Body Surface Area Calculator

How to use this body surface area calculator

The tool has two modes because "body surface area" means two different things depending on who is asking. Oncology and cardiology mean the whole body in square metres. Dermatology and burn care mean the fraction of the skin that is involved, as a percentage. Both are here.

1. For total BSA in m², enter height, weight, age, and sex. Age is not cosmetic. It decides which formula gets recommended, because one of the seven is validated in children and one of them should never be used on them. Sex only affects the Schlich equation, the single formula in this set that has separate versions for men and women.
2. Read the big number, then read the spread. The headline figure comes from the formula appropriate to your inputs. The chip beside it says how far apart the highest and lowest estimates are. For an average-sized adult that gap is around 5%. At the extremes it gets much wider, and the tool says so.
3. For percentage of body affected, switch to the second tab. Pick what you are measuring, then either count handprints or set how much of each body region is involved. The result is a percentage with a severity band.
4. Save it if you are taking it to an appointment. The PDF export lays out the inputs, every formula's result, and the sources on one page. The share link keeps your values in the URL.

Why this tool is different from other BSA calculators

Most body surface area pages do one of two things: return a single number from a single formula without saying which one, or list eight formulas side by side and leave you to pick. Neither answers the question a person actually has. Here is what is different:

• It tells you which formula to use. Mosteller for adults, Haycock for children, with the reasoning stated rather than assumed. Every other calculator either hides the choice or hands you a menu.
• It shows the disagreement instead of hiding it. Each formula gets a position on a track between the lowest and highest estimate, so you can see at a glance whether the equations are clustered or scattered. At a BMI of 39 they scatter by roughly 15%, and that is worth knowing.
• It excludes a formula rather than quietly misapplying it. The Schlich equation was fitted to 3-D body scans of adults. On a newborn it returns a figure about 45% below every other formula, so the tool drops it entirely for anyone under 18 rather than letting it drag the average.
• It covers percentage of body affected too. Google's own related searches for this term include "burn" and "psoriasis", from people looking for how much of the body is involved rather than its total area. Every page currently ranking ignores that, and one of them explicitly declines to handle it.
• No login, no newsletter wall, no ads, and it embeds free. Two of the ten results on page one put a registration form between you and a 1916 equation.

How it works (the math)

Every formula here estimates surface area from just two measurements, height and weight, because measuring the real thing means wrapping a body in paper strips and it has been done to only a few hundred people in the past century. Each equation is a curve fitted to one of those small samples, which is precisely why they disagree. With W in kilograms and H in centimetres:

• Mosteller (1987): BSA = √(W × H ÷ 3600)
• Du Bois & Du Bois (1916): BSA = 0.007184 × W^0.425 × H^0.725
• Haycock (1978): BSA = 0.024265 × W^0.5378 × H^0.3964
• Gehan & George (1970): BSA = 0.0235 × W^0.51456 × H^0.42246
• Boyd (1935): BSA = 0.0003207 × H^0.3 × Wg^{(0.7285 − 0.0188 × log₁₀Wg)}, where Wg is weight in grams
• Takahira (1925): BSA = 0.007241 × W^0.425 × H^0.725
• Schlich (2010): women 0.000975482 × W^0.46 × H^1.08; men 0.000579479 × W^0.38 × H^1.24

Mosteller is the one that won clinical practice, and the reason is unglamorous: it is a square root, so a pharmacist can check it on a phone calculator and catch an error before a dose leaves the room. Du Bois came first and was fitted to nine subjects, one of whom was a child with a chronic illness; it is still the most widely cited and it reads low at high body weights. Gehan and George used 401 direct measurements, the largest sample behind any of these. Haycock was built specifically to work in infants and children.

For the percentage mode, the handprint rule holds that a person's own palm with the fingers closed covers about 1% of their body surface. That is the version taught, and it is convenient, but measured values come in at roughly 0.8% for men and 0.7% for women, so counting at 1% overestimates by 10–20%. Both options are offered, because the 1% figure is what a clinician will expect to hear. The region method uses the Wallace Rule of Nines: head 9%, front of the trunk 18%, back 18%, each arm 9%, each leg 18%, groin 1%.

One formula was left out on purpose. Fujimoto (1968) appears on most competing calculators. When this tool was cross-validated on 28 August 2026, the largest of those calculators returned a Fujimoto result that did not match the Fujimoto coefficient printed on its own page. With the reference implementation and its own published equation in conflict, shipping it would have meant guessing which was right, so it is not here. The other seven all agree with that reference to two decimal places.

Worked examples

A 70 kg, 175 cm adult

Mosteller returns 1.85 m². So does Du Bois, and so does Haycock. Gehan and George and Boyd both land on 1.86, Takahira on 1.87, Schlich on 1.77. The spread is 5.8%, driven almost entirely by Schlich sitting below the pack. For an average-sized adult the choice of formula is close to irrelevant, and any of them is fine for a back-of-envelope figure.

A 120 kg, 175 cm adult

Now the picture changes. Boyd returns 2.48 m², Haycock 2.47, Mosteller 2.42, and Du Bois only 2.32. The spread has widened to 14.7%. These equations were fitted to mostly average-sized people, and they extrapolate differently once you leave that range. This is the case where it genuinely matters which formula a hospital has standardised on, and it is why the tool flags a BMI at or above 30 rather than quietly averaging.

