How Body Surface Area Is Calculated

Body surface area (BSA) is the total area of your skin, measured in square meters. It sounds like a fact for a quiz — an average adult's is 1.6–1.9 m² — but it's one of the most used numbers in medicine, because many drug doses, chemotherapy protocols, and cardiac measurements are scaled to it rather than to body weight.

Why surface area instead of weight?

Body weight is the obvious way to scale a dose, but physiology doesn't scale with volume alone. Skin area grows roughly with the square of body dimensions, while weight grows with the cube. A tall, thin person and a short, heavy person can weigh the same yet have noticeably different surface areas — and for drugs cleared by the liver and kidneys, or processes like heat exchange through the skin, that difference matters. Dosing by BSA reduces the spread between under- and over-dosing compared with weight alone, which is why chemotherapy protocols are written in mg/m².

BSA also shows up outside pharmacy: cardiac output is divided by BSA to get the cardiac index, kidney function is reported per 1.73 m² of BSA, and burn teams estimate damaged skin area against it. One number, many jobs.

The Mosteller formula

The most widely used modern formula is Mosteller's (1987), popular precisely because it fits on one line:

BSA (m²) = √( height (cm) × weight (kg) / 3600 )

Worked example: a person 175 cm tall weighing 70 kg gets √(175 × 70 / 3600) = √(12,250 / 3600) = √3.403 ≈ 1.84 m². In imperial units the same idea is written with a pre-combined constant:

BSA (m²) = √( height (in) × weight (lb) / 3131 )

Someone 5'10" (70 in) and 180 lb lands at √(70 × 180 / 3131) ≈ 2.01 m² — the same answer you'd get converting to metric first, which is exactly what the calculator does under the hood.

Four formulas, slightly different answers

Mosteller isn't the only game. The original Du Bois formula dates to 1916 and is still the reference in pharmacology; several others were fitted to different populations decades later:

Du Bois:        0.007184 × W^0.425 × H^0.725
Mosteller:      √( H × W / 3600 )
Haycock:        0.024265 × W^0.5378 × H^0.3964
Gehan & George: 0.0235   × H^0.42246 × W^0.51456
                (W = weight in kg, H = height in cm)

For the 175 cm / 70 kg example the calculator returns Du Bois 1.85, Mosteller 1.84, Haycock 1.85, and Gehan & George 1.85 m² — all within about 1% of each other. The formulas disagree most for very small or very large bodies (infants, bariatric patients), where each equation's original study population matters. The tool shows all four plus their mean, with Du Bois as the headline figure because so many published dose tables were built on it.

Typical values and how BSA scales

Most adults fall between 1.6 and 1.9 m²; taller or heavier people run higher, and children much lower. A child of 100 cm and 20 kg works out to roughly 0.72–0.77 m² depending on the formula. That's the clearest illustration of the scaling point: the child is less than a third of the adult's weight, but only about 40% of the skin area — area lags behind mass as bodies grow.

BSA vs BMI and body fat

BSA is easy to confuse with BMI, but they answer different questions. BMI (weight relative to height squared) screens for whether weight is in a typical range — see How the BMI Formula Works. BSA estimates the skin's actual area for dosing and physiology. Both take only height and weight, so neither can tell muscle from fat — for that you'd need a body-fat method such as the US Navy tape approach in How Body Fat Is Measured. Two people with identical BSA values can have completely different body compositions.

Try it yourself

The Body Surface Area Calculator runs all four formulas at once, in metric or imperial, and shows the mean alongside each individual estimate. To compare with other health numbers from the same two inputs, the BMI Calculator gives your screening category, and the BMR Calculator estimates daily resting energy burn — a nice demonstration of how three very different metrics all fall out of a height and a weight.