The formula
How to calculate body surface area
Body surface area is used in medicine to scale doses and physiological measurements to body size. Chemotherapy dosing, cardiac index and burn assessment all work in units per square metre rather than per kilogram.
Mosteller is the formula most used in practice because it can be done on any calculator and agrees closely with the older Du Bois equation. Where the two disagree meaningfully, it is usually at the extremes of weight.
What to enter:
- Weight (kg)
- Height (cm)
The result updates on every keystroke. The URL updates too, which makes the filled-in version easy to bookmark or send to someone else.
Consistency in how the underlying measurements are taken matters more than precision to the last decimal — measuring at the same time of day, in the same state of rest or hydration, makes readings taken weeks apart genuinely comparable in a way that a single very precise reading on its own is not.
Why body surface area matters
The formula behind body surface area comes from published research rather than being invented for this page, and it is presented here exactly as it appears in the literature it is drawn from — the value of a calculator is applying it correctly and instantly, not reinventing it.
This tends to get checked before or after a change in routine — a new training block, a change in diet, a different stage of a pregnancy — as a way of putting a figure on something that would otherwise only be felt rather than measured.
A single number like this is most useful placed in context rather than read in isolation. Reference ranges exist because bodies vary considerably and still count as healthy across a wide spread of values, so a result sitting outside a commonly quoted range is a prompt to look more closely rather than a verdict on its own. The same figure, tracked consistently over time, tends to say more than any single reading ever will.
In practice, this kind of figure is often calculated more than once with slightly different inputs to see how sensitive it is — trying a measurement taken this morning against one taken yesterday evening, for instance, to see how much normal daily variation actually moves the result. That habit is a good one: it builds a feel for how much a single reading can be trusted on its own before drawing any conclusion from it.
Worked example
Take the figures the calculator starts with:
- Weight: 75 kg
- Height: 175 cm
That gives:
- BSA, Mosteller: 1.909 m²
- BSA, Du Bois: 1.903 m²
- Difference between the two: 0.006 m²
Those starting figures exist only to show the calculation working, not as a target or a reference value to compare yourself against — replace them with your own measurements in the calculator above and the same formula applies exactly as shown.
Reading the result
A typical adult falls between 1.6 and 2.0 m². Doses are then quoted per square metre, so a drug given at 100 mg/m² means roughly 190 mg for a 1.9 m² patient.
Where this goes wrong. Formula choice matters clinically. Different equations can differ by 5% or more, which is why protocols specify which one to use. Never use a web calculator to determine a medication dose — that is a decision for the prescribing clinician.
As with any general-purpose formula, the result here is an estimate derived from population averages, not a measurement specific to you — a reasonable starting point for a conversation with a qualified professional, not a replacement for one. Nothing on this page is medical advice.
Because metabolic rate and organ function scale closer to surface area than to mass. For narrow-therapeutic-index drugs such as cytotoxics, that difference is clinically significant.
They are all approximations from small validation studies. Mosteller is preferred for simplicity and reproducibility; Du Bois remains common in cardiology. Consistency within a treatment matters more than the choice.
The answer it gives you is bSA, Mosteller. With 75 kg weight and 175 cm height, that comes to 1.909 m². Change any field and the figure moves with it.
It uses a published formula applied exactly as defined, so the arithmetic itself is precise — but the formula is a population-level estimate, and individual results can vary from it for reasons the formula has no way to account for. Treat the output as a reasonable estimate rather than a measured or diagnostic value.
For most people, yes — a single reading is a snapshot, while the same measurement repeated consistently over weeks or months shows a trend, which is generally more informative than any one figure taken in isolation.
Small differences between calculators usually come down to which published formula or reference dataset each one uses — several equations in this area exist in more than one accepted version. This page states the specific formula used in the section above, so it is possible to check directly whether another source is using the same one.