Health
Body Surface Area Calculator
The four formulas agree closely at an average build. This page shows the spread, and where it grows — the sizes at which the choice of formula starts to matter.
The link carries your figures, so it reopens on exactly these numbers.
Where the four formulas agree, and where they do not
The spread stays near a percent across ordinary adult builds and climbs at the extremes — the small child and the short, heavy adult — which are exactly the sizes BSA is most often used to dose.
| Build | Body surface area (average of four) | Spread as a share of the average |
|---|---|---|
| Small child (15 kg, 95 cm)The four spread about 5% — and BSA doses paediatric chemotherapy. | 0.63 | 5 |
| Small adult (55 kg, 165 cm) | 1.59 | 0.9 |
| Average adult (80 kg, 180 cm)The default above. Under 1% apart. | 2 | 0.7 |
| Short, heavy adult (140 kg, 155 cm)Around 12% — the widest gap, at a build BSA handles worst. | 2.45 | 11.5 |
| Tall adult (95 kg, 195 cm) | 2.27 | 0.3 |
The formula
Worked example
An adult 180 cm tall weighing 80 kg
- Mosteller: √(80 × 180 ÷ 3600) = √4 = 2.00 m²
- Du Bois 2.00, Haycock 2.01, Gehan–George 2.01
- the four agree to 0.7% here — a 0.01 m² spread
- but for a short, heavy adult they pull 12% apart, and for a small child 5%
Where this goes wrong
Trusting a single BSA at the extremes of size
At 180 cm and 80 kg the four formulas fall within 0.01 m² of each other — 0.7% — and any one of them will do. That agreement is why the number feels solid. It is not solid everywhere. BSA is what many chemotherapy doses are scaled to, and those doses matter most for the smallest and largest patients — precisely where the formulas diverge. For a small child the four spread about 5%; for a short, heavy adult, around 12%. On a drug with a narrow margin between a working dose and a toxic one, choosing Du Bois over Mosteller can move the dose by more than that margin allows. BSA also estimates an average body shape and cannot see an unusual one. Where it will drive a real dose, it is a decision for the clinician, not the calculator.
Questions
- Which BSA formula should I use?
- Du Bois is the historical default and Mosteller the one most used at the bedside because it is easy to check by hand — √(weight × height ÷ 3600). At an ordinary adult build the choice barely matters; the four agree to about a percent. At the extremes of size it matters more, and Haycock, which was fitted with children in its sample, tends to hold up best at small sizes.
- Why is BSA used to dose chemotherapy instead of body weight?
- Because BSA tracks metabolic mass — the body’s energy turnover — more closely than weight does, and many drugs clear at a rate closer to BSA than to weight. It is not a perfect basis: for drugs with a narrow therapeutic index the estimate may not be precise enough, and BSA loses accuracy at the extremes of height and weight. But across most patients it gives more consistent effects than weight alone.
- What is a normal body surface area?
- An average adult is roughly 1.7 m²: a little under for women, a little over for men, and around 2 m² for a larger adult. A newborn is near 0.25 m² and a ten-year-old near 1.1 m². These are averages, not targets — BSA is a size measure, not a health one.
- Do these formulas work for children?
- The Haycock and Mosteller formulas were validated in infants and children as well as adults, so they extend to small sizes better than the others. All four agree less at small sizes than at adult ones, which is why the page shows the spread — and why a paediatric dose is a clinical decision, not a single calculator output.