How do you calculate body surface area?
Body surface area is estimated from weight in kg and height in cm. The calculator offers four published equations. The simplest, and the default, is Mosteller:
BSA (m²) = √(weight × height ÷ 3600)
The other three use powers of weight and height:
- Du Bois: 0.007184 × weight^0.425 × height^0.725
- Haycock: 0.024265 × weight^0.5378 × height^0.3964
- Gehan & George: 0.0235 × weight^0.51456 × height^0.42246
Du Bois (1916) was built on nine people, one of them a child. Gehan & George (1970) used several hundred measurements, and Haycock (1978) was validated in infants, children and adults. Mosteller (1987) is a simplification designed for a pocket calculator. Our guide on which BSA formula to use explains when each one fits.
How to use the BSA calculator
- Enter the Weight in kg and the Height in cm.
- Under Equation, choose the one your protocol specifies. Mosteller is selected by default.
- Read the BSA for that equation, to two decimal places, and compare it with the other three shown underneath.
Worked examples
Example 1 – Mosteller in an adult. 70 kg, 170 cm. 70 × 170 = 11,900; 11,900 ÷ 3600 = 3.306; √3.306 = 1.818 → 1.82 m². Du Bois gives 1.81 m², Haycock and Gehan & George 1.83 m².
Example 2 – a child. 20 kg, 110 cm. Mosteller: 20 × 110 ÷ 3600 = 0.611; √0.611 = 0.78 m². Du Bois and Haycock also give 0.78 m², Gehan & George 0.8 m².
Example 3 – when the equations disagree. 120 kg, 165 cm. Mosteller: 120 × 165 ÷ 3600 = 5.5; √5.5 = 2.35 m². Du Bois gives 2.23 m², Haycock 2.41 m² and Gehan & George 2.39 m². That is a spread of 0.18 m², roughly 8%. At high weights, the protocol’s choice of equation matters.
Quick reference
BSA in m² for a few weight and height combinations, rounded to two decimals as in the calculator.
| Weight, height | Mosteller | Du Bois | Haycock | Gehan & George |
|---|---|---|---|---|
| 10 kg, 75 cm | 0.46 | 0.44 | 0.46 | 0.48 |
| 20 kg, 110 cm | 0.78 | 0.78 | 0.78 | 0.8 |
| 50 kg, 165 cm | 1.51 | 1.53 | 1.51 | 1.52 |
| 60 kg, 160 cm | 1.63 | 1.62 | 1.64 | 1.65 |
| 70 kg, 170 cm | 1.82 | 1.81 | 1.83 | 1.83 |
| 90 kg, 180 cm | 2.12 | 2.1 | 2.14 | 2.14 |
| 120 kg, 165 cm | 2.35 | 2.23 | 2.41 | 2.39 |
Common mistakes
- Height in metres. Entering 1.70 instead of 170 gives a BSA about ten times too small with Mosteller. The calculator expects centimetres.
- Pounds or inches entered as kg or cm. Convert first: 1 kg = 2.2046 lb, 1 inch = 2.54 cm.
- Forgetting the square root. 70 × 170 ÷ 3600 = 3.31, which is not a plausible adult BSA. The square root brings it to 1.82 m².
- Using an old weight. BSA-based doses need a current, measured weight and height, not a figure copied from an earlier admission.
A quick sense check
An adult of average build should come out between about 1.5 and 2.2 m². A value above 3 m² or below 1 m² in an adult points to a unit error. For a dose in mg/m², multiply the dose per m² by the BSA and compare the total with what you expect. If the protocol doses on weight instead, the mg/kg dose calculator does that sum.
In obesity some protocols cap the BSA or use an adjusted body weight, so it’s worth checking the result against the prescription and your local protocol.
Frequently asked questions
What is the Mosteller formula?
BSA (m²) = √(weight in kg × height in cm ÷ 3600). It needs only a square root, so it is the one most often used at the bedside and the default in this calculator. For 70 kg and 170 cm it gives 1.82 m². Du Bois gives 1.81 m² for the same person.
Which BSA equation should I use?
It’s best to use the one named in your protocol. For most adults of average build the four equations agree within a few hundredths of a m². They drift apart at the extremes of weight and height, and that is exactly when the protocol’s choice matters. Our guide on which BSA formula to use compares them in detail.
Why does the calculator show four results?
So you can see at a glance whether the choice of equation matters for this patient. If the four values are close, the choice makes little difference. If they spread out, as at 120 kg and 165 cm where they range from 2.23 to 2.41 m², it’s worth checking which equation your protocol requires.
What is a normal body surface area?
Most adults fall roughly between 1.6 and 2.0 m², and a newborn is about 0.2 m². There is no single normal value, because BSA simply reflects body size. What matters is whether the figure is plausible for the person in front of you, which is why a quick sense check is worth doing.
Can I use pounds and inches?
Not directly: the calculator expects kilograms and centimetres. Divide pounds by 2.2046 to get kg, and multiply inches by 2.54 to get cm, before you enter them. For example, 154 lb is 69.9 kg and 67 inches is 170.2 cm. Entering pounds as kilograms would inflate the BSA considerably.
Why are drug doses given per m²?
Because some body functions track surface area better than weight. Cardiac output and kidney function, for example, scale more closely with BSA. For this reason several medicines, especially in oncology, are prescribed in mg/m². Multiply the dose per m² by the BSA to get the total dose; it’s worth checking that against the prescription.
Is BSA reliable in people with obesity?
Less so. All four equations were derived from fairly small groups of people of average build. At very high weights they are less certain and disagree the most. Protocols handle this in different ways: some cap the BSA, others use an adjusted weight. It’s advisable to follow the one that applies to the drug.
Sources
- Mosteller RD. Simplified calculation of body-surface area. N Engl J Med 1987
- Du Bois D, Du Bois EF. A formula to estimate the approximate surface area if height and weight be known (1916, reprinted 1989)
- Haycock GB, Schwartz GJ, Wisotsky DH. Geometric method for measuring body surface area. J Pediatr 1978
- Gehan EA, George SL. Estimation of human body surface area from height and weight. Cancer Chemother Rep 1970