ABC Tools

Body Surface Area, Explained

After reading this you will know what body surface area (BSA) measures, how five common formulas compute it from your height and weight, why they mostly agree, and how clinicians use the number to scale drug doses.

What body surface area is

Body surface area is the total area of the outside of your body, reported in square meters (m²). A typical adult sits near 1.7 m². A small child might be near 0.5 m², and a large adult can exceed 2.2 m².

You cannot measure BSA directly without wrapping a body in a grid, so it is estimated from two numbers that are easy to record: height and weight. That estimate turns out to be useful because many physiological quantities scale with surface area rather than with weight alone. Heat loss, some measures of metabolic rate, and the volume in which certain drugs distribute all track surface area more closely than they track mass.

Here is the hook. A drug dosed at 500 mg per m² gives a person with a BSA of 1.86 m² a dose of 930 mg, while a person at 1.50 m² receives 750 mg. Same drug, same protocol, doses that differ by 24 percent. That is why the number matters.

When to use it, and when not

BSA is the standard scaling factor for chemotherapy doses and for a handful of other drugs with narrow safety margins. It is also used to index cardiac output (liters per minute per m²) and glomerular filtration rate, so that a small person and a large person can be compared on the same footing.

It is a poor tool for judging body composition. BSA rises with both height and weight, so a tall lean person and a shorter heavier person can share the same BSA while differing greatly in fat and muscle. For body composition questions use the BMI Calculator, the Body Fat Estimator, or the Waist-to-Hip & Waist-to-Height Calculator instead.

These formulas were fitted to adult and general pediatric data. They lose accuracy at the extremes: very small infants, severe obesity, and amputations all break the height-and-weight assumption. In those cases a clinician uses adjusted methods, not a bedside formula.

The formulas and the intuition

Every formula here follows the same physical idea. Surface area grows roughly as the square of a linear dimension, while volume (and therefore weight) grows as the cube. Combine height and weight and you can back out an area. The formulas differ only in the exponents and constants fitted to their original data.

The simplest is Mosteller, which is exact enough for most bedside work:

\text{BSA} = \sqrt{\dfrac{H \times W}{3600}}

Here H is height in centimeters and W is weight in kilograms. The result is in m². The constant 3600 is 60^2, chosen so the units come out right.

Du Bois and Du Bois (1916) is the oldest and still widely cited:

\text{BSA} = 0.007184 \times H^{0.725} \times W^{0.425}

The exponents 0.725 and 0.425 are close to the 2/3 and 1/3 you would guess from pure geometry, but not identical, because real bodies are not smooth solids.

Haycock (1978) was fitted with more infants and children in the sample:

\text{BSA} = 0.024265 \times H^{0.3964} \times W^{0.5378}

Gehan and George (1970):

\text{BSA} = 0.0235 \times H^{0.42246} \times W^{0.51456}

Boyd's formula uses weight in grams and a weight exponent that itself depends on weight, so it is the most elaborate of the five. All five take the same two inputs, so any disagreement between them comes from their fitted constants, not from extra information.

A worked example with the demo data

Height 175 cm, weight 70 kg

These are the tool's default values. Work the Mosteller formula by hand:

  1. Multiply height by weight: 175 × 70 = 12250.
  2. Divide by 3600: 12250 / 3600 = 3.4028.
  3. Take the square root: \sqrt{3.4028} = 1.845 m².

Now Du Bois for the same person:

  1. 175^{0.725} = 41.83.
  2. 70^{0.425} = 6.147.
  3. Multiply: 0.007184 × 41.83 × 6.147 = 1.847 m².

The two methods differ by 0.002 m², about 0.1 percent. Running all five gives a tight cluster.

BSA for 175 cm and 70 kg by five formulas
FormulaBSA (m²)Difference from Mosteller
Mosteller1.8450
Du Bois1.847+0.002
Haycock1.852+0.007
Gehan-George1.851+0.006
Boyd1.856+0.011

The spread across all five is 0.011 m², about 0.6 percent. For a normal adult body the choice of formula rarely changes a dose meaningfully.

