Body Surface Area Calculator

What Body Surface Area Measures

Body surface area is the area of a person's skin, reported in square meters. It is a geometric description of size, not a score of health, fitness, or body composition. A tall, lean adult and a shorter adult with more body fat can land on the same BSA, because the number reflects the outer envelope of the body rather than what is inside it.

The measurement earns its place in medicine because several physiological quantities track body size more closely than they track weight. Blood volume, heat exchange, resting energy use, and the clearance of many drugs all scale with the size of the body rather than with kilograms alone. BSA gives clinicians a single denominator that makes those quantities comparable between a small adult and a large one.

Typical values sit between roughly 1.5 and 2.0 m². Adult men average near 1.9 m² and adult women near 1.6 m², though the range within each group is wide. A value outside that band is not automatically abnormal: a very tall person, a child, or someone with a large frame can be well outside it and still be healthy.

Three uses account for most of the demand. Chemotherapy regimens are frequently written in milligrams per square meter. Kidney function is reported per 1.73 m² so that a filtration rate can be compared across body sizes. Cardiac output is divided by BSA to produce the cardiac index. Each of those conventions exists because the raw measurement would otherwise be dominated by how large the patient is.

The number this page produces is an estimate from a formula, not a laboratory measurement. That distinction matters at the edges of the population, in people with atypical body proportions, and in any situation where a dose depends on the result. The sections below explain how the estimate is built, which equations are used, and where the arithmetic stops being reliable.

How the Estimate Is Built

Direct measurement of skin area is awkward. Researchers have wrapped bodies in paper or foil, coated them in a material that could be peeled off and measured, and, more recently, used three-dimensional scanners to build a mesh of the body surface. None of those methods belongs in a clinic, so routine practice relies on equations fitted to measurements made in a study sample.

Every formula on this page takes the same two inputs: height in centimeters and weight in kilograms. The output is in square meters. If you work in feet and pounds, convert before entering the numbers. A height of 5 feet 10 inches is 177.8 cm, and 150 pounds is 68.0 kg. Mixing a metric height with a pound weight produces a result that is not in any unit at all.

The equations are power laws. In log-log form, each one is a straight line: the logarithm of BSA is a constant plus one coefficient times the logarithm of weight plus another coefficient times the logarithm of height. That structure is why the exponents look arbitrary. They are not derived from geometry; they are regression coefficients chosen to fit the measured surface areas of the study group.

Geometry suggests a different exponent. If bodies were scaled copies of one another, surface area would grow with the two-thirds power of mass, since area scales as length squared and volume as length cubed. Real bodies are not scaled copies: infants have proportionally larger heads, adults carry different amounts of trunk and limb mass, and body shape changes with weight. The fitted exponents absorb those shape differences, which is also why different study populations produce slightly different coefficients.

Worked example: a person 177.8 cm tall who weighs 68.0 kg. Mosteller gives the square root of (177.8 × 68.0 ÷ 3600), which is about 1.83 m². Du Bois gives 0.007184 × 68.0^0.425 × 177.8^0.725, about 1.85 m². The two answers differ by roughly one percent, which is typical at average body size.

Measurement error travels straight through. A two-centimeter error in height or a two-kilogram error in weight changes the result by a few tenths of a percent, which is trivial for a trend but can matter when a dose is written per square meter. Use a current measurement rather than a remembered one, and do not round the inputs before calculating.

The Formulas in the Result Table

In the equations below, W is weight in kilograms, H is height in centimeters, and the result is in square meters. The calculator applies all of them to the same inputs, which is useful because the size of the disagreement is itself information: at average body size the answers cluster tightly, while at the extremes they can separate by several percent.

Two details in the table are easy to miss. Boyd's exponent is not a constant: the term (0.6157 − 0.0188 × log₁₀W) changes with weight, which is an attempt to model the way body shape shifts as mass increases. Schlich's equations are the only ones here that take sex as an explicit input, and their coefficients differ enough that using the wrong row produces a visibly wrong answer.

The Fujimoto and Takahira equations were fitted to Japanese subjects, and several later comparisons found that they read low in European and North American populations. That does not make them wrong; it makes them local. A formula is a description of the sample it was built from, and the further a patient sits from that sample, the more the estimate deserves skepticism.

