Body Surface Area Calculator

What body surface area is, and why it matters

Body surface area, or BSA, is exactly what it sounds like: the total area of the outside of your body, measured in square metres. On its own that might sound like a curiosity, but in medicine it is quietly one of the most useful numbers there is.

The reason is that many things in the body scale with surface area better than with plain weight. So BSA turns up wherever a dose or a measurement needs to be matched to body size. Oncologists use it to work out chemotherapy doses. Cardiologists index heart measurements to it. Kidney specialists use it to adjust measures of kidney function. This calculator gives you that figure, and lets you choose which of the established formulas to use.

The formulas this calculator uses

There is no single agreed BSA formula. Over the years researchers have published more than forty, and this tool offers five of the best known. You enter your weight in kilograms and your height in feet and inches, which the tool converts to centimetres, then it applies whichever you select:

• Du Bois and Du Bois: 0.007184 × weight^0.425 × height^0.725
• Mosteller: the square root of (weight × height ÷ 3600)
• Haycock: 0.024265 × height^0.3964 × weight^0.5378
• Gehan and George: 0.0235 × height^0.42246 × weight^0.51456
• Boyd: a longer form that varies its own weight exponent, giving a close fit across a wide size range

Weight is in kilograms and height in centimetres throughout. They look different, but for an average adult they land remarkably close to one another, which is reassuring rather than confusing.

A worked example

Take someone 5 feet 9 inches tall, weighing 70 kg. That height is 175 cm. Using the Mosteller formula, which is the easiest to follow:

BSA = √(70 × 175 ÷ 3600)
BSA = √(12250 ÷ 3600) = √3.40
BSA ≈ 1.85 m²

Run the same person through the older Du Bois formula and you get almost exactly the same answer, about 1.85 square metres. That closeness is the point: for most adults the common formulas agree to within a percent or two, so the choice rarely changes the result much.

Which formula should you pick?

For most people it genuinely does not matter, since they cluster so tightly. But if you want a reason to choose:

• Du Bois and Du Bois (1916) is the historical standard. It has been the reference for over a century, and many oncology and cardiology tables were built around it. Worth knowing it was derived from just nine people.
• Mosteller (1987) is the one most widely used today, mainly because it is simple enough to work out on any calculator. If you are unsure, this is a safe default.
• Haycock and Gehan and George were validated to work well in children as well as adults, so they see use in paediatrics.

If a clinician or a protocol specifies a particular formula, use that one. Otherwise, Mosteller and Du Bois are the two you will meet most often, and they will hand you nearly identical numbers.

Accuracy, and a word on dosing

These formulas are estimates, and it is worth being clear-eyed about that. The classic ones were derived from very small samples, some from only a handful of people, and validated against limited data. They work well enough for routine use across average adults, but they lose accuracy at the extremes of size, and they cannot account for an unusual build.

One thing needs saying plainly, because BSA is tied to drug dosing. This tool is a reference and a learning aid, not a prescribing tool. Real medication dosing, chemotherapy above all, involves far more than a surface area number: the specific drug, the protocol, kidney and liver function, and a clinician's judgement. Never use a BSA figure from any online calculator to set or change a real dose. That decision belongs with the prescriber and the pharmacy, every time.

Questions people ask

Which BSA formula is most accurate?

No single one wins cleanly, because none is measuring you directly; they are all estimates fit to old data. Du Bois has been the long-standing reference and Mosteller is the most used for its simplicity, and for most adults they agree within a percent or two. For a specific clinical purpose, the formula the protocol names is the one that matters.

What is an average body surface area?

For an average adult it is roughly 1.7 square metres, with most adults falling somewhere between about 1.5 and 2.0. Taller and heavier people sit higher, which is exactly why the figure is used to scale doses and measurements to body size.

Can I use this to calculate my medication dose?

No. BSA is one input a clinician may use, but a real dose depends on the drug, the protocol, your organ function and clinical judgement. Use this to understand the concept, and leave actual dosing to your doctor and pharmacist.

References

Where the figures come from. The formulas this tool offers are reproduced above as it applies them, and trace to their original publications: Du Bois and Du Bois for the classic 1916 equation, Mosteller for the simplified 1987 version, and Haycock and colleagues for the formula validated in children and adults.

1. Du Bois D, Du Bois EF. A formula to estimate the approximate surface area if height and weight be known. Archives of Internal Medicine. 1916;17(6):863-871.
2. Mosteller RD. Simplified calculation of body-surface area. New England Journal of Medicine. 1987;317(17):1098. https://doi.org/10.1056/NEJM198710223171717
3. Haycock GB, Schwartz GJ, Wisotsky DH. Geometric method for measuring body surface area: a height-weight formula validated in infants, children, and adults. Journal of Pediatrics. 1978;93(1):62-66. https://doi.org/10.1016/s0022-3476(78)80601-5