Psoriasis on both elbows and one shin

Using the handprint method, someone finds it takes about four handprints to cover the affected skin. At the taught 1% value that is 4% of body surface, which lands in the moderate band. At the corrected 0.8% it is 3.2%, still moderate but only just. Either way it sits above the 3% line where the National Psoriasis Foundation stops calling psoriasis mild, which is the threshold that tends to change what gets prescribed. If itch is the dominant symptom, an anti-itch cream addresses the scratching that widens the patches in the first place.

Frequently asked questions

What is a normal body surface area?

Roughly 1.9 m² for an average adult man and 1.6 m² for an average adult woman, about 1.14 m² for a ten-year-old and around 0.25 m² for a newborn. But "normal" is the wrong frame: BSA is a size measurement, not a health measurement. A tall person has a large BSA and that tells you nothing about their health. If you want a number that says something about body composition, look at lean body mass or ideal body weight instead.

Which BSA formula is most accurate?

None of them is "most accurate" in a way that survives contact with a real patient, because all of them are estimates from two measurements. Mosteller is the practical default for adults and Haycock for children. What matters more than picking the best equation is picking one and staying with it: if a treatment was calculated on Du Bois, re-checking it later on Mosteller will show a difference that is an artefact of the formula rather than a change in the person.

Why is BSA used for chemotherapy dosing instead of weight?

Because surface area tracks metabolic mass more closely than weight does, and metabolic mass is what governs how fast a drug is cleared. It is an imperfect proxy. For drugs with a narrow margin between an effective dose and a toxic one, BSA-based dosing is actively debated, and it becomes less reliable at the extremes of height and weight. It remains more consistent than dosing on body weight alone, which is why it persists.

How do I calculate what percentage of my body is affected by a rash?

Use the second tab. The quickest method is the handprint rule: the affected person's own hand, palm and fingers together, covers about 1% of their body. Count how many handprints it takes to cover the affected skin. For larger or more scattered areas, the region method is more reliable, since it lets you say something like "half of one arm and a quarter of the back" rather than counting dozens of handprints.

Can I use this to work out how bad a burn is?

No, and please do not try. The percentage mode will accept a burn and give you a number to describe over the phone, but the Rule of Nines counts only partial- and full-thickness burns, does not apply to young children, and says nothing about depth, which is what actually determines treatment. Any burn on the face, hands, feet, genitals, or a major joint needs emergency care regardless of size, as does any electrical or chemical burn. This tool calculates no fluid volumes and no doses.

Related tools

• Ideal Body Weight Calculator: four published formulas with the same show-the-disagreement approach.
• Lean Body Mass Calculator: fat-free mass, the other way of adjusting for body size.
• TDEE Calculator: daily energy expenditure, which scales with surface area.
• Browse all free tools by Dr. Taylor →

Dr. Taylor's recommendations

Two of these follow directly from what the calculator does. The rest depend on what you found:

• Best Bathroom Scales. Every formula on this page is driven by your weight, and most home scales drift. If the BSA figure is going anywhere near a clinical conversation, it is worth being weighed on something accurate.
• Best Eczema Creams. Barrier-repair emollients are the usual first step when the affected area is under 3%.
• Best Anti-Itch Creams. Itch drives scratching, and scratching is what turns a small patch into a larger one.
• Best Antifungal Creams. A spreading ring-shaped rash is often fungal, and it will get worse rather than better under a steroid cream.

Sources & methodology

• Mosteller RD. Simplified calculation of body-surface area. N Engl J Med 1987;317:1098.
• Du Bois D, Du Bois EF. A formula to estimate the approximate surface area if height and weight be known. Arch Intern Med 1916;17:863-71.
• Haycock GB, Schwartz GJ, Wisotsky DH. Geometric method for measuring body surface area: a height-weight formula validated in infants, children and adults. J Pediatr 1978;93:62-66.
• Gehan EA, George SL. Estimation of human body surface area from height and weight. Cancer Chemother Rep 1970;54:225-235.
• Boyd E. The Growth of the Surface Area of the Human Body. University of Minnesota Press, 1935.
• Schlich E, Schumm M, Schlich M. 3-D-Body-Scan als anthropometrisches Verfahren zur Bestimmung der spezifischen Körperoberfläche. Ernährungs Umschau 2010;57:178-183.
• Rule of Nines, StatPearls (NCBI Bookshelf): region percentages, and the 0.8%/0.7% correction to the palm rule.
• National Psoriasis Foundation: the under 3% / 3–10% / over 10% severity bands, and the position that high-impact sites count as severe regardless of area.
• American Burn Association: burn centre referral criteria.

Every formula on this page is checked against a fixture file on each build, and each one was cross-validated against an independent reference calculator to two decimal places on 28 August 2026. This tool is reviewed annually. About Dr. Taylor · Last reviewed August 28, 2026.

Embed this tool on your site

Free for nursing and medical educators, dermatology and oncology practices, patient-education sites, and anyone teaching BSA. Required attribution is built into the snippet. No fee, no account, no rate limit.

<iframe
  src="https://bestrateddocs.com/body-surface-area-calculator/embed/"
  width="100%"
  height="820"
  loading="lazy"
  style="border:1px solid #e8dfeb; border-radius:16px; max-width:760px;"
  title="Body Surface Area Calculator (BestRatedDocs)">
</iframe>
<p style="font-size:12px;color:#666;margin-top:8px">
  Body surface area calculator by
  <a href="https://bestrateddocs.com/body-surface-area-calculator/">BestRatedDocs</a>
  &middot; Reviewed by Dr. David Taylor, MD, PhD
</p>