BMI and BSA both start from height and weight. They are not two names for the same result. Using one when a protocol asks for the other changes the number a clinician or a dosing reference expects.
Body mass index (BMI)
BMI is weight divided by height squared. Public health charts use it to group adults into broad weight categories. It does not measure body fat, muscle, or fitness. Athletes, older adults, children, and pregnancy need a different reading of the same math. A BMI calculator is a screening figure, not a diagnosis.
Body surface area (BSA)
BSA estimates the skin surface of the body in square meters. Several formulas exist; Mosteller is widely used because it is simple: the square root of (height in cm times weight in kg, divided by 3600). Some drug references, chemotherapy protocols, and physiology indexes ask for BSA specifically. The protocol names the formula. Do not swap in BMI.
Which to open
- Weight category, population charts, or “is this BMI high?” — BMI.
- A reference that says m², body surface area, or names Mosteller or Du Bois — BSA.
- A dose, a fluid rate, or a chemotherapy order — follow the source that named the formula. Do not calculate a dose from a general article.