About the Body Surface Area Calculator
Body surface area is used to dose chemotherapy, to index cardiac output, and to estimate burns. It matters clinically in a way most body measurements do not — a drug dose can depend directly on it.
It also cannot be measured on a living person without considerable effort, so every figure is a regression. And the one most of medicine still uses, Du Bois and Du Bois from 1916, was fitted on nine subjects, one of whom was a child.
This body surface area calculator runs four formulas and applies a test that separates them in a way the literature rarely mentions.
How to Use the Body Surface Area Calculator
Enter height and weight in metric or imperial.
Pick a formula for the headline figure — all four are reported regardless.
Dose per square metre is optional. Enter a rate and you will see the dose, and how much the choice of formula would change it.
Step-by-Step Example
180cm, 80kg.
Du Bois: 0.007184 × 180^0.725 × 80^0.425 = 2.00 m²
Mosteller: √(180 × 80 ÷ 3600) = 2.00 m²
Haycock: 0.024265 × 180^0.3964 × 80^0.5378 = 2.01 m²
Gehan-George: 0.0235 × 180^0.42246 × 80^0.51456 = 2.01 m²A spread of 0.013 m², under 1%. For an average adult the formulas agree closely, which is exactly what you would expect given they were all fitted around average adults.
The Scaling Test
Here is the structure worth knowing, and it is the most useful thing on this page.
Scale a body geometrically by a factor k. Then:
height goes as k (a length)
weight goes as k³ (it follows volume)
area MUST go as k² (it is an area)Now take a formula of the general shape BSA = c · H^a · W^b and substitute:
BSA ∝ k^a · (k³)^b = k^(a + 3b)For that to equal k², dimensional consistency demands:
a + 3b = 2That is a hard requirement, not a preference. And the four formulas divide sharply on it:
| Formula | a + 3b | |
|---|---|---|
| Du Bois | 0.725 + 3(0.425) = 2.00000 | exact |
| Mosteller | 0.5 + 3(0.5) = 2.00000 | exact |
| Haycock | 0.3964 + 3(0.5378) = 2.00980 | close |
| Gehan-George | 0.42246 + 3(0.51456) = 1.96614 | not quite |
Two of them are exactly right. That is unlikely to be luck. Mosteller's √(H·W/3600) was deliberately constructed as the simplest form satisfying it, and Du Bois's exponents land on it precisely.
The other two were fitted purely to data with no constraint imposed, so they drift from the geometry — which is why they diverge from the rest at extreme body sizes even while agreeing closely in the middle.
The calculator verifies this behaviourally as well as algebraically: scale a body by 1.2 and area must rise by exactly 1.44. Du Bois and Mosteller do. Haycock gives 1.4426 and Gehan-George gives 1.4311.
Nine Subjects, One of Them a Child
Du Bois and Du Bois published in 1916 from measurements on nine people. It is still the reference formula across much of medicine.
That is not automatically disqualifying — it agrees with later, better-sampled formulas to within 1% for average adults, and it passes the scaling test exactly, which suggests the underlying form is sound even if the sample was tiny.
But it is worth knowing when a chemotherapy dose depends on it, and it is worth knowing that the agreement holds for average adults specifically.
Where the Formulas Actually Disagree
At the extremes.
For an average adult the spread is under 1%. At infant sizes it can exceed 10%, which is the range where the choice genuinely matters — and where Haycock has the advantage, having been fitted specifically to include infants and children. Du Bois, fitted on nine mostly adult subjects, is on the weakest ground there despite being the usual reference.
The same widening happens at high body weights, where BSA-based dosing has been criticised on separate grounds anyway.
What This Means for Dosing
If a drug is prescribed at 350 mg/m², the formula choice shifts the dose by a few milligrams for an average adult. Small.
The practical conclusion is not that one formula is right but that a clinical setting should fix one and stay with it. Consistency between a baseline measurement and a follow-up matters more than which formula is marginally better, and switching mid-course introduces a change that has nothing to do with the patient.
Understanding Your Result
Body surface area is the figure from your chosen formula.
All four formulas shows each one.
How far apart they are gives the spread in m² and as a percentage.
The scaling test reports the exponent sum for each, and which pass exactly.
Worth knowing flags an infant size, a wide spread, or the dose consequence.
When Should You Use This Calculator?
To check a dose calculation. And to see how much the formula matters.
For cardiac index. Output divided by BSA, against a typical 1.73 m².
To understand a clinical figure. Now you know which formula and what it rests on.
Not for self-medication. Dosing is a clinical decision.
Common Mistakes
Assuming the formulas are interchangeable. They agree for average adults and diverge at the extremes.
Using Du Bois for an infant. Haycock was fitted to include them; Du Bois was not.
Switching formulas mid-treatment. The change is in your arithmetic, not the patient.
Treating BSA as a measurement. Every one of these is a regression.
Forgetting it is an area. It grows as roughly the two-thirds power of mass, not in proportion to it.
Body surface area is used in clinical dosing and this calculator is for understanding and checking that arithmetic, not for making treatment decisions. Nothing here is medical advice — dosing is a matter for a qualified clinician.