Body Surface Area Calculator

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What body surface area is, and why medicine uses it instead of weight

Body surface area is exactly what it sounds like: the total area of skin wrapping the body, in square metres. A typical adult falls somewhere between 1.5 and 2.1 m². It was originally established by measurement rather than calculation (Du Bois and Du Bois covered their subjects in moulded paper strips in 1916 and worked out the area of the flattened pieces), but that is an experiment, not a bedside procedure, so every clinical figure since has been an estimate from height and weight.

The reason medicine reaches for it at all is that many of the processes a doctor needs to scale do not scale with mass. Resting metabolic rate, cardiac output, glomerular filtration and blood volume all rise with body size more slowly than weight does, and across the range of adult sizes they track surface area much more closely. A person of 100 kg does not have twice the drug-clearing capacity of a person of 50 kg, so a dose set per kilogram will tend to overdose the larger person and underdose the smaller one. Body surface area is a single number that folds height and weight together into something closer to metabolic size.

That is the case for it. The case against (and it is a serious one, argued in the oncology literature for decades) is that even BSA explains only a modest share of the variation in how fast individuals clear most drugs. It survives because it is better than weight alone, because it is computable from two measurements anyone can take, and because fifty years of dose-finding trials were designed around it.

The six formulas, and where each one came from

Du Bois and Du Bois (1916) is the ancestor. Published as the tenth paper in a series on clinical calorimetry, it fitted a power equation (0.007184 × height^0.725 × weight^0.425) to direct measurements of just nine people, one of them a child with growth failure. That tiny sample is the standard criticism of it, and it is also a remarkable piece of work that has held up for a century.

Takahira (1925) refitted the same equation shape on a Japanese sample and changed only the leading constant, from 0.007184 to 0.007241, which is why its answers sit consistently within about one per cent of Du Bois. Fujimoto (1968) also worked with a Japanese population and arrived at different exponents entirely (0.008883 × height^0.663 × weight^0.444), which gives it a noticeably different slope across the size range.

Gehan and George (1970) attacked the sample-size problem directly by pooling 401 direct surface-area measurements from the published literature, and their equation became common in oncology. Haycock, Schwartz and Wisotsky (1978) built a geometric method validated from premature infants through to adults, which is why paediatric services often prefer it. Mosteller (1987) is the outlier: a single-paragraph letter to the New England Journal of Medicine proposing a simplification (the square root of height times weight over 3600) that needs no exponent key on a calculator and still matches Du Bois closely across the usual adult range. Its simplicity is why it is now the default in most hospitals.

Why the formulas disagree, and by how much

Each equation is a curve fitted to a different sample of directly measured bodies, using different statistical methods, in populations with different average builds. They are estimating the same physical quantity, so they cannot diverge wildly, but they were never going to land on the same number either. For a mid-range adult the six typically fall within two to four per cent of each other. At 175 cm and 78 kg, this page computes a span of 1.89 to 1.96 m², a spread of about 3.8% around a mean of 1.94.

The disagreement widens at the edges. Very small bodies, very large bodies and unusual height-to-weight combinations are where the fitted curves pull apart, because that is where each sample had the fewest observations to constrain it. This matters most in paediatrics and in the treatment of obese adults, which is exactly where the choice of formula gets argued over.

Three to four per cent sounds negligible until it is multiplied by a dose. A drug prescribed at 500 mg/m² comes out around 35 mg apart at the two ends of that span for one 175 cm, 78 kg adult. For most agents that is well inside the margin the dose was chosen with; for a few, with a narrow therapeutic window, it is not. This is why the answer to 'which formula is right' is always 'the one the protocol specifies', and why a number from a website is not a substitute for a prescriber's calculation.

Chemotherapy dosing, and the argument about capping

Dosing cytotoxic drugs per square metre dates from mid-century work translating doses between animal species, where surface area gave better agreement than weight. It carried over into human phase I trials and became the convention for most cytotoxics, which is why oncology paperwork is full of figures like 175 mg/m² and why patients receiving chemotherapy usually know their own BSA.

