Body Surface Area (BSA) Calculator
Why Body Surface Area Beats Body Weight
Weight alone is a blunt instrument for describing how much of a person there is. Two people at 70 kg can differ substantially in the amount of skin, and therefore in the amount of metabolic surface, they carry. Body surface area (BSA) captures that dimension in a single figure expressed in square metres, and it has become the reference denominator in contexts where scaling to body size matters more than scaling to mass alone — chemotherapy protocols, cardiac index, and burn assessment among them. This calculator applies the Mosteller formula, which survives because it is the rare clinical shortcut that is both defensible and computable in your head.
The Formula
This calculator uses the Mosteller equation exactly as published:
The elegance is in the constant. 3600 is simply 60 × 60, which means a person 60 cm tall weighing 60 kg would have a BSA of exactly 1.00 m². Everything else scales from that anchor as the square root of the height-weight product — which is why BSA rises much more gently than weight does.
Mosteller BSA Reference Grid
Every value below was computed with the same formula this calculator runs:
| Height | 50 kg | 60 kg | 70 kg | 80 kg | 90 kg | 100 kg |
|---|---|---|---|---|---|---|
| 150 cm | 1.44 | 1.58 | 1.71 | 1.83 | 1.94 | 2.04 |
| 160 cm | 1.49 | 1.63 | 1.76 | 1.89 | 2.00 | 2.11 |
| 170 cm | 1.54 | 1.68 | 1.82 | 1.94 | 2.06 | 2.17 |
| 180 cm | 1.58 | 1.73 | 1.87 | 2.00 | 2.12 | 2.24 |
| 190 cm | 1.62 | 1.78 | 1.92 | 2.05 | 2.18 | 2.30 |
Read across a row and note how doubling weight from 50 kg to 100 kg raises BSA by only about 42% — the square-root relationship at work.
The Square-Root Curve
BSA at 170 cm across a 50 kg range of body weight. The bars climb, but they flatten — surface area is far less sensitive to mass than mass is to itself.
Where BSA Is Actually Used
- Oncology dosing — many cytotoxic agents are prescribed in mg/m², making BSA the direct multiplier that turns a protocol dose into a patient dose.
- Cardiac index — cardiac output divided by BSA normalises pump performance across body sizes, so a 1.5 m² patient and a 2.2 m² patient can be compared on the same scale.
- Renal reporting — eGFR is conventionally reported per 1.73 m², a historical "average adult" BSA, which is why that odd-looking number appears in every lab report.
- Fluid and burn calculations — surface-area-based estimates underpin several established resuscitation and metabolic formulas.
Mosteller Versus the Alternatives
Mosteller is not the only BSA equation — Du Bois, Haycock, and Boyd all predate or compete with it. Mosteller earned its place in 1987 by producing results in close agreement with the older equations while reducing the arithmetic to a single square root. When a formula is going to be used at a bedside under time pressure, tractability is a feature, not a compromise.
How to Use This Calculator
- Enter Height (cm). If you have height in inches, multiply by 2.54 first.
- Enter Weight (kg). From pounds, divide by 2.20462.
- Select Calculate. The result returns BSA rounded to two decimal places, with the intermediate square-root step shown so the arithmetic can be checked by hand.
Related Calculations
BSA is the denominator behind eGFR, which reports kidney function per 1.73 m². For the mass-based alternative to surface-area dosing, see the Medication Dosage Calculator.