Calculadora BSA

The body surface area (BSA) calculator estimates the total surface area of the body from height and weight. BSA is used in medicine more often than body weight alone for dosing certain medications, particularly chemotherapy, and for indexing measurements such as cardiac output to a person's size.

A fórmula

Du Bois: BSA (m2) = 0.007184 x weight(kg)^0.425 x height(cm)^0.725. Mosteller: BSA (m2) = square root of (height(cm) x weight(kg) / 3600)

• weight = body weight in kilograms
• height = standing height in centimetres
• Both formulas return BSA in square metres (m2)

Du Bois formula, 1916; Mosteller formula, New England Journal of Medicine, 1987

How the calculation works

The Du Bois formula was developed in 1916 from direct surface measurements of a small number of individuals and has remained in clinical use for over a century, despite its age and small original sample. It raises weight and height to specific fractional powers and multiplies by a constant.

The Mosteller formula, published in 1987, is a simpler approximation: it multiplies height in centimetres by weight in kilograms, divides by 3600, and takes the square root. It is easier to compute by hand and tends to produce results close to the Du Bois formula across most adult body sizes, which is why many hospitals now default to it.

Both formulas treat the body as a smooth geometric shape and derive surface area from just two measurements, height and weight, rather than measuring the body directly. They are estimates, not physical measurements.

How to read your result

Typical BSA values cluster in a fairly narrow range for adults, with more variation at the extremes of age and body size. The table below gives commonly cited reference figures.

Why medicine uses BSA instead of weight alone

Many chemotherapy drugs are dosed per square metre of BSA rather than per kilogram of body weight, on the reasoning that BSA correlates better with blood volume, cardiac output, and metabolic rate than weight alone does, which matters for drugs with a narrow safety margin. A dose expressed as milligrams per square metre is converted to a total dose using the calculated BSA.

BSA is also used to index cardiac measurements. Cardiac output divided by BSA gives the cardiac index, which allows clinicians to compare heart performance across people of different sizes on a common scale, since a larger body generally requires a higher absolute cardiac output to meet its needs.

Outside oncology and cardiology, BSA appears in burn assessment, kidney function calculations, and some pediatric drug dosing protocols.

Because both formulas were derived from adult body proportions, hospital pharmacies and oncology units generally standardize on one formula, most often Mosteller, and use it consistently for a given patient across an entire treatment course, rather than mixing formulas between visits.

Why the two formulas disagree, and how obesity is handled

For a person of average adult size, say 170 cm and 70 kg, Du Bois and Mosteller return nearly the same figure, both close to 1.8 square metres. The gap opens up at the extremes: at the same height but 140 kg, Du Bois gives roughly 2.43 square metres while Mosteller gives roughly 2.57 square metres, a difference of about 0.14 square metres, or nearly 6 percent. For a drug dosed at 100 milligrams per square metre, that gap alone shifts the calculated total dose by around 14 milligrams, which is why a single formula is fixed for a patient's entire course rather than alternated.

The reason the two diverge is that they weight height and weight differently. Du Bois raises weight to the power 0.425 and height to the power 0.725, giving relatively more influence to height, while Mosteller treats height and weight as interchangeable inside a single square root, giving weight comparatively more influence.

Because fat mass does not carry the same metabolic activity or blood supply as lean mass, using an uncapped BSA calculated from actual body weight can overstate the appropriate dose in a person with obesity. For this reason, some institutional chemotherapy protocols cap the BSA value used in dose calculations, commonly somewhere around 2.0 to 2.2 square metres regardless of a higher calculated figure, or substitute an adjusted or ideal body weight into the formula instead of actual weight. The specific convention varies by institution and by drug, which is another reason BSA-based dosing for obese patients is set by an oncology pharmacist or prescriber rather than by a general calculator.

• Du Bois and Mosteller agree closely for adults near an average build and diverge more as weight moves away from that range in either direction.
• A capped BSA is a dosing safeguard, not a different measurement of body size; the person's actual BSA is still whatever the formula calculates.
• Height is measured standing without shoes when possible, and weight should be current, not from a previous visit, since both feed directly into the result.

Limitações

• Du Bois and Mosteller can disagree by a small but clinically relevant margin, particularly at the extremes of height and weight; the two formulas are not interchangeable within a single treatment course.
• BSA-based estimates are not well validated for people with obesity, since fat mass does not scale with surface area the same way lean mass does, which can overstate the appropriate drug dose.
• They are not validated for amputees, since standard formulas assume a complete body and do not adjust for missing limbs.
• Pediatric and neonatal BSA estimation using adult-derived formulas carries more uncertainty, and many pediatric protocols use dedicated nomograms or weight-based dosing instead.
• BSA is an estimate derived from two measurements, not a direct measurement of the body; it does not account for body composition, fluid retention, or edema, which can materially change true surface area.
• Chemotherapy and other BSA-dosed medications should only be dosed by a clinician or pharmacist using validated protocols; this calculator is for general reference, not for calculating an actual treatment dose.

Perguntas frequentes

Which BSA formula should I use, Du Bois or Mosteller?

Clinically, Mosteller is now more commonly used because it is simpler to calculate and produces results close to Du Bois for most adults. The formula actually used for a specific treatment is determined by the prescribing clinician or institutional protocol, not by preference.

Why is chemotherapy dosed by body surface area instead of weight?

BSA is thought to correlate with blood volume and metabolic rate better than weight alone, which historically made it a preferred way to scale doses of drugs with a narrow margin between an effective dose and a toxic one.

What is a normal BSA for an adult?

Most adults fall somewhere between about 1.4 and 2.0 square metres, with 1.73 square metres often used as a reference average in drug labeling, though individual values vary with height and weight.

Is BSA the same as the body surface area used for burn assessment?

Burn assessment typically uses a percentage of total body surface area affected, often estimated with tools like the Rule of Nines or Lund and Browder charts, rather than the absolute BSA figure in square metres calculated here.

Does BSA-based dosing use a different number for someone with obesity?

Often yes. Because fat mass does not scale metabolically the same way lean mass does, many institutional protocols cap the BSA figure used for dosing, or substitute an adjusted or ideal body weight into the formula, rather than using the full calculated BSA from actual weight. The exact approach is set by the treating institution and prescriber, not by a general calculator.

Referências

1. Du Bois D, Du Bois EF. A formula to estimate the approximate surface area if height and weight be known. Reprinted in Nutrition, 1989.
2. Mosteller RD. Simplified calculation of body-surface area. New England Journal of Medicine, 1987.
3. National Heart, Lung, and Blood Institute. Heart Failure.