If you only remember one equation, remember Mosteller. It is the square root of height times weight divided by 3600, which can be done on any phone and is accurate enough for a quick sanity check. If you are reconciling a number with a hospital record, use the equation named in the record rather than substituting a different one.

Where These Equations Came From

The Du Bois formula appeared in 1916, based on measurements of nine subjects. A sample that small would not pass review today, yet the equation has survived more than a century of use because it predicts measured surface area well across a wide range of adults. Its persistence is a useful reminder that statistical elegance and clinical durability are different things.

Boyd's work in the 1930s took a different approach, measuring children as well as adults and letting the weight exponent vary with weight. The resulting equation is more flexible but harder to compute by hand, which is probably why it never displaced Du Bois at the bedside. Gehan and George published their fit in 1970 using a substantially larger dataset, and their equation is often preferred in research settings where the small original sample is a concern.

Haycock and colleagues published in 1978 with infants and children specifically in mind. Pediatric bodies change proportions rapidly, and an equation fitted mostly to adults can drift when applied to a growing child. The Haycock exponents differ enough from Du Bois that the two can disagree by several percent in a small child, which is one reason pediatric units tend to standardize on it.

Mosteller's 1987 paper was a deliberate simplification. The square-root form is mathematically tidy, easy to remember, and close enough to the older equations for most clinical purposes. It became popular in intensive care and in settings where a calculation has to be done quickly and checked mentally.

Fujimoto and Takahira belong to a parallel Japanese literature from the mid-twentieth century. Both were fitted to Japanese subjects, and both use exponents that differ from the Western equations. Schlich's work is the most recent entry in this table: it used three-dimensional scanning to measure body surface directly, then fitted separate equations for men and women. That methodology avoids the assumptions of the older wrapping studies, but the resulting equations are still population-specific.

Read as a group, these equations tell a consistent story. There is no single true BSA formula, only formulas that fit particular samples more or less well. The spread between them is small for an average adult and larger for anyone whose body shape is unusual, which is exactly where a clinician needs to know which equation is in use.

Which Formula to Use

In a hospital, the choice is usually made for you. The electronic record, the chemotherapy protocol, or the pharmacy's dosing system names a formula, and the correct move is to use that one. Switching equations between visits creates a step change in the trend that looks like a change in the patient but is only a change in arithmetic.

For general adult work, Du Bois remains the common default and is the equation against which the others are usually compared. Mosteller is the practical choice when speed matters and the result will be checked against a range rather than used to write a dose. Gehan and George is a reasonable research alternative because it was fitted to a larger sample. Haycock is the usual choice for infants and children.

Consistency matters more than theoretical accuracy. If a patient's BSA is being tracked during a course of treatment, the same equation applied to the same quality of measurements will show the real trend. Alternating formulas can move the number by more than the treatment does. Ask which equation your center uses and stay with it.

One caution applies to all of them: a formula chosen for convenience is still an estimate. If the result will be used to prescribe, the decision belongs to the clinical team, and the dose should be checked against the protocol's caps, the patient's organ function, and the pharmacy's preparation. The calculator supplies a number, not a prescription.

BSA and Chemotherapy Doses

Cytotoxic chemotherapy is often prescribed in milligrams per square meter, so the dose is multiplied by the patient's BSA before the pharmacy prepares it. The convention grew up because early trials found that a dose scaled to surface area produced more consistent blood levels than a fixed dose, and because many of these drugs have a narrow window between the therapeutic and toxic range.

The relationship is looser than the convention suggests. BSA explains only part of the variation in how a given patient handles a drug. Age, kidney and liver function, body composition, genetic differences in metabolizing enzymes, and drug interactions all move the same needle. Two patients with the same BSA can have substantially different exposure to the same dose.

Practice has responded in several ways. Some protocols cap BSA at a fixed value, commonly 2.0 m², so that a patient with a very large body size is not automatically given a proportionally huge dose. Carboplatin is usually dosed with the Calvert formula, which uses kidney function and a target area under the curve rather than BSA alone. Busulfan, methotrexate, and fluorouracil are examples of drugs for which therapeutic drug monitoring can replace or refine a BSA-based estimate.

