What body surface area is for
Body surface area (BSA) is the total area of the skin, in square metres. Doctors use it more than you might expect, because it tracks how much blood the body pumps and how fast it handles many medicines better than weight alone does. Some drug doses, kidney tests and heart measurements are scaled to it.
Measuring skin directly is impractical, so BSA is estimated from height and weight. This body surface area calculator uses the Mosteller formula, which is simple and widely used, and shows the older Du Bois formula next to it.
How to use it
- Choose metric or US units.
- Enter height and weight, measured as accurately as you can.
- Pick the formula to show first; the other one appears underneath.
- Read the surface area in square metres, with the difference between the two formulas.
The two formulas
Mosteller: BSA (m²) = √(height cm × weight kg ÷ 3600)
Du Bois: BSA (m²) = 0.007184 × weight kg^0.425 × height cm^0.725Du Bois and Du Bois published their formula in 1916 from careful measurements of just 9 people. Mosteller's 1987 version was designed so it could be worked out on a pocket calculator, and gives very similar results for most adults. Both are described in the NIH StatPearls chapter on BSA.
Worked example: 175 cm and 70 kg
- Mosteller: 175 × 70 = 12,250; ÷ 3600 = 3.4028; √3.4028 = 1.8447 m²
- Du Bois: 0.007184 × 70^0.425 × 175^0.725 = 1.8481 m²
- Difference: 0.0035 m²
This person has a body surface area of 1.8447 m² by Mosteller and 1.8481 m² by Du Bois. The two agree to within a fraction of a percent here, which is typical for adults of average build.
BSA for some heights and weights
| Height | Weight | Mosteller | Du Bois |
|---|---|---|---|
| 160 cm | 55 kg | 1.56 | 1.56 |
| 170 cm | 70 kg | 1.82 | 1.81 |
| 180 cm | 85 kg | 2.06 | 2.05 |
| 190 cm | 100 kg | 2.3 | 2.28 |
The formulas drift apart slightly at larger sizes. Research suggests that in people with obesity both formulas can be less accurate, so clinics may choose a formula on purpose or cap the value for some treatments.
Why surface area, not weight?
Weight grows with the cube of body size, while surface area grows roughly with the square. Many things the body does, such as losing heat through the skin and pumping blood, follow surface area more closely than weight. That is why a dose or a test scaled to BSA can suit both a small adult and a large one better than a dose scaled to weight alone. Even so, the formulas are estimates, and clinicians sometimes adjust them for very large or very small patients.
Good to know
- BSA rises with both height and weight, but more slowly than weight does.
- Children use the same idea, but this page is set up for adults.
- For energy needs rather than size, see the BMR calculator; for body weight in context, the BMI calculator.
Frequently asked questions
What is a normal body surface area?
There is no single normal value, because BSA depends on height and weight. The table on this page gives some examples: about 1.56 square metres for 160 cm and 55 kg, rising to about 2.3 for 190 cm and 100 kg. Taller and heavier people have a higher BSA, and smaller people a lower one.
Which BSA formula is best?
Mosteller is the most widely used today because it is simple and gives results close to Du Bois for most adults. Different clinics and studies use different formulas, so when a value is used for treatment, the clinician chooses the formula. This calculator shows both so you can compare them.
Why is BSA used for medicine doses?
Body size affects how a body handles some medicines, and surface area follows blood flow and the work of the kidneys more closely than weight alone. For a small number of treatments, especially some cancer drugs, doses are set per square metre. Only a clinician should work out a dose.
How do I convert BSA to square feet?
Divide the square metres by 0.09290304, the exact size of one square foot in square metres, or multiply by about 10.764. A BSA of 1.85 square metres is about 19.9 square feet. The calculator shows this conversion with every answer.
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