Because dose scales with BSA and BSA rises with weight, very large patients generate very large calculated doses. Historically many services capped the BSA used in the calculation, often at around 2.0 m², out of a fear of excess toxicity. The evidence did not support the habit: the American Society of Clinical Oncology's guideline on dosing obese adults concluded that full weight-based doses should be used, and that capping risks undertreatment without a demonstrated safety benefit. Practice has moved, but local variation persists, and it is one of the reasons a dose on a real chart may not match a number computed from height and weight alone.

None of this changes what the calculation on this page is for. It gives you the figure a protocol would start from, so that the number on your paperwork makes sense. It does not and cannot tell you what dose is appropriate.

Cardiac index, and why an echo report divides by body surface area

Cardiac index is cardiac output in litres per minute divided by body surface area, and it is the everyday reason BSA appears on cardiology and critical care reports. A cardiac output of five litres a minute means something very different in a 1.5 m² person and a 2.2 m² one, so dividing by BSA puts hearts of different sized bodies on one scale. The usual adult range for cardiac index at rest is roughly 2.5 to 4.0 L/min/m².

The same logic runs through echocardiography more broadly. The chamber quantification recommendations used by sonographers index left ventricular mass, chamber volumes, stroke volume and aortic root dimensions to BSA, so that 'enlarged' means enlarged for that body rather than enlarged in absolute centimetres. When a report says something is indexed, BSA is almost always the denominator, and the formula used for it is stated in the laboratory's protocol.

The illustration in the result panel divides a textbook five litres a minute by your BSA. It is arithmetic showing how the indexing works, not a measurement of your heart, which cannot be inferred from a height and a weight.

Burns, fluid resuscitation and renal normalisation

Burn assessment uses the percentage of total body surface area involved rather than the absolute figure. The rule of nines assigns nine per cent (or multiples of it) to each major region of an adult body (head, each arm, front and back of the torso, each leg), with one per cent for the perineum; the Lund and Browder chart does the same job more accurately, and with age-specific proportions for children, whose heads are a much larger share of their surface. That percentage then drives fluid resuscitation formulas, which are written as millilitres per kilogram for each per cent of the body burned.

Kidney function meets BSA from the other direction. Estimated glomerular filtration rate is reported per 1.73 m², a normalisation constant that lets one person's filtration be compared with another's and with the published thresholds for chronic kidney disease staging. For most purposes the normalised figure is the right one to read. For dosing a renally cleared drug with a narrow therapeutic window in someone whose body size is far from average, the calculation may need to be de-normalised back to absolute millilitres per minute using that person's own BSA, a step pharmacists take deliberately, and a good illustration of why the same number is used differently in different rooms.

Why body surface area has no healthy range

Every other number on this site has an interpretation attached: a band, a threshold, a direction that is better. BSA has none. It is a size measure, and larger people have larger surface areas the same way taller people have longer femurs. There is nothing to aim for, nothing to reduce, and a change in it means only that your height or weight changed.

What it does have is conventions. The 1.73 m² used to normalise kidney function is a historical average retained for continuity rather than a current description of anybody. Measured populations run larger: a multicentre study of adult cancer patients in the United Kingdom found a mean BSA of 1.79 m² (1.91 m² for men and 1.71 m² for women), and noted that the older figures still quoted in cost and dosing models understate the patients actually being treated. Those are useful anchors for reading your own result, not targets.

Why you must never adjust a dose from this page

This is the one hard line on the page, and it is not boilerplate. A prescribed dose depends on which BSA formula the protocol validated, on renal and hepatic function, on age and performance status, on dose-banding policies that round doses to standardised vial sizes, on capping rules where a service still applies them, on interactions with everything else the person is taking, and on clinical judgement that no calculator has access to. Height and weight are two inputs among many.

If a figure on your paperwork looks inconsistent with what you see here, the useful response is a question, not a change. Pharmacists and prescribing clinicians are used to being asked how a dose was worked out and will generally walk through the calculation with you. Adjusting a dose on the strength of a website, in either direction, is the single most dangerous thing anyone could do with this number.

• A different formula can shift the answer by a few per cent; protocols specify which one to use.
• Dose banding means real doses are often rounded to standard quantities, so an exact per-square-metre figure is not expected to match.
• Paediatric doses use the same equations but belong entirely with the prescriber, working from a current measured weight.

