Body Surface Area Calculator
Body surface area is the measure clinicians use where weight alone is too crude — chemotherapy dosing, cardiac index, burn assessment and paediatric fluids. This calculator applies the Mosteller formula, which is the most widely used because it is simple enough to check by hand.
Inputs
Result
1.87 m²
Mosteller formula
How it works
BSA = √(height × weight / 3600)
How the body surface area calculator works
Mosteller: BSA in m² = square root of (height in cm × weight in kg ÷ 3600).
Du Bois and Haycock formulas exist and produce slightly different values; institutions standardise on one because dosing must be reproducible.
Typical adult BSA is 1.6–2.0 m². Many protocols cap dosing BSA at 2.0 or 2.2 m² to avoid overdosing very large patients.
Worked example: 172 cm, 70 kg
- 172 × 70 = 12,040.
- 12,040 ÷ 3,600 = 3.344.
- BSA = √3.344 = 1.83 m².
- A drug dosed at 350 mg/m² would give 350 × 1.83 = 640 mg.
- The same patient at 90 kg would have a BSA of 2.07 m², a 13% higher dose.
Common mistakes to avoid
Mixing units
Mosteller requires centimetres and kilograms. Feeding inches or pounds into the same divisor produces a wildly wrong area.
Switching formulas mid-treatment
Du Bois and Mosteller can differ by a few percent. Dosing must use whatever the protocol specifies, consistently.
Treating it as a fitness metric
BSA is a clinical scaling factor, not a health indicator. It says nothing about body composition.
Frequently asked questions
What is a normal body surface area?
About 1.7 m² for an average adult woman and 1.9 m² for an average adult man. Children scale down with size.
Why is BSA used for drug dosing?
It correlates better than weight with metabolic rate, cardiac output and renal clearance, which govern how a drug is processed.
Which BSA formula is most accurate?
Mosteller is the most used for its simplicity; Du Bois is older and slightly underestimates in obesity. Accuracy differences are small relative to dosing tolerances.
Is BSA capped for large patients?
Many protocols cap it at 2.0–2.2 m², though practice varies and some guidelines now recommend dosing on actual BSA.
How does BSA relate to cardiac index?
Cardiac index is cardiac output divided by BSA, which allows comparison between patients of very different sizes.
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