Body surface area (BSA) calculator

Body surface area (BSA) is the area of skin covering the body, in square meters. Nobody measures it with a tape: it is estimated from weight and height.

The most used is Mosteller, because it fits in one square root:

BSA = √(height_cm × weight_kg ÷ 3600)

Step by step (1.75 m, 70 kg)

1. Multiply height by weight: 175 × 70 = 12,250
2. Divide by 3,600: 12,250 ÷ 3,600 = 3.403
3. Take the square root: √3.403 ≈ 1.84 m²

A typical adult lands between 1.6 and 2.0 m².

Why medicine doses per m² instead of per kilogram

Because body surface area tracks metabolism, blood volume and kidney filtration better than weight alone. Two 70 kg people of very different heights process a drug differently — and weight does not capture that.

• Chemotherapy — many cytotoxic doses are prescribed in mg/m².
• Cardiac index — cardiac output divided by BSA (normally about 2.5 to 4.0 L/min/m²), so hearts in different-sized bodies can be compared.
• Kidney function and fluid replacement, especially in pediatrics and major burns.

Mosteller vs Du Bois: where they agree

Person	Mosteller	Du Bois	Gap
Newborn — 50 cm, 3.5 kg	0.22 m²	0.21 m²	~5%
Child — 75 cm, 10 kg	0.46 m²	0.44 m²	~4%
Child — 1.20 m, 25 kg	0.91 m²	0.91 m²	—
Adult — 1.60 m, 55 kg	1.56 m²	1.56 m²	—
Adult — 1.75 m, 70 kg	1.84 m²	1.85 m²	~1%
Adult — 1.80 m, 95 kg	2.18 m²	2.15 m²	~1%
Adult — 1.70 m, 120 kg	2.38 m²	2.28 m²	~4%

The point of the table: the further a body sits from the range these formulas were built on — very small infants, marked obesity —, the less the formulas agree with each other. They came from small samples (Du Bois's original 1916 study involved nine people) and were never meant for the extremes.

The 2 m² cap

Because of that uncertainty, many chemotherapy protocols cap the BSA used in dosing at around 2.0 m², or work with standardized dose bands instead of applying the formula with no ceiling. That is a clinical decision, made case by case.