How To Calculate Body Surface Area

Body surface area, written BSA, is the total external surface of the body measured in square metres. It cannot be measured directly with any accuracy, so it is estimated from height and weight.

The Mosteller formula is the most widely used. Multiply height in centimetres by weight in kilograms, divide by 3600 and take the square root. A 170 centimetre, 70 kilogram adult gives 1.8181 square metres.

Mosteller became the standard because it is simple and agrees well with the older formulas. It was published in 1987 and is now the default in most hospital protocols and chemotherapy regimens.

The Du Bois formula is the classic, dating from 1916, and uses fractional powers of height and weight. It gives 1.3868 for the same adult, noticeably lower than Mosteller at this size.

Haycock's formula, from 1987, uses different fractional exponents and gives 1.8257 for the same person. The three formulas agree within a few percent for most adults, which is why the choice rarely changes a clinical decision.

BSA is used because metabolism and heat loss scale with surface area rather than with weight. Two people of the same weight but different heights have different metabolic rates, and BSA captures that.

Chemotherapy doses are classically prescribed per square metre. The dose per square metre is multiplied by the BSA, so 100 milligrams per square metre on a 1.8181 square metre adult is 181.81 milligrams.

Burn treatment estimates the burned area as a percentage of total BSA. The rule of nines divides the body into regions worth 9 percent each, and the percentage is then applied to the BSA to guide fluid resuscitation.

Cardiac index is cardiac output divided by BSA. Normalising by surface area lets clinicians compare a small and a large patient on the same scale, which is why heart function is reported this way.

Kidney function is indexed to BSA as well. A glomerular filtration rate is often reported per 1.73 square metres, the average adult value, so results can be compared between people of different sizes.

Pediatric dosing relies on BSA more than weight for some drug classes, because children have a much higher surface area relative to their mass than adults do.

The formulas assume a normally proportioned body. In obesity, amputation or significant edema the estimates drift, and specialised adjustments are used instead.

The 1.73 square metre figure used to index kidney function comes from the average adult body surface area, which is why results read per 1.73 square metres.

For children the Boyd formula is sometimes preferred, and paediatric dosing charts exist precisely because the adult fits drift at small sizes.

Amputations change the estimate because the formulas assume a complete body, and adjusted versions reduce the height or weight accordingly.

Body surface area also predicts evaporative water loss, which is why burn patients are warmed and humidified according to their area.

In research, normalising by body surface area lets trials compare doses across volunteers of very different builds without a separate arm per body size.

Height and weight should be measured on the same day where possible, because a stale weight shifts the estimate by a few percent in either direction.

Mosteller is deliberately chosen for its simplicity, so a hand calculation on a ward round gives the same number as the hospital system.