Body Surface Area (BSA) Calculator
Body surface area (BSA) is an estimate of the total surface area of the human body, calculated from height and weight rather than measured directly. Clinicians use it instead of weight alone because it correlates more closely with metabolic rate, blood volume, and cardiac output, which makes it a more reliable way to scale drug doses, IV fluids, and other treatments across patients of different sizes. This calculator is provided for informational and educational use only and is not a substitute for a professional medical calculation or a clinician's judgment.
Average BSA by age
These reference values show how average body surface area scales with age, from newborn to adult:
| Age group | Average BSA |
|---|---|
| Newborn | ≈ 0.25 m² |
| 2-year-old | ≈ 0.5 m² |
| 10-year-old | ≈ 1.14 m² |
| Adult female | ≈ 1.6 m² |
| Adult male | ≈ 1.9 m² |
What is Body Surface Area?
Body surface area is a calculated estimate of how much skin covers the human body, expressed in square meters (m²). It can't be measured directly with any practical clinical tool, wrapping a person in a tape measure or scanning their entire surface isn't feasible in a hospital setting, so BSA is instead derived mathematically from height and weight using one of several validated formulas.
The reason BSA is used instead of body weight alone comes down to physiology. Weight is a poor proxy for how a body actually processes drugs, fluids, and heat, because it doesn't account for the ratio of surface area to volume. A tall, lean person and a shorter, heavier person can weigh the same amount but have very different metabolic rates, blood volumes, and rates of heat loss. BSA correlates more closely with these physiological variables, including cardiac output, renal clearance, and basal metabolic rate, than weight does on its own, which is why it became the standard scaling factor for many medical calculations during the 20th century.
Because BSA is estimated rather than measured, different formulas (derived from different study populations, at different points in history, using different measurement techniques) produce slightly different results for the same person. None of them is definitively correct, they're all approximations, and the differences between formulas are usually small enough to be clinically insignificant for most adults of average build.
What is BSA used for?
BSA shows up most often in three clinical contexts:
- Drug dosing, especially chemotherapy. Many chemotherapy protocols calculate dose per square meter of BSA (e.g., “175 mg/m²”) rather than per kilogram of body weight, on the theory that BSA-based dosing better normalizes drug exposure and toxicity across patients of different sizes. This practice dates back to the 1950s and remains standard in oncology today.
- Cardiac index. Cardiac output (how much blood the heart pumps per minute) is divided by BSA to calculate cardiac index, which lets clinicians compare heart performance across patients of very different body sizes on a level playing field.
- Burn assessment and fluid resuscitation. Burn severity is often expressed as a percentage of total body surface area affected, and BSA is used to calculate fluid replacement needs in burn patients.
It's worth being upfront about the limitations. BSA-based chemotherapy dosing has been debated in the oncology literature for decades, particularly for drugs with a narrow therapeutic index, two patients with an identical BSA can still metabolize and clear the same drug very differently, because BSA doesn't account for organ function, body composition, or genetic variation in drug metabolism. BSA formulas are also generally less accurate at the extremes of height and weight (very tall, very short, or significantly over- or under-weight), since most formulas were derived from average-sized adult populations. For these reasons, BSA is one input among several that a treating clinician weighs, not a formula to be applied mechanically.
BSA formulas explained
Every formula below takes weight in kilograms (W) and height in centimeters (H), and returns BSA in square meters. They differ in the study population they were derived from and the mathematical relationship each assumes between body size and surface area.
Mosteller
BSA = √( (W × H) / 3600 )
Mosteller, 1987
Published by Dr. R.D. Mosteller in 1987, this is the simplest BSA formula to compute by hand and the one most commonly used in modern clinical software and calculators. It produces results very close to Du Bois for typical adult body sizes, which is why it's treated as the default clinical formula today.
Du Bois
BSA = 0.007184 × W^0.425 × H^0.725
Du Bois & Du Bois, 1916
Derived by Dr. Delafield Du Bois and Eugene Du Bois from direct surface measurements of just nine people, this 1916 formula became the original clinical standard. Over a century later, it's still the most widely cited BSA formula in published research.
Haycock
BSA = 0.024265 × W^0.5378 × H^0.3964
Haycock, Schwartz & Wisotsky, 1978
Published in 1978, this formula was validated specifically across infants, children, and adults, and is generally preferred in pediatric contexts where Du Bois and Mosteller may be less reliable.
Gehan & George
BSA = 0.0235 × W^0.51456 × H^0.42246
Gehan & George, 1970
Developed in 1970 from a larger and more diverse dataset than Du Bois's original study, this formula appears frequently in oncology chemotherapy-dosing literature as an alternative to Du Bois.
