Body Surface Area Calculator

This estimates the total surface area of a human body. Direct measurement of BSA is difficult, so a long line of researchers has published formulas that approximate it from height and weight. The calculator runs eight of the best known at once and shows the answer in square metres, square feet and square inches.

Typical values

BSA scales with body size, so it is worth knowing roughly where a figure should land:

Person	m²	ft²
Newborn child	0.25	2.69
Two-year-old child	0.5	5.38
Ten-year-old child	1.14	12.27
Adult female	1.6	17.22
Adult male	1.9	20.45

Notice that surface area roughly doubles between birth and age two, but only grows by about half again between ten and adulthood. Area does not keep pace with mass as a body grows, which is the reason BSA matters clinically at all.

Why clinicians use BSA rather than weight

BSA is often preferred to body weight because it is a better indicator of metabolic mass, the body's demand for energy. Metabolic mass can be approximated by fat-free mass, meaning everything that is not fat: bone, tendon, internal organs, muscle, blood, nerve tissue and the rest. Body fat is not metabolically active, so excluding it gives a reasonable estimate of the tissue that actually does the work.

There is a deeper reason too. Metabolic rate does not scale in proportion to mass; it scales closer to surface area, which is why a small child burns far more energy per kilogram than an adult does. Two people of the same weight but different heights present different amounts of tissue to be perfused and different amounts of skin to lose heat through, and BSA captures that where a single weight cannot.

Beyond metabolism, BSA is used to calculate the cardiac index, which relates cardiac output to body size so that a heart's performance can be compared across people. It also underpins fluid and electrolyte calculations in paediatrics, and burn assessment works on the same principle, the "rule of nines" estimates the percentage of total body surface affected.

BSA and chemotherapy dosing

The best-known clinical use is dosing chemotherapy, where doses are commonly expressed in milligrams per square metre.

The practice has real critics. Drugs with a narrow therapeutic index (where the dose that treats and the dose that harms sit close together) may not be well served by a BSA estimate, and the risk is producing toxicity rather than benefit. There is also evidence that BSA formulas become less accurate at the extremes of height and weight, where BMI may be the better guide.

Those limits are worth stating plainly, but so is the reason the practice persists: chemotherapy effects dosed by BSA are still more consistent than those dosed by body weight alone. It is a better instrument than the obvious alternative, not a perfect one. Several agents have moved to flat dosing or to dosing by measured drug clearance where the evidence supports it, and carboplatin is dosed by kidney function rather than BSA at all.

The eight formulas

Each takes weight in kilograms and height in centimetres and returns BSA in square metres.

Du Bois (1916) is the most widely used, and it has been shown to estimate body fat effectively in both obese and non-obese patients, which BMI does not:

BSA = 0.007184 × W^0.425 × H^0.725

Its origin is striking: Du Bois and Du Bois derived it from just nine subjects, one of whom was a child with severe malnutrition. That it is still the reference standard more than a century later says something about how slowly this field moves.

Mosteller (1987) is the one clinicians can do in their heads, because it reduces to a square root:

BSA = 0.016667 × W^0.5 × H^0.5 = √(W × H ÷ 3600)

Its simplicity is why it dominates bedside use, and it agrees with Du Bois closely across the normal range.

Haycock (1978) was validated in infants, children and adults, which makes it a common choice in paediatrics:

BSA = 0.024265 × W^0.5378 × H^0.3964

Gehan & George (1970) was fitted to 401 direct measurements, a far larger sample than Du Bois had:

BSA = 0.0235 × W^0.51456 × H^0.42246

Boyd (1935) is the unusual one, because its weight exponent itself depends on weight:

BSA = 0.0333 × W^{(0.6157 − 0.0188 × log₁₀W)} × H^0.3

Fujimoto (1968) and Takahira (1968) both come from Japanese research and were fitted to Japanese populations, which is worth knowing when applying them elsewhere:

Fujimoto: BSA = 0.008883 × W^0.444 × H^0.663

Takahira: BSA = 0.007241 × W^0.425 × H^0.725

Takahira shares Du Bois's exponents exactly and differs only in the leading constant, which is why it tracks Du Bois about 0.8% higher at every size.

Schlich (2010) is the newest and the only one that differs by sex, having been built from 3D body scans rather than the wrapping-and-measuring methods of the older work:

Men: BSA = 0.000579479 × W^0.38 × H^1.24

Women: BSA = 0.000975482 × W^0.46 × H^1.08

Its exponents are the giveaway: height carries far more weight than in any of the older formulas, so Schlich diverges most for people who are unusually tall or short.

Why it is estimated rather than measured

Measuring a body's surface area directly is genuinely awkward, which is why a century of researchers kept publishing formulas instead of measurements. The early work used coating methods (covering the skin in strips of paper or adhesive tape, then peeling them off, flattening them and measuring the area) or wrapping the body in moulds and doing the same. It is slow, uncomfortable, and imprecise around the hands, feet and face where most of the fiddly area hides.

That is why sample sizes were so small: Du Bois managed nine subjects, and Gehan and George's 401 measurements represented an enormous effort by the standards of 1970. Modern 3D optical scanning finally made large samples practical, which is what Schlich's 2010 work used, and it is the main reason a formula from 2010 disagrees noticeably with one from 1916 despite both being fitted to real bodies.

How much do they disagree?

On the defaults the eight answers span 1.80 to 1.89 m², a spread of about 0.09 m², or roughly 5% of the mean. The first chart shows that spread directly.

Five per cent sounds small until you attach it to a drug. A cytotoxic dose of 500 mg/m² differs by about 45 mg between the lowest and highest formula for the same patient. This is why hospitals standardise on one formula and stick to it rather than letting each prescriber pick, and why a BSA figure should always carry the name of the formula that produced it.

The disagreement also grows at the extremes. All eight were fitted to populations near average size, so they agree well in the middle and diverge for the very small, the very tall, and the severely obese, precisely the patients where dosing accuracy matters most.

Reading your result

The panel leads with Du Bois because it remains the default in most clinical settings, and lists all eight so you can see the range rather than a single number presented with false confidence. The second chart puts your figure against the reference values above, which is the quickest sanity check there is: an adult result far outside 1.5 to 2.2 m² usually means a units mistake in the input.

One caution worth repeating. This is a reference tool, not a prescribing tool. Chemotherapy and other BSA-based dosing must come from the treating team using their institution's chosen formula and their own verified measurements. Our BMI Calculator and Body Fat Calculator cover the other common ways of describing body size.