Why the formulas agree, and where they part

For ordinary adult sizes the five formulas stay within a few percent, as the table shows. They diverge as you move toward the edges of the data they were fitted to. At very low weight, Haycock and Gehan-George (both fitted with more children) tend to read higher than Du Bois, because Du Bois had only a handful of small subjects.

The chart below shows Mosteller BSA across a weight range for a fixed height of 175 cm. The curve is concave: each extra kilogram adds less area than the one before, because you are taking a square root.

BSA rises with weight but flattens: going from 70 to 80 kg adds 0.127 m², while 110 to 120 kg adds only 0.103 m².

At 175 cm and 70 kg the five formulas give 1.845, 1.847, 1.852, 1.851 and 1.856 m². Increasing weight to 120 kg raises the Mosteller value to 2.415 m²; dropping to 40 kg lowers it to 1.394 m². The formulas stay within roughly one percent of each other across normal adult sizes and spread apart below 15 kg.

Reading and interpreting your result

Read BSA as a scaling number, not a health score. A larger BSA is not better or worse; it just means more surface to lose heat across and, for BSA-dosed drugs, a larger dose.

To sanity-check any result, remember three anchors: an average adult is near 1.7 m², a large adult near 2.0 m², and a newborn near 0.25 m². If your number falls far outside the range those anchors suggest for the size you entered, recheck the units.

BSA
Body surface area, the estimated outer area of the body in m².
Indexed value
A physiological measure divided by BSA, for example cardiac index in L/min/m², so people of different sizes can be compared.
mg/m²
A dose expressed per unit of surface area; multiply by BSA to get the total dose.

Common mistakes

The errors here are almost always input errors, not formula errors.

  • Mixing units. These formulas take centimeters and kilograms. Enter height in inches and 70 becomes a fraction of the true value. A 175 cm person entered as 69 (inches) would return roughly 1.16 m², obviously too low.
  • Confusing weight and mass units. Boyd's formula internally uses grams, but the tool handles the conversion for you. Enter kilograms.
  • Over-trusting the fourth digit. The formulas were fitted to small samples in the early and mid twentieth century. Two methods that print 1.845 and 1.856 are not really that precise; the true value carries an uncertainty of a few percent.
  • Using BSA for body composition. It cannot separate fat from muscle. See the related tools below.

If two formulas disagree by more than a few percent for a normal adult, one of your inputs is likely wrong. Recompute the Mosteller value by hand as a quick check: it needs only a multiply, a divide and a square root.

Related tools on this site

BSA answers one narrow question. These tools cover the rest of body and metabolism work:

Frequently asked questions

Which BSA formula should I use?

Mosteller is the usual bedside choice because it is easy to compute and agrees with the others within a few percent for adults. Many oncology protocols specify Du Bois. For infants and small children, Haycock and Gehan-George were fitted with more pediatric data.

Why do different formulas give slightly different numbers?

Each was fitted to its own set of measured bodies with different constants and exponents. For the 175 cm, 70 kg default the five range from 1.845 to 1.856 m², a spread of about 0.6 percent. The gap widens for very small bodies.

What is a normal BSA?

Around 1.7 m² for an average adult, roughly 1.9 m² for a large adult, and near 0.25 m² for a newborn. There is no healthy or unhealthy BSA; it simply scales with body size.

Can I use BSA to check if my weight is healthy?

No. BSA rises with both height and weight and cannot tell you about body fat. Use BMI, a body fat estimate, or waist ratios for that question.

Does BSA change my drug dose in real life?

For BSA-dosed drugs, yes. A protocol of 500 mg/m² gives 923 mg at 1.845 m² and 850 mg at 1.70 m². Only a clinician sets these doses; this tool is for education, not treatment.

Try it yourself in the tool

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