The human body is the hardest surface to measure in medicine. You cannot wrap a tape measure around it cleanly, and yet knowing its surface area — the body surface area, or BSA — is essential for dosing many medications, interpreting heart and kidney function, and comparing patients of very different sizes. This calculator estimates BSA from just two measurements, height and weight, using the six most popular published formulas, so you can see the standard clinical estimate in seconds.
What Is Body Surface Area?
Body surface area is the total area of the outer surface of the body, expressed in square meters. An average adult sits between about 1.4 and 2.2 square meters; an adult woman typically measures around 1.6 and an adult man around 1.9. Newborns start near a quarter of a square meter, grow to about half a square meter at age two and reach roughly 1.14 square meters by age ten. Because it cannot be measured directly during a routine visit, every BSA you see is an estimate built from height and weight.
Why surface area and not just body weight? Because the body's metabolic demands scale more closely with its surface than with its mass. A large body loses heat, burns fuel and circulates blood at a rate that tracks its surface area, which makes BSA a better proxy for the body's active tissue than a raw weight number.
Why BSA Matters More Than Body Weight
BSA is a better indicator of metabolic mass than body weight because it reflects fat-free mass — bone, muscle, organs, blood and nerves — more closely than the scale does. Body fat is largely metabolically idle, so a heavy, high-fat body carries the same surface area as a leaner one of the same dimensions even though their weights differ. Dosing scaled to BSA therefore lands closer to the metabolically active tissue that actually processes the drug.
In the clinic this matters for cardiac index, which relates heart output to body size, and for reference values used across kidney and respiratory measurements. Wherever a physiological quantity needs to be normalized to the patient, BSA is the normalizer of choice.
Body Surface Area and Chemotherapy Dosing
The most widely known use of BSA is chemotherapy dosing. Cancer drugs typically have a narrow therapeutic index: the margin between a dose that helps and a dose that harms is small, and an overdose can be toxic while an underdose may fail to treat the disease. Dosing scaled to body surface area made it possible to give reliably effective doses across patients whose weights varied enormously, and it remains the clinical standard for most cytotoxic agents.
The method is not perfect. Critics point out that BSA captures size but not how an individual liver or kidney clears a drug, and that the estimates lose accuracy at the extremes of height and weight. Some research suggests BMI is a better guide for very large patients. Still, when the choice is between dosing by weight alone and dosing by BSA, the evidence consistently favors BSA, which is why oncology continues to calculate it before every cycle.
How to Use the Calculator
Start with the gender control, which only affects the Schlich formula. Then enter your weight, choosing kilograms or pounds, and your height, choosing centimetres or inches. Both unit choices are converted to the metric values the formulas need before any calculation runs. The results panel lists six estimates in square meters, each labelled with its formula, so the Du Bois and Mosteller values sit at the top and the rest follow.
For a 70 kilogram, 170 centimetre adult the Du Bois formula returns about 1.81 square meters and Mosteller about 1.82. The gauge beside the results places that figure against the typical adult range, so a normal reading sits comfortably inside the green band. Drag the sliders and watch all six estimates move together — the formulas agree on most body shapes and only diverge at the edges.
The Six Formulas and How They Differ
Each formula is a different curve fitted to a different group of measured bodies.
- Du Bois (1916) — the classic: 0.007184 × W^0.425 × H^0.725, derived from a small number of early direct measurements and still the most cited.
- Mosteller (1987) — the square root of height times weight over 3600; simple enough to compute mentally and the workhorse of routine practice.
- Haycock (1978) — tuned for children as well as adults, with exponents of 0.5378 and 0.3964.
- Gehan and George (1970) — fitted to a large cancer patient dataset, exponents 0.51456 and 0.42246.
- Fujimoto (1968) — developed on a Japanese sample with exponents 0.444 and 0.663.
- Schlich (2010) — derived from three-dimensional body scans, with separate coefficients for men and women.
The differences between them are typically a few hundredths of a square meter for an average adult. Clinical teams therefore standardize on one formula and use it consistently, because the value used for dosing is only meaningful if it is always computed the same way.
How to Calculate Body Surface Area by Hand
The Mosteller formula is the one worth memorizing. Multiply height in centimetres by weight in kilograms, divide by 3600, and take the square root. For a 70 kilogram adult who is 170 centimetres tall, the product is 11,900; dividing by 3600 gives 3.306, and the square root is 1.82 square meters. The Du Bois formula needs fractional powers, so it is effectively calculator work, which is exactly what this tool automates across all six methods at once.
BSA and Medication Safety
Because BSA drives dosing for drugs with narrow safety margins, small errors in height or weight translate directly into dose errors. The calculator converts pounds and inches for you, but the number you enter still has to be right — a two-centimetre mistake in height moves the estimate by a measurable fraction of a square meter. For children, the adult-derived formulas are approximations and pediatric-specific methods are preferred. Whenever a real prescription is involved, the dosing calculation must be confirmed by the prescribing clinician rather than taken from an estimate.
Comparing BSA with BMI
Body mass index and body surface area both start from height and weight but answer different questions. BMI classifies body size into weight categories and screens whole populations; BSA estimates a physical area used to scale doses and physiological references. A tall, muscular athlete and a short, heavier person can share a BMI while owning very different surface areas. That is why screening tools use BMI and dosing tools use BSA — and why this calculator pairs naturally with the body mass index calculator when you want both views of the same measurements.
Typical BSA Values Across Age
| Reference | Approximate BSA |
|---|---|
| Newborn child | 0.25 m² |
| Two-year-old child | 0.50 m² |
| Ten-year-old child | 1.14 m² |
| Adult female (average) | 1.60 m² |
| Adult male (average) | 1.90 m² |
A child's surface area grows faster than weight in the early years, then settles into the adult range through the growth spurt. Anyone whose personal estimate lands far from the average for their age should treat that as a prompt to check the height and weight entered rather than a medical finding — the formulas can only be as good as the measurements fed into them.
How the Calculator Compares with a Medical Measurement
True BSA was originally measured by covering bodies in film or paper and summing the pieces, and later by geometric approximations and modern three-dimensional scanning. All six formulas in this tool are shortcuts fitted to those measurements, so the results are estimates, not gold-standard readings. For an average adult the shortcuts are excellent, staying within a few hundredths of a square meter of each other; the spread widens only for unusual builds, which is precisely when a clinician should pick the best-validated formula rather than whatever number is handiest.
Common Mistakes
- Entering weight in pounds without converting, which dramatically inflates the estimate — the formulas only accept kilograms.
- Mixing inches and centimetres between the two fields, producing a nonsense result.
- Assuming the gender setting changes every result, when it only affects the Schlich formula.
- Expecting all six formulas to agree exactly; they are different curves and diverge away from average builds.
- Rounding height or weight aggressively, which moves the dose-sized estimate more than it seems.
- Using an adult formula for a child and treating it as precise, rather than as an approximation.
Key Assumptions
- Weight is in kilograms and height in centimetres; other units are converted before the formulas run.
- The formulas are statistical estimates, most accurate for typical adult body shapes.
- Only the Schlich formula uses sex-specific coefficients.
- The six results differ slightly and no single value is universally "correct" — consistency matters more than the choice.
From the oncology ward to the physiology lab, body surface area is the scale on which the human body is compared and dosed. Whether you are checking your own figure against the typical range or preparing numbers for a clinical discussion, these six established formulas give you the estimate in the units medicine actually uses — square meters of living surface.
Disclaimer
Results are provided as estimates for informational purposes only and may be inaccurate. Always verify outcomes with a qualified professional before making financial or personal decisions based on these calculations.