Body Surface Area Calculator

Why surface area instead of weight

Many physiological quantities — resting metabolic rate, cardiac output, glomerular filtration, drug clearance — track body surface area more closely than they track body mass. Dosing a cytotoxic drug per kilogram would overdose larger patients, because clearance does not rise in proportion to weight. Expressing the dose per square meter keeps exposure closer to constant across body sizes, which is why oncology protocols are written in mg/m².

Mosteller is the practical default

Mosteller's 1987 formula — the square root of height in centimeters times weight in kilograms, divided by 3600 — is simple enough to do on any calculator and agrees closely with the older, more complex equations. Du Bois is the historical standard from 1916, derived from just nine subjects. Haycock and Gehan-George were fitted to larger samples including children. In clinical practice the differences between them are usually small, but they are not zero.

The formulas disagree, and that matters at the margins

For an average adult the four results typically sit within a few percent of each other. At the extremes — very small children, very high body weights — the gap widens, and Du Bois in particular is known to underestimate in obesity. The spread shown above makes that disagreement visible rather than hiding it behind a single number. Where a protocol specifies a formula, use that one.

Capping and adjusting in real practice

Many institutions cap calculated BSA at 2.0 or 2.2 m² for chemotherapy dosing in very large patients, on the grounds that the linear relationship between surface area and drug clearance breaks down at the top end. Others dose on actual BSA and monitor toxicity. There is genuine clinical disagreement here, and it is a decision for the prescribing team rather than a calculator. This page reports the raw formula outputs with no cap applied, so if your protocol caps, apply it after reading the figure here.

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