BMI and BSA look alike because both start from height and weight, but they answer different questions. Body mass index (BMI) is a size ratio — weight divided by height squared — that sorts adults into screening categories. Body surface area (BSA) is a surface estimate — the square root of height times weight divided by 3600 — reported in square meters. The BSA page makes the separation explicit: BSA estimates surface area in square meters, while BMI compares weight with height squared and is more commonly used as a screening measure.
What each calculator does
The BMI calculator takes age (adults 20 and older only), a metric or imperial unit system, height, and weight. It converts height to meters — centimeters divided by 100, or inches times 0.0254 — and weight to kilograms — pounds times 0.453592 — then computes BMI = weight kg ÷ height m². The result is rounded to one decimal place and paired with CDC-style screening categories: underweight below 18.5, normal weight below 25, overweight below 30, and obese at 30 or higher. The page also computes the healthy weight range for that height, 18.5 × h² to 24.9 × h², displayed in kilograms or converted back to pounds. Age does not appear in the formula; it only gates the result to adults.
The BSA calculator takes a metric or imperial unit system, height, and weight. In imperial mode it converts inches to centimeters (× 2.54) and pounds to kilograms (× 0.453592), then applies the Mosteller formula: BSA = sqrt(height cm × weight kg ÷ 3600). The result is rounded to two decimals in square meters, and the output also lists the formula name, the height used, and the weight used so you can see whether a conversion happened. The page notes that for many adults BSA falls near 1.6 to 2.2 m², but that observation is only descriptive: the calculator does not label a BSA as healthy or unhealthy.
Side-by-side
| BMI calculator | BSA calculator | |
|---|---|---|
| Question answered | What size category is this height-weight combination? | How much external surface area does this body have? |
| Core formula | weight kg ÷ height m² | sqrt(height cm × weight kg ÷ 3600) |
| Inputs | Age (20+ gate), height, weight | Height, weight |
| Main result | BMI to one decimal, category, healthy weight range | BSA to two decimals in m², with formula name and converted values |
| Interpretation | CDC screening categories, not a diagnosis | A body-size scale value, not a health score |
| Unit handling | Height and weight converted to SI before the formula | Inches and pounds converted to centimeters and kilograms before the formula |
When to use which
Use the BMI calculator when the question is body-size classification: whether a height and weight combination falls into the common adult screening bands, or what weight range at a given height corresponds to a healthy BMI. It is a screening measure, and the page is explicit that the categories are not diagnoses and can mislead for muscular people because BMI uses total weight rather than fat mass.
Use the BSA calculator when the question is surface area: a body-size number in square meters used for understanding how a common clinical body-size calculation is made. The page warns that BSA should not be used to choose a medication dose on your own — protocols may specify an equation, dose cap, organ-function adjustment, or rounding rule — and that the number does not tell you whether weight, body composition, or health is appropriate.
Limits and disclaimer
Both pages are educational only and are not medical advice. The BMI calculator uses fixed adult categories for age 20 and older, is not for children, pregnancy, or diagnosis, and its hard category cut points mean a small measurement difference near 18.5, 25, or 30 can change the displayed label. The BSA result is an equation-based estimate: different published equations (Mosteller, Du Bois, Haycock, Boyd) can differ slightly, and standard height-and-weight formulas are less representative in situations such as pregnancy, edema, amputation, severe obesity, or infant body proportions. Neither page diagnoses or prescribes; seek individualized guidance from a qualified healthcare professional.