How it's calculated

Worked example: an adult of 175 cm and 78 kg

1. Mosteller, step one: multiply height by weight, which gives 175 × 78 = 13,650.
2. Step two: divide by 3600, which gives 13,650 ÷ 3600 = 3.7917.
3. Step three: take the square root, which gives √3.7917 = 1.947, so 1.95 m² to the two decimals clinical paperwork uses.
4. Du Bois for comparison: 175^0.725 = 42.29 and 78^0.425 = 6.37, so 0.007184 × 42.29 × 6.37 = 1.94 m².
5. Run all six and they land at 1.95 (Mosteller), 1.94 (Du Bois), 1.96 (Haycock), 1.96 (Gehan and George), 1.89 (Fujimoto) and 1.95 (Takahira) m².
6. The spread: 1.96 − 1.89 = 0.07 m², which is about 3.8% of the 1.94 m² mean of the six.
7. Read against the conventions: 1.95 m² is about 12.6% above the 1.73 m² normalisation constant used for kidney function, and above the 1.79 m² mean measured in UK adult cancer patients.
8. Cardiac index illustration: if cardiac output were the textbook resting 5 L/min, 5 ÷ 1.95 = 2.6 L/min/m², which sits at the lower end of the usual 2.5–4.0 range.

Where this number is used in the real world

• Oncology, where most cytotoxic chemotherapy is prescribed in milligrams per square metre and the BSA formula is named in the protocol.
• Cardiology and echocardiography, where cardiac output, stroke volume, ventricular mass and valve areas are all indexed to BSA so they can be compared between bodies of different sizes.
• Intensive care, where cardiac index rather than raw cardiac output is the figure tracked at the bedside.
• Burns care, where injury extent is recorded as a percentage of total body surface area and drives the fluid resuscitation calculation.
• Nephrology, where estimated glomerular filtration rate is reported per 1.73 m² and occasionally de-normalised to a patient's own surface area for drug dosing.
• Paediatrics, where maintenance fluid and some drug doses are worked per square metre, usually with the Haycock or Mosteller equation.
• Clinical research, where normalising physiological measurements to BSA removes body size as a confounder between study participants.

Frequently asked questions

Keep going

A single number rarely tells the whole story. Alongside the BSA result, the BMI calculator, the lean body mass calculator, the ideal weight calculator, the child BMI calculator and the healthy weight range calculator each add a different angle on the same measurements. For the reasoning behind the numbers, read BMI explained and BMI vs body fat vs waist.

Sources

1. Mosteller RD. Simplified calculation of body-surface area. N Engl J Med 1987;317(17):1098. doi.org/10.1056/NEJM198710223171717
2. Du Bois D, Du Bois EF. Clinical calorimetry: tenth paper. A formula to estimate the approximate surface area if height and weight be known. Arch Intern Med 1916;17(6_2):863–71. doi.org/10.1001/archinte.1916.00080130010002
3. 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(1):62–6. doi.org/10.1016/S0022-3476(78)80601-5
4. Gehan EA, George SL. Estimation of human body surface area from height and weight. Cancer Chemother Rep 1970;54(4):225–35. pubmed.ncbi.nlm.nih.gov/5527019/
5. Sacco JJ, Botten J, Macbeth F, Bagust A, Clark P. The average body surface area of adult cancer patients in the UK: a multicentre retrospective study. PLoS One 2010;5(1):e8933. doi.org/10.1371/journal.pone.0008933
6. Lang RM, Badano LP, Mor-Avi V, et al. Recommendations for cardiac chamber quantification by echocardiography in adults: an update from the American Society of Echocardiography and the European Association of Cardiovascular Imaging. J Am Soc Echocardiogr 2015;28(1):1–39. doi.org/10.1016/j.echo.2014.10.003
7. Griggs JJ, Bohlke K, Balaban EP, et al. Appropriate systemic therapy dosing for obese adult patients with cancer: ASCO guideline update. J Clin Oncol 2021;39(18):2037–48. doi.org/10.1200/JCO.21.00471