The Mosteller formula, and what it needs
Body surface area estimates the total area of your skin in square metres from just height and weight. The Mosteller formula is the one most clinics use because you can do it on any calculator. Give it centimetres and kilograms; if you have feet or pounds, convert first (1 in = 2.54 cm, 1 lb = 0.4536 kg).
The four formulas give slightly different numbers
All four take height and weight and return square metres. They agree closely for average adults and diverge at the extremes, which is why the formula is chosen to fit the patient.
| Formula | Equation | Best for |
|---|---|---|
| Mosteller | √(H×W / 3600) | General use, adults and children |
| Du Bois | 0.007184 × W0.425 × H0.725 | Adults, clinical trials |
| Haycock | 0.024265 × H0.3964 × W0.5378 | Infants and children |
| Boyd | 0.0333 × W0.6163 × H0.7236 | Very large or very small patients |
H is height in cm, W is weight in kg. Du Bois tends to overestimate BSA in infants, which is why Haycock exists.
Typical BSA by age and size (Mosteller)
| Person | Height & weight | BSA |
|---|---|---|
| Newborn | 50 cm, 3.5 kg | 0.25 m² |
| 1 year | 75 cm, 10 kg | 0.47 m² |
| 5 years | 107 cm, 18 kg | 0.87 m² |
| 10 years | 135 cm, 30 kg | 1.22 m² |
| Adult woman | 165 cm, 62 kg | 1.69 m² |
| Adult man | 178 cm, 78 kg | 1.96 m² |
A drug given at, say, 400 mg/m² means a 1.8 m² patient receives 720 mg. BSA is re-checked during treatment because losing 5–10% of body weight changes the dose.
BSA is not BMI
- BSA measures skin area in square metres and is used for dosing and fluids.
- BMI is weight divided by height squared, and is used to screen for under- and over-weight.
- The same person can have a high BMI and a large BSA — they answer different questions.
Common questions
Which BSA formula should I use?
For adults, Mosteller is the usual choice because it is simple and accurate. Du Bois gives almost the same number and appears in older guidelines. Haycock is preferred for children. If a clinical protocol names a specific formula, use that one.
What is a normal body surface area for an adult?
Most adults fall between about 1.5 and 2.0 square metres. A typical man is around 1.9 and a typical woman around 1.6. BSA is descriptive, not a health score, so there is no ideal value to aim for.
Why do doctors dose drugs by BSA instead of body weight?
Two people of the same weight but different heights can have quite different surface areas, and many drugs distribute in a way that tracks surface area better than weight. Chemotherapy in particular is dosed in milligrams per square metre because the safe window between too little and too much is narrow.
