Body surface area (BSA) calculator
Body surface area estimates the total external area of the body in square metres. It is widely used in medicine to scale drug doses (notably chemotherapy), cardiac index and other values to body size, because it often reflects physiology better than weight alone.
Why BSA instead of weight?
Weight and BSA both scale with body size, but they do so differently. A person who doubles their weight does not double their BSA — surface area grows more slowly. For drugs that distribute by body surface or are eliminated via organ systems whose capacity scales with surface area, BSA-based dosing keeps the exposure more consistent across body sizes than weight-based dosing would. Chemotherapy agents in particular are dosed in milligrams per m² because their therapeutic window is narrow and their toxicity correlates with surface exposure.
BSA also underpins the cardiac index — cardiac output (litres per minute) divided by BSA — which lets clinicians compare heart function between people of different sizes. A cardiac output of 5 L/min is adequate for a 50 kg person but borderline for a 100 kg person; cardiac index normalises this.
The three formulas
All three formulas take height in centimetres and weight in kilograms (imperial inputs are converted first) and return BSA in m²:
Du Bois: 0.007184 × height^0.725 × weight^0.425 Mosteller: √(height × weight ÷ 3600) Haycock: 0.024265 × height^0.3964 × weight^0.5378
The Du Bois & Du Bois equation (1916) is the historical reference; it was derived from direct measurements of nine subjects and has been the implicit standard in many dosing protocols for over a century. The Mosteller formula (1987) is mathematically simpler and easier to verify mentally, which is why it is the most common choice in clinical practice today. The Haycock formula (1978) was calibrated against a wider range of body sizes including children and is often preferred in paediatric settings.
The tool computes the selected formula live and lets you switch between them to compare.
Worked example
Height 178 cm, weight 75 kg:
| Formula | BSA |
|---|---|
| Du Bois | ≈ 1.93 m² |
| Mosteller | ≈ 1.93 m² |
| Haycock | ≈ 1.93 m² |
For illustration — Mosteller: √(178 × 75 ÷ 3600) = √3.708 ≈ 1.93 m². The three agree closely for a typical adult in the middle of the height-weight distribution. Differences between formulas become more apparent at the extremes — very short or tall individuals, or very low or high body weight.
Practical notes
- Always confirm which formula is specified in the dosing protocol you are following — substituting one formula for another without checking can change the calculated dose.
- For paediatric patients, Haycock is the most commonly recommended formula, but local institutional guidelines take precedence.
- BSA is a calculated estimate, not a measurement. It assumes an idealised body shape and the actual surface area of any individual differs.
- Imperial inputs (feet, inches, pounds) are internally converted to centimetres and kilograms before the formula is applied; the result is always returned in m².
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