Body Surface Area Calculator

How this is calculated

⚠️ THIS IS NOT A DOSING TOOL AND MUST NOT BE USED AS ONE. Body surface area is used clinically to work out doses of chemotherapy agents and certain antibiotics. That calculation belongs to the people prescribing it, who also weigh kidney and liver function, other medication, previous responses and a great deal else that no calculator sees. If you want to know why your dose is what it is, ask the team who set it - they will tell you, and they will be pleased you asked. Never adjust anything based on a number from this page. WHAT BSA IS FOR: it indexes body size in a way that weight alone does not. Beyond drug dosing it is used to calculate the cardiac index and to index physiological measurements such as estimated glomerular filtration rate, which is how kidney function gets compared meaningfully between a large adult and a small one. THE MOSTELLER FORMULA IS THE SIMPLE ONE and the reason it caught on: BSA equals the square root of height in centimetres multiplied by weight in kilograms, divided by 3600. It is memorable and can be done on any calculator, which is exactly why it is recommended for clinical and laboratory work. THE DU BOIS FORMULA IS THE OLDER AND MORE WIDELY CITED ONE: 0.007184 multiplied by height in centimetres to the power 0.725, multiplied by weight in kilograms to the power 0.425. ⚠️ THEY DISAGREE, AND THE DISAGREEMENT IS THE POINT OF SHOWING BOTH. At an average adult size the two land within about half a percent of each other, which looks reassuring. They diverge at the extremes of height and weight, and across the range of formulas in use BSA values can vary by as much as 0.5 m2 for the same person. That is not a rounding difference - it is a large fraction of a typical adult's total. Any number derived from BSA inherits that uncertainty, which is one more reason the calculation belongs with clinicians who know which formula their protocol specifies. BSA IS A MODEL OF A BODY, NOT A MEASUREMENT OF ONE. Both formulas were fitted to small samples of people and predict surface area from two numbers. They cannot see body composition, proportions, limb length, amputation, pregnancy or oedema, all of which change the real answer. For children in particular, dosing practice varies and paediatric protocols often differ from adult ones.

Common questions

Can I use this to check my medication dose?

No. Body surface area is genuinely used to calculate doses of chemotherapy agents and certain antibiotics, but that calculation belongs to the clinicians prescribing them - and it is only one input among many. Kidney and liver function, other medicines you take, how you responded to previous cycles, your blood counts and the specific protocol all feed into a dose, and none of them appear on this page. If you want to understand why your dose is what it is, ask the team who set it; they will explain it and they will be glad you asked. Nothing here should ever prompt you to change anything.

Why show two formulas?

Because they disagree, and knowing that is more useful than a single confident-looking number. At an average adult size Mosteller and Du Bois land within about half a percent of each other, which is reassuring - but they diverge at the extremes of height and weight, and across the various formulas in clinical use, body surface area values can vary by as much as 0.5 m² for the same person. That is a substantial fraction of a typical adult's total, and anything calculated from BSA inherits it. Which formula applies to you is determined by the protocol being followed, not by preference.

What is BSA actually used for?

Indexing body size in a way that weight alone cannot. The best known use is dosing chemotherapy and some antibiotics, but it is also used to calculate the cardiac index - cardiac output scaled to body size - and to index physiological measurements such as estimated glomerular filtration rate, which is how kidney function can be compared meaningfully between a large adult and a small one. In each case the point is the same: the raw figure means different things in different-sized bodies, and BSA is the scaling factor that makes them comparable.

Which formula is better?

Neither, in the abstract - it depends on what is being calculated and what the protocol specifies. Mosteller is recommended for clinical and laboratory work largely because it is simple enough to remember and to evaluate on any calculator, which matters more than it sounds when the alternative is a formula with two fractional exponents. Du Bois is the older and most widely cited. The honest answer is that if a number matters to your care, the formula was chosen by whoever wrote the protocol, and that is the one that applies to you.

How accurate is it?

It is a model rather than a measurement. Both formulas predict surface area from just height and weight, fitted to small samples of people, and they cannot see body composition, proportions, limb length, amputation, pregnancy or fluid retention - all of which change the real answer. Two people with identical height and weight can have genuinely different surface areas and will get the same number here. That is a limitation of the approach rather than of this page, and it is why the figure is used as a scaling factor within a clinical judgement rather than as a fact on its own.

Does it work for children?

The formulas are applied to children as well as adults, but paediatric practice differs in ways that matter, and dosing protocols for children are often not the adult ones scaled down. Weight-based dosing is common in paediatrics precisely because it avoids some of the problems BSA has at small body sizes. If you are trying to understand a child's dose, that is a question for their clinical team rather than for a calculator - the difference between paediatric and adult protocols is not something a formula reveals.

Why do the two answers differ more for some people?

Because the formulas have different shapes. Mosteller is a square root of the product of height and weight, while Du Bois raises each to its own fractional power - so they agree closely across the middle of the range they were fitted to and drift apart towards the edges. Someone very tall and light, or short and heavy, sits where the two curves have separated, and the gap between the outputs widens accordingly. Seeing that gap grow is a useful signal that a single BSA figure is carrying more uncertainty than usual.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Body Surface Area Calculator at https://www.bfcbrilliance.com/tools/body-surface-area-calculator.

What I entered:
- Height (cm or in): ___
- Height units: ___
- Weight (kg or lb): ___
- Weight units: ___

What it calculated:
- Body surface area - Mosteller: ___
- Body surface area - Du Bois: ___
- Difference between the two: ___
- — as a percentage: ___
- Height used: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Keep this general and do not give medical advice — where it matters, tell me what to raise with a clinician.

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