What body surface area is and why it is used
Body surface area is the total area of skin, about 1.7–2.0 m² for an adult. It cannot be measured directly in practice, so it is estimated from height and weight with formulas fitted to careful measurements of small numbers of people. BSA matters because metabolic rate, cardiac output, kidney filtration and drug clearance all scale more closely with surface area than with weight — a 100 kg person does not have twice the metabolism of a 50 kg one — so doses and physiological indices are normalised per square metre.
The example adult, 5′10″ and 170 lb, by each formula:
| Formula | BSA | In ft² |
|---|---|---|
| Mosteller | 1.952 m² | 21.0 ft² |
| Du Bois | 1.948 m² | 21.0 ft² |
| Haycock | 1.958 m² | 21.1 ft² |
| Gehan & George | 1.962 m² | 21.1 ft² |
The formulas
Du Bois and Du Bois (1916) fitted their equation to nine subjects and it remained the standard for most of a century. Mosteller (1987) simplified it to a square root that can be done mentally, √(height × weight ÷ 3600), and is now the most used in oncology. Haycock (1978) was derived for infants and children as well as adults and is preferred in paediatrics. Gehan and George (1970) used 401 subjects and is common in some chemotherapy protocols. For adults of ordinary build they agree within about 2–3%; they diverge at the extremes of size.
- Mosteller: BSA = √(cm × kg ÷ 3600)
- Du Bois: BSA = 0.007184 × kg^0.425 × cm^0.725
- Haycock: BSA = 0.024265 × kg^0.5378 × cm^0.3964
- Gehan & George: BSA = 0.0235 × kg^0.51456 × cm^0.42246
BSA by height and weight
Weight moves BSA more than height does at adult sizes, but both matter — which is the point of using BSA rather than weight.
| Weight | 5′4″ | 5′8″ | 6′0″ |
|---|---|---|---|
| 110 lb | 1.50 | 1.55 | 1.59 |
| 130 lb | 1.63 | 1.68 | 1.73 |
| 150 lb | 1.75 | 1.81 | 1.86 |
| 170 lb | 1.87 | 1.92 | 1.98 |
| 190 lb | 1.97 | 2.03 | 2.09 |
| 220 lb | 2.12 | 2.19 | 2.25 |
| 250 lb | 2.26 | 2.33 | 2.40 |
Children
Children have far more surface area per kilogram than adults — a newborn about 0.06 m²/kg against 0.025 for an adult — which is why paediatric fluid and drug calculations often use BSA. Haycock's formula was validated down to infant sizes; Mosteller also performs well in children.
| Child | Mosteller | Haycock |
|---|---|---|
| Newborn, 3.5 kg, 50 cm | 0.22 | 0.22 |
| 1 year, 10 kg, 75 cm | 0.46 | 0.46 |
| 5 years, 18 kg, 110 cm | 0.74 | 0.74 |
| 10 years, 32 kg, 140 cm | 1.12 | 1.11 |
| Adult reference | 1.73 | — |
Where BSA is used
The 1.73 m² figure that appears in kidney function reports is the surface area of a 'standard' adult from the 1920s; GFR is reported normalised to it so results are comparable across body sizes. Chemotherapy dosing per m² dates from the 1950s, when it was found to scale toxicity better than weight between species; many protocols cap the BSA used at 2.0 or 2.2 m² for very large patients.
| Use | Unit | Note |
|---|---|---|
| Chemotherapy dosing | mg per m² | Most cytotoxic drugs; often capped at 2.0–2.2 m² |
| Cardiac index | L/min per m² | Cardiac output normalised to body size |
| Kidney function (GFR) | mL/min per 1.73 m² | Reported relative to the standard adult |
| Burn assessment | % of BSA | Rule of nines; fluid resuscitation |
| Paediatric fluids and drugs | mL or mg per m² | Scales better than weight in children |
Limits
All BSA formulas are approximations from small historical samples and assume ordinary body proportions. In people with obesity, amputations, or very short or tall stature the estimates can be off by 5–10%, and different formulas disagree most there. For clinical use, follow the formula and rounding rules specified by the protocol or institution; this page reports several so that a figure from any of them can be checked.