√(height x weight / 3600). The usual clinical choice.
The equation, and where it comes from
Mosteller: BSA = √(height in cm x weight in kg / 3600)
Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317(17):1098. Du Bois D, Du Bois EF. Arch Intern Med. 1916;17:863-871.
How to read the result
An average adult is around 1.7 m². BSA has no normal range in the way a lab value does: it is a size measure used to index other quantities, such as cardiac index or glomerular filtration rate.
Your patient's numbers never leave this device
The bsa (body surface area) arithmetic runs in JavaScript inside your own browser. There is no server call when you type, nothing is written to a database, nothing is logged, and no account is involved. Once the page has loaded you can disconnect from the network and it still works.
That matters more here than on an ordinary calculator. Values entered on a hospital workstation come off a real chart, and this tool deliberately does not remember them: close the tab and they are gone, including for the next person who uses that machine.
Frequently asked questions
- Which BSA formula should I use, Mosteller or Du Bois?
- Mosteller is the usual choice in practice because it is a single square root you can do on any calculator and it agrees closely with Du Bois across normal builds. Du Bois dates from 1916 and was derived on only nine subjects, and it tends to underestimate at the extremes of size. This tool offers both so you can see the difference for your own numbers.
- What is the Mosteller formula?
- BSA in m² = the square root of (height in cm multiplied by weight in kg, divided by 3600). Mosteller published it as a one-line simplification in a 1987 letter to the New England Journal of Medicine, and its arithmetic simplicity is exactly why it displaced the older exponent-based equations at the bedside.
- What is a normal body surface area?
- There is no normal range in the diagnostic sense. An average adult sits near 1.7 m², which is why glomerular filtration rate is conventionally indexed to 1.73 m². BSA is a descriptor of size, so the useful comparison is against the population, not against a reference interval.
- Why is BSA used at all rather than just weight?
- Several physiological quantities scale better with surface area than with mass, among them basal metabolic rate, cardiac output and renal clearance. Indexing to BSA lets you compare a small adult with a large one on the same scale.
- Can I enter height in feet and inches?
- Enter centimetres here. One inch is 2.54 cm, so a person who is 5 feet 9 inches is 175.3 cm. Keeping a single unit on the field avoids the mixed-unit mistakes that unit switchers can introduce on a two-input formula.