Newer agents are not automatically dosed the same way. Many targeted therapies and immunotherapies have wider therapeutic windows and are given at fixed doses, because the target is a biological pathway rather than a body surface. The dosing statement in the prescribing information is the authority for any specific drug, and it may differ from the historical BSA convention.

What BSA does not do is replace monitoring. Blood counts, kidney function, liver tests, and the patient's actual toxicity still determine whether the next cycle proceeds unchanged. A calculator can produce the starting number; it cannot see the patient.

Cardiac Index and Other Indexed Measures

Cardiac output is the volume of blood the heart pumps per minute, usually 4 to 8 liters at rest. Dividing by BSA gives the cardiac index, typically 2.5 to 4.0 liters per minute per square meter. The normalization matters because a large person naturally pumps more blood; an absolute output that looks adequate in a small adult may be low for a large one, and the reverse is also true.

Worked example: a cardiac output of 5.0 L/min in a patient with a BSA of 1.8 m² gives an index of 2.78 L/min/m². The same output in a patient with a BSA of 2.2 m² gives 2.27, below the usual range. Neither number is a diagnosis on its own, but the indexed value is the one that can be compared with a reference range.

The same logic applies to stroke volume, ventricular mass, and valve area. Left ventricular mass index divides mass by BSA and is reported against sex-specific thresholds; an elevated value suggests hypertrophy and carries prognostic weight. Aortic valve area index below 0.6 cm²/m² is one criterion for severe aortic stenosis, particularly when the absolute valve area sits in an ambiguous range.

Indexing is not beyond criticism. In obesity, dividing by a BSA that has been inflated by adipose tissue can make cardiac output or ventricular mass appear more normal than the underlying physiology warrants. Some investigators prefer height-based indexing or lean body mass for specific questions. The practical point is that an indexed value should be read alongside the absolute measurement and the clinical picture, not instead of them.

BSA and BMI Are Not Rivals

Body mass index is weight in kilograms divided by height in meters squared. It is a screening tool: a quick way to sort populations into weight categories and to flag individuals for further assessment. BSA is a size descriptor used as a denominator for physiological measurements and drug doses. They are correlated because both use height and weight, but they answer different questions and cannot be converted into each other.

BMI deliberately removes height from the comparison by dividing by height squared. That is useful when the question is whether a person's weight is high or low relative to their stature. BSA keeps height and weight in a power relationship that approximates the body's outer surface. That is useful when the question is how large the body is for the purpose of scaling a dose or a flow.

Two people can share a BMI and differ in BSA. A 1.60 m adult weighing 64 kg has a BMI of 25.0 and a Mosteller BSA of about 1.69 m². A 1.85 m adult weighing 85.6 kg has the same BMI of 25.0 but a BSA of about 2.10 m². The second person is simply larger, and any per-square-meter dose or indexed cardiac measurement will reflect that.

Neither number measures body fat. BMI can misclassify a muscular athlete as overweight, and BSA says nothing about the proportion of muscle to fat. Waist circumference, body composition measurement, and clinical assessment fill gaps that both indices leave open. Treating either as a complete picture of health is a category error.

In practice, BMI belongs to screening and population health, while BSA belongs to dosing and physiological normalization. A clinic may record both on the same visit without either one being redundant. If you are choosing which to track for your own purposes, choose based on the question you are asking, not on which number sounds more scientific.

Children: Different Bodies, Different Formula

Children are not small adults in geometric terms. An infant's head is a large fraction of total surface area, the limbs are proportionally shorter, and body proportions change continuously through adolescence. That is why the Haycock equation, fitted to infants and children, is generally preferred in pediatric settings over equations derived from adult samples.

Pediatric drug dosing often uses milligrams per kilogram rather than per square meter, but BSA remains important for chemotherapy, some antimicrobials, and certain biologic agents. A common historical method expresses a child's dose as a fraction of the adult dose using the ratio of the child's BSA to 1.73 m². That reference value represents a conventional adult surface area, not a universal target, and modern protocols usually specify the calculation directly.

Neonates and premature infants are a special case. Their surface-area-to-mass ratio is high, their skin is thin, and their fluid compartments differ from those of older children. Formulas fitted to older children may drift in this group, and dosing is typically guided by neonatal-specific protocols and close monitoring rather than by a single BSA estimate.

