How to calculate body surface area

Body surface area estimates the total area of the body in square metres, and it takes one short formula. Here are the Mosteller and Du Bois methods, worked examples, typical values, and what BSA is used for, with the note that clinical decisions belong to professionals.

By the Calculator Den team · Updated 2026-10-28

Body surface area (BSA) estimates the total external area of a person's body in square metres. It is a staple of medicine because it tracks overall body size better than weight alone, which is why it is used for things like scaling drug doses and assessing burns. You cannot measure it directly, so it is estimated from height and weight with a short formula. This guide shows the two most common formulas, Mosteller and Du Bois, with worked examples, typical values and what BSA is used for. It is a general explainer, not medical advice: any clinical decision that depends on BSA belongs to a qualified professional.

What body surface area is

BSA is a single number, in square metres (m²), that represents how much total surface a body has. Two people can weigh the same but have different BSAs if their heights differ, which is the point: BSA combines height and weight into one measure of size. It is estimated rather than measured because actually measuring the skin area of a body is impractical, so over the last century researchers fitted formulas that predict it well from height and weight.

How to calculate BSA with the Mosteller formula

The Mosteller formula is the one most calculators and clinicians reach for, because it is simple enough to do on any calculator:

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

Worked example: a person 180 cm tall weighing 80 kg. Multiply height by weight: 180 × 80 = 14,400. Divide by 3600: 14,400 ÷ 3600 = 4. Take the square root: √4 = 2.0 m². That is the whole calculation. If your height is in metres or inches, or your weight in pounds, convert first (1 m = 100 cm, 1 inch = 2.54 cm, 1 lb ≈ 0.4536 kg).

The Du Bois formula

The older Du Bois and Du Bois formula (1916) is still widely cited and gives very similar answers:

BSA (m²) = 0.007184 × height^0.725 × weight^0.425 (height in cm, weight in kg)

For the same 180 cm, 80 kg person, Du Bois gives about 0.007184 × 43.2 × 6.44 = about 2.0 m², matching Mosteller to the first decimal place. Because the formulas agree closely for most adults (they can diverge a little at the extremes, such as small children or very high weights), Mosteller is usually preferred for being easier to compute, and different clinical settings may standardise on different formulas. Other formulas exist too (Haycock, Gehan and George, Boyd), and they rarely differ by more than a few percent.

Worked examples

A couple more, using Mosteller, to lock in the method.

• A smaller adult. 165 cm, 65 kg: √(165 × 65 ÷ 3600) = √(10,725 ÷ 3600) = √2.979 = about 1.73 m².
• A child. 120 cm, 25 kg: √(120 × 25 ÷ 3600) = √(3,000 ÷ 3600) = √0.833 = about 0.91 m².

What BSA is used for

BSA matters because many bodily functions scale with surface area rather than weight. Its main uses are clinical, and in each case the figure feeds a professional's judgement rather than replacing it:

• Drug dosing. Some medicines, chemotherapy in particular, are dosed per square metre of BSA (for example mg/m²) so the dose suits body size.
• Cardiac index. Heart output is often divided by BSA to compare people of different sizes on a like-for-like basis.
• Burns. The extent of a burn is described as a percentage of total body surface area (estimated with charts or the rule of nines, a related idea rather than the Mosteller formula itself), which guides fluid and treatment decisions made by trained clinicians.
• Kidney and metabolic measures. Several values, such as eGFR, are normalised to a standard BSA of 1.73 m² so results are comparable between people. That 1.73 is a convention, not a personal target.

Because these uses are clinical, treat a BSA figure as an input, not an instruction. Dosing and treatment are set by healthcare professionals using the full picture, not a formula alone. Never work out a medication dose from this page or a calculator; that is done with institutional protocols.

Typical BSA values

Most adults fall between about 1.5 and 2.2 m², with roughly 1.7 m² a common average (men average roughly 1.9 m² and women roughly 1.6 m², and children lower in line with their size). The widely used reference value of 1.73 m² comes from an average adult and is the standard to which many lab results are normalised. There is no single "healthy" BSA in the way there is a healthy BMI range; BSA is a measure of size, not of body composition, so a bigger number is not better or worse, just bigger.

Common mistakes

• Wrong units. The Mosteller formula needs height in centimetres and weight in kilograms. Put height in metres or weight in pounds and the answer is far off; convert first.
• Confusing BSA with BMI. BMI (weight ÷ height²) describes weight for height; BSA describes total surface area. They use the same inputs but answer different questions.
• Forgetting the square root. Mosteller ends in a square root. Skipping it leaves you with a number many times too large.
• Treating the number as a decision. A BSA value supports a clinical calculation; it does not make one. Leave dosing and treatment to a professional.

Keep height in centimetres, weight in kilograms, and remember the square root, and BSA is a one-line calculation. When you want it done instantly, the free body surface area calculator uses the Mosteller formula to return your BSA in square metres, and the BMI calculator covers the separate weight-for-height question. This article is a general explainer, not medical advice; any decision that relies on BSA should be made with a qualified healthcare professional.