BSA Calculator - Body Surface Area
Enter your height and weight to calculate your body surface area (BSA) in square metres. Choose from five widely used clinical formulas including Du Bois and Mosteller, switch between metric and imperial units, and see where your result sits against typical adult reference values. BSA is used in oncology, cardiology, and pharmacology to scale medication doses and physiological measurements to body size.
Formula
Worked example
A person weighing 70 kg at 175 cm: Du Bois gives 0.007184 x 70^0.425 x 175^0.725 = approximately 1.85 m². Mosteller gives sqrt(175 x 70 / 3600) = sqrt(3.403) = approximately 1.84 m². Both formulas agree closely for average-build adults.
What is body surface area?
Body surface area (BSA) is the total external surface of the human skin, measured in square metres. Unlike body weight, it scales in a way that closely tracks many physiological functions including renal filtration, basal metabolic rate, and cardiac output. The concept was first formalised by Du Bois and Du Bois in 1916, who used plaster casts to measure the actual skin surface of nine subjects and derived a formula to predict it from height and weight alone. Modern clinical practice continues to use BSA-based dosing because it correlates better than body weight with the volume of distribution and clearance of many drugs, particularly cytotoxic agents used in chemotherapy.
Which BSA formula should I use?
The Du Bois formula remains the most widely cited in clinical literature and is the default for most institutional protocols. The Mosteller formula (1987) is popular because of its simple square-root form and gives results within 2-3% of Du Bois for most average-build adults. The Haycock formula was derived from a broader weight range and performs better at the extremes, making it the preferred choice in many paediatric contexts. Gehan and George used a larger regression dataset and tend to produce slightly higher estimates. Fujimoto derived his formula specifically from a Japanese adult population, so it may be more appropriate for East Asian patients where body proportions differ. For most healthy adults, all five formulas agree within 0.05 m².
How is BSA used clinically?
The main clinical use of BSA is chemotherapy dosing. Most cytotoxic drugs have a narrow therapeutic window, and dosing in mg per m² of BSA rather than mg per kg of body weight reduces the variation in drug exposure between patients. The reference adult BSA of 1.73 m² (from 1938 pharmacology data) is still used as the denominator when normalising renal function to calculate the estimated GFR (eGFR). Cardiac index, the standard measure of heart function, is cardiac output divided by BSA, allowing comparisons between people of different body sizes. Some radiation therapy protocols and burn assessment methods also use BSA as a scaling factor.
BSA versus BMI
BSA and BMI measure different things. BMI is a weight-to-height ratio used to screen for underweight, healthy weight, overweight, and obesity at a population level. It says nothing about the physical size of the body surface. BSA is a two-dimensional measure of how much external skin a person has. A very muscular person and a very obese person of the same weight and height will have the same BMI, but their BSA will differ slightly because fat distributes differently from muscle. BSA is not used as a health indicator. It is a physiological scaling factor whose value depends almost entirely on height and weight.
Average BSA by age and sex
| Population | Average BSA (m²) |
|---|---|
| Newborn | 0.25 |
| 2-year-old child | 0.50 |
| 10-year-old child | 1.14 |
| Adult female (average) | 1.60 |
| Adult male (average) | 1.90 |
| Reference adult (pharmacology) | 1.73 |
Reference values used in pharmacology and clinical dosing guidelines.
Frequently asked questions
What is a normal BSA for an adult?
The average BSA for an adult woman is approximately 1.6 m² and for an adult man approximately 1.9 m². The classic pharmacological reference adult of 1.73 m² was derived from early 20th-century data on a small group of subjects and persists as a normalisation constant in renal and cardiac formulas. Typical healthy adults range from about 1.5 to 2.3 m², with values outside that range occurring at the extremes of height and weight.
Which BSA formula is the most accurate?
No single formula is universally most accurate because each was derived from a different reference population. Du Bois and Mosteller perform similarly for average-build adults and differ by under 3%. Haycock is preferred in children and very light or very heavy patients. If you are using BSA for clinical purposes, check which formula is specified in the relevant protocol or institutional guidelines, as the choice can affect drug doses by several percent.
How does BSA affect chemotherapy dosing?
Most chemotherapy agents are prescribed in milligrams per square metre (mg/m²) of BSA rather than milligrams per kilogram of body weight. This normalises drug exposure to body size, reducing the risk of toxicity in smaller patients and under-dosing in larger ones. Many institutions cap the BSA used for dosing at 2.0 m² regardless of the calculated value to limit toxicity, since the relationship between BSA and drug clearance breaks down at very high body surface areas. Always follow the prescribing clinician and institution protocol.
What is the reference adult BSA of 1.73 m² used for?
The value 1.73 m² comes from 1938 data published by McIntosh, who averaged the BSA of a group of young adults. It has since been adopted as a normalisation constant for kidney function tests. When a laboratory reports your eGFR (estimated glomerular filtration rate), it expresses creatinine clearance per 1.73 m² of BSA, making the result comparable between people of different body sizes. The same constant appears in equations for cardiac index and some drug pharmacokinetic models.
Can I use this calculator for children?
Yes, but choose the Haycock formula, which was specifically derived from and validated in a paediatric population. The Du Bois formula tends to overestimate BSA at low body weights, which can lead to over-dosing if used for paediatric drug calculations. For the smallest infants (under about 3 kg), even Haycock has wider uncertainty, and a specialist paediatric pharmacist should verify any dose calculation.
Why do different formulas give different results?
Each formula was fitted to a different dataset using statistical regression. The Du Bois dataset from 1916 had only nine subjects. Later researchers used larger and more diverse samples, which shifted the regression coefficients. At an average adult size (70 kg, 175 cm) most formulas converge, but at the extremes of height or weight the differences can reach 5-10%, which matters when doses are in the range of mg/m².