Growth also means the number has a short shelf life. A BSA calculated at the start of a long treatment course may be stale by the next cycle. Recalculate with a current length and weight at each visit, and use the same equation throughout so that the trend is interpretable. If a child's growth is atypical because of illness, hormone deficiency, or a genetic syndrome, the formula is still arithmetic; the clinical interpretation is not.

Burns and the Rule of Nines

Burn size is usually described as a percentage of total body surface area, which sounds like the same quantity this page calculates. It is not. The burn percentage is the share of skin that has been injured, assessed by inspection, while the BSA formulas estimate the total surface area of an intact body. The two are related but not interchangeable, and burn protocols use their own charts.

The Rule of Nines is a rapid adult estimate. The head accounts for 9%, each arm 9%, the front of the trunk 18%, the back of the trunk 18%, each leg 18%, and the perineum 1%. The percentages sum to 100. A patient with burns over the entire right arm and the front of the trunk would be estimated at 9% plus 18%, or 27%.

Children need a different chart because their heads are proportionally larger and their legs shorter. The Lund-Browder chart adjusts the percentages by age and is the usual choice in pediatric burn care. Applying the adult Rule of Nines to a small child will overestimate the head and underestimate the legs.

Fluid resuscitation is the reason the percentage matters so much. The Parkland formula is a common starting point: 4 mL of lactated Ringer's solution per kilogram of body weight per percent of burned surface area over the first 24 hours, with half given in the first 8 hours from the time of injury. It is a starting estimate, not a fixed prescription; urine output and the patient's response guide subsequent adjustments.

Several caveats belong with that formula. Superficial sunburn is not counted the same way as a full-thickness burn. Electrical and inhalation injuries can cause damage that the visible surface percentage underestimates. Fluid overload is a real risk, particularly in older patients and those with heart or kidney disease. Burn resuscitation is a monitored clinical procedure, not a home calculation.

Where the Estimate Breaks Down

All of these equations were fitted to groups of people, and the fit is best near the middle of those groups. At the extremes of height and weight, the formulas diverge from one another and from measured surface area. A patient with severe obesity may get answers that differ by several percent depending on the equation, which is one reason protocols often cap the BSA used for dosing.

Fluid changes distort weight without changing skin area. Edema, ascites, and rapid weight gain from heart or kidney failure add kilograms that the formula treats as tissue. The result is an overestimate, sometimes a large one. The same problem appears in reverse after dehydration or amputation: standard equations assume an intact body and will overestimate surface area when a limb is missing.

Height is not stable across a lifetime. Adults lose stature with age as vertebral discs compress, and a height recorded a decade ago may be two or three centimeters too high. For a patient whose BSA is being tracked, remeasure height rather than copying it forward. Small errors are tolerable in a trend; they are less tolerable when the number is multiplied into a drug dose.

Body composition is invisible to these formulas. Two people with the same height and weight receive the same estimated BSA even if one is muscular and the other has a high proportion of fat. That is not a flaw in the arithmetic so much as a limit of the inputs: surface area is being inferred from two numbers that cannot describe shape. Three-dimensional scanning can measure the surface directly, but it remains a research and specialty tool rather than a routine one.

Finally, a formula cannot know the clinical context. It does not know about pregnancy, a recent amputation, a pleural effusion, or a course of steroids that has changed body water. Use the estimate as one input among several, and let the clinical team decide when it is close enough and when it is not.

What to Take Away

Body surface area is a size index expressed in square meters. It is estimated from height and weight, not measured directly in routine care, and the result depends on which equation is used. At average adult size the common formulas agree closely; at the extremes they can differ enough to matter.

Use the formula your clinic or protocol uses, keep using it consistently, and measure height and weight afresh rather than relying on old values. Treat the number as a starting point for a clinical decision, not as a substitute for one. The 1.73 m² figure you see in kidney function reports is a conventional adult reference, not a target that an individual is supposed to reach.

The calculator is most useful for understanding the arithmetic and for checking a number you have already been given. It is not a dosing tool for patients at home. If a medication dose depends on BSA, the prescription and its verification belong to the clinical team and the pharmacy.