Boyd
BSA = 0.0333 × W^(0.6157 − 0.0188 × log₁₀W) × H^0.3
Boyd, 1935
Edith Boyd's 1935 formula is unusual in that the weight exponent itself changes with weight, an attempt to better account for how surface area scales at different body sizes. It remains an early, less commonly used alternative to Du Bois.
Fujimoto
BSA = 0.008883 × W^0.444 × H^0.663
Fujimoto et al., 1968
Derived from measurements of Japanese adults and published in 1968, the Fujimoto formula is sometimes preferred in East Asian clinical literature over Western-derived formulas like Du Bois.
Takahira
BSA = 0.007241 × W^0.425 × H^0.725
Fujimoto et al., 1968
Published alongside the Fujimoto formula from the same 1968 Japanese study, Takahira's equation uses the same exponents as Du Bois with a slightly different constant, making it a close regional variant of the original 1916 formula.
Schlich (male)
BSA = 0.000579479 × W^0.38 × H^1.24
Schlich, Schumm & Schlich, 2010
Published in 2010, this is the most recent formula on this page, derived from 3-D body-scan data rather than manual measurement, with separate equations for men and women.
Schlich (female)
BSA = 0.000975482 × W^0.46 × H^1.08
Schlich, Schumm & Schlich, 2010
The female counterpart to the Schlich male equation above, also derived from 3-D body-scan measurements taken in the same 2010 study.
Frequently asked questions
What is a normal BSA for an adult?
Average adult BSA is roughly 1.9 m² for men and 1.6 m² for women, though it varies with height and weight. There isn't a single "normal" value the way there is for BMI ranges — BSA is used as an input to other calculations (like drug dosing) rather than as a health score on its own.
Which BSA formula is most accurate?
No single formula is universally "most accurate" — each was derived from a different measured population. Du Bois (1916) remains the historical reference standard and is still widely cited in research. Mosteller is the most commonly used in modern clinical practice because it is simpler to compute and produces results very close to Du Bois for most adults. Haycock is generally preferred for infants and children because it was validated specifically on pediatric data.
How is BSA different from BMI?
BMI (Body Mass Index) uses weight and height to estimate body fat and screen for underweight/overweight categories. BSA estimates the total external surface area of the body and is used mainly to scale medical treatments — such as chemotherapy doses or cardiac output — to a person's size. They're calculated from the same two inputs but answer different clinical questions.
Can I calculate BSA by hand?
Yes. The Mosteller formula is simple enough to do with a basic calculator: multiply weight in kg by height in cm, divide by 3600, then take the square root. The other formulas involve fractional exponents (like W^0.425), which are impractical without a scientific calculator — that's what this tool is for.
Why do doctors use BSA for chemotherapy dosing?
BSA correlates more closely with metabolic rate, blood volume, and cardiac output than body weight alone does, which made it an appealing way to scale chemotherapy doses across patients of different sizes. It has been standard practice since the 1950s. That said, it's a debated method for some narrow-therapeutic-index drugs, since two patients with the same BSA can metabolize a drug very differently — dosing decisions are always made by a treating clinician, not by this or any online calculator.
Does BSA calculation differ for children versus adults?
The underlying formulas are the same power equations, but some (like Haycock) were specifically validated against pediatric measurements and are generally preferred for infants and young children. Average BSA also scales dramatically with age — a newborn's BSA (≈0.25 m²) is roughly a sixth of an adult's.
What units does this BSA calculator use?
You can enter values in metric (kilograms and centimeters) or imperial (pounds and feet/inches) — toggle between them above the form. All formulas internally use kilograms and centimeters, so imperial inputs are converted automatically before the calculation runs.
References
- 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(6):863–871. PMID 2520314
- Mosteller RD. “Simplified calculation of body-surface area.” N Engl J Med. 1987;317:1098. PMID 3657876
- 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 Institute of Child Welfare Monograph Series No. X. Oxford University Press, 1935.
- Fujimoto S, et al. “Studies on the physical surface area of Japanese. 18.” Nippon Eiseigaku Zasshi. 1968;5:443–450 (also the source of the Takahira formula).
- Schlich E, Schumm M, Schlich M. “3-D-Body-Scan als anthropometrisches Verfahren zur Bestimmung der Körperoberfläche.” Ernährungs Umschau. 2010;57:178–183.
Reviewed by the TecDit team · Last updated September 20, 2026
This calculator is provided for informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or dosing decisions.
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