BSA Calculator (Body Surface Area)

A typical adult's body surface area is about 1.71 m² for women and 1.91 m² for men, and the four validated formulas agree within about 0.7% at that size — but they diverge by about 9% at 170 cm and 140 kg.

What body surface area tells you

Body surface area (BSA) estimates the total area of your skin in square metres from your height and weight. It matters because many physiological quantities — cardiac output, kidney filtration, drug clearance — scale better with surface area than with weight alone. That's why chemotherapy doses are prescribed per m² and kidney function (eGFR) is reported per 1.73 m² of body surface. Unlike BMI, BSA isn't a health classification — it's a scaling measurement.

The default here is the Mosteller formula — the square root of height times weight divided by 3,600 — which became the clinical favourite for its simplicity. The classic Du Bois formula dates to 1916, fitted to direct tape measurements of just nine people, yet it agrees with Mosteller within about 1–2% for typical adults. For body-composition questions, pair BSA with lean body mass and FFMI.

Four formulas compared (180 cm, 80 kg)

For a typical adult the four validated formulas agree within ~0.7%. They diverge more at the extremes — very light or very heavy adults, and young children — and the direction depends on height as well as weight. At 170 cm Du Bois reads the highest of the four below about 61 kg and the lowest above about 66 kg.

The four formulas only agree if you are average-sized

That comparison uses 180 cm and 80 kg, and at that size the four land within 0.7% of each other. It is worth being clear about why. The agreement is a property of that body, not a property of the formulas. Hold height at 170 cm, change nothing but the weight, and the four move apart in both directions.

All four equations evaluated at a fixed height of 170 cm and rounded to two decimal places, exactly as the calculator prints them. Spread is the gap between the highest and lowest of the four.

The narrowest point sits near 65 kg, where the four fall within 0.011 m² of each other — close enough that only Gehan-George moves the second decimal, 1.76 against 1.75 for the other three. Step away in either direction and the gap opens: 3.0% at 50 kg, and 0.22 m² or 9.1% at 140 kg, about fifteen times the spread at 65 kg. The disagreement is not spread evenly across body sizes. It is concentrated at the ends.

Which formula reads high also flips. At 170 cm, Du Bois returns the highest of the four below about 61 kg and the lowest of the four above about 66 kg. It crosses Mosteller exactly at 65.8 kg, where both return 1.76 m²; at 180 cm that crossing sits at 78.1 kg, both returning 1.98 m². So "Du Bois reads high" has no fixed answer — it depends which side of a mid-sized body you are standing on. The window of agreement drifts with height too. Minimising the four-formula spread puts it near BMI 20 at 150 cm, BMI 22 at 170 cm, BMI 23 at 180 cm and BMI 25 at 190 cm: the taller you are, the heavier you have to be before the formulas start to argue.

This matters, but the direction depends on how the per-square-metre figure is built. A quantity worked out as an amount per square metre — a mg/m² dose — is linear in BSA: double the surface area and you double the amount. A figure that is instead indexed to BSA, like eGFR per 1.73 m², moves the opposite way, because BSA is the divisor. At 170 cm and 140 kg, a figure worked out per m² on Du Bois comes out 8.3% smaller than the same figure worked out on Haycock — the same disagreement the table records as 9.1%, which expresses it as a share of the lowest of the four rather than the highest. At 180 cm and 80 kg the same substitution moves it by 0.5%. That is the practical takeaway. If your BMI is near the middle, pick any of the four and stop thinking about it. If you sit well above or below that, the single headline number is hiding a real disagreement, and the breakdown panel — which prints all four — is the more honest read. None of which is a dosing instruction: this tool is educational, and dose selection is a clinician's decision made from protocols, not from a web page.

Limits worth knowing

BSA is an estimate from two numbers; it can't see body composition, and every formula carries the fingerprint of the population it was fitted on. Results are shown to two decimal places by clinical convention — more precision than that would be false. And one limit above all: this tool is educational. It must never be used to calculate or adjust medication doses — that's a decision for clinicians working from protocols, not from a web page.

Sources: Mosteller, NEJM (1987); Du Bois & Du Bois, Arch Intern Med (1916); Haycock, J Pediatr (1978); Gehan & George, Cancer Chemother Rep (1970); StatPearls (NCBI); Sacco et al. UK average-BSA study; Heaf, on the 1.73 m² convention. Educational estimates, not medical advice.