Now take a taller, heavier adult of 180 cm and 80 kg: Du Bois gives 2.00 m squared and Mosteller gives exactly 2.00 m squared, because 180 x 80 / 3600 works out to 4 and its square root is 2. Seeing the numbers converge builds confidence that no single formula is an outlier. The number changes real decisions. An oncology pharmacist calculating a carboplatin or doxorubicin dose multiplies the drug's per-square-metre dose by the patient's BSA, so a 1.6 m squared patient and a 2.1 m squared patient receive meaningfully different amounts. An intensivist divides a measured cardiac output by BSA to get cardiac index, the number that actually tells them whether perfusion is adequate for that person's size. A paediatrician reaches for Mosteller precisely because it is quick and reliable in small children, where a 20 kg child at 100 cm lands near 0.75 m squared. A researcher normalising kidney function reports it per 1.73 m squared, the historical average adult BSA, so the same surface-area concept underlies the eGFR you may have seen on a blood test. A few cautions keep this honest. BSA formulas are estimates fitted to populations, so they lose accuracy at the extremes of body size and in people whose proportions differ from the average, and none of them accounts for body composition. Rounding at the wrong step is the usual reason two websites disagree, so this tool keeps full precision internally and rounds only the displayed value to two decimals. Never use a BSA figure to self-adjust a medication; drug dosing is a clinical decision that depends on far more than surface area. Results here are for general information and education, not a substitute for professional judgement, so consult a qualified healthcare professional for any medical decision. The whole calculation runs inside your browser, so your height and weight are never uploaded, stored or shared.

Body Surface Area (BSA) Calculator Formula & Method

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