Body Surface Area Calculator
Calculate BSA using Du Bois, Mosteller, and Haycock formulas for drug dosing.
Why Dose by Surface Area
Body surface area correlates better with metabolic rate, cardiac output and glomerular filtration than weight does. Because most cytotoxic drugs are cleared by organs whose capacity scales with those variables, BSA scales dose more closely to elimination than weight alone.
| Formula | Expression | Note |
|---|---|---|
| Mosteller | √(height × weight / 3600) | Simplest; most widely used clinically |
| Du Bois | 0.007184 × H0.725 × W0.425 | Original 1916; derived from only nine subjects |
| Haycock | 0.024265 × H0.3964 × W0.5378 | Better in paediatrics |
The formulas typically agree within about 5%, but they diverge at extremes of body habitus — which is precisely where dosing decisions are most difficult. Mosteller has become the default largely because it can be computed reliably at the bedside.
| Patient | Approximate BSA (m²) |
|---|---|
| Neonate | 0.2–0.25 |
| Child, 5 years | 0.7–0.8 |
| Average adult woman | 1.6 |
| Average adult man | 1.9 |
| Capped for dosing | Often 2.0–2.2 |
The Limits of BSA Dosing
BSA dosing is convention rather than settled science. Studies have found it explains relatively little of the variability in drug clearance for many agents, and it has been criticised for decades — yet it persists because no simple alternative has proven better across the range of cytotoxics.
Capping is common practice in obese patients, typically at 2.0 to 2.2 m², on the reasoning that adipose tissue contributes to surface area without proportionally increasing clearance. The practice varies by protocol and remains contested, with some evidence suggesting capping causes underdosing.
Certain drugs bypass BSA entirely. Carboplatin uses the Calvert formula based on renal function, because it is almost entirely renally cleared — a direct measure of elimination beats a surrogate.
Worked Examples
Common Mistakes
They typically fall within 5%, but diverge at extremes of height and weight — exactly where dosing decisions matter most. Protocols usually specify which to use.
Carboplatin is dosed by the Calvert formula from renal function, because measured clearance beats a surface area surrogate for a drug eliminated by one route.
Haycock performs better in small children. Adult-derived equations systematically misestimate BSA at very low body sizes.
It explains less clearance variability than often assumed. It persists as convention because no simple alternative has proven consistently better.