BSA Calculator

What body surface area is and why it is used

Body surface area (BSA) is an estimate of the total surface area of your skin, expressed in square metres. An average adult is about 1.7 to 1.9 m². It sounds like an odd thing to measure, but it turns out to be one of the better ways to scale a drug dose or a physiological measurement from one person to another.

The reason is that many body processes, heat loss, drug clearance, blood flow, scale more closely with surface area than with weight. A 220-pound person does not need 60% more of a chemotherapy drug than a 140-pound person, because surface area grows more slowly than mass. That is why oncology doses are written as mg/m² rather than mg/kg.

BSA is used for:

• Chemotherapy and some biologics, doses per square metre of body surface.
• Cardiac index, cardiac output divided by BSA, so a small and a large patient can be compared.
• Glomerular filtration rate, kidney function is often indexed to 1.73 m², a standard adult BSA.
• Nutrition and fluid requirements, particularly in burns and critical care.
• Paediatric dosing, children have far more surface area per kilogram than adults.

The National Cancer Institute's definition of body surface area is a good short reference for how it is used clinically, and the Mosteller formula itself was published in the New England Journal of Medicine in 1987.

Worked example: a 5-foot-4 adult weighing 150 pounds

Start by converting to metric, because every BSA formula uses centimetres and kilograms.

• Height: 5 ft 4 in = 64 in = 162.6 cm
• Weight: 150 lb / 2.205 = 68.0 kg
• Mosteller: square root of (162.6 x 68.0 / 3600) = square root of 3.072 = 1.75 m²

The other three formulas give almost the same answer:

• Du Bois: 0.007184 x 162.6^0.725 x 68.0^0.425 = 1.73 m²
• Haycock: 0.024265 x 162.6^0.3964 x 68.0^0.5378 = 1.77 m²
• Boyd: 1.78 m²

So the spread across four published formulas is 0.05 m², about 3%. For most clinical purposes that is well within tolerance, and it is why Mosteller, with its single square root, is the formula used when someone needs an answer quickly.

Now use it. If a protocol calls for 100 mg/m² of a drug, this patient's dose is 1.75 x 100 = 175 mg. At 1.75 m² their kidney function would be indexed against the 1.73 m² standard, so a measured GFR of 90 mL/min would be reported as roughly 91 mL/min/1.73 m².

Worked example: a 35-pound child, 40 inches tall

Children are where BSA earns its keep, because their body proportions are so different from an adult's.

• Height: 40 in = 101.6 cm
• Weight: 35 lb / 2.205 = 15.9 kg
• Mosteller: square root of (101.6 x 15.9 / 3600) = square root of 0.4487 = 0.67 m²
• Haycock: 0.67 m², Boyd: 0.69 m²

Put that next to the 1.75 m² adult. The child weighs 23% as much as the adult but has 38% of the surface area. That gap is the whole reason paediatric dosing is not "half an adult dose", a 17.5 kg child given half an adult dose would be under-dosed on a per-m² basis.

Cardiac index works the same way. If that child had a cardiac output of 2.4 L/min, the index would be 2.4 / 0.67 = 3.58 L/min/m², comfortably inside the normal 2.5 to 4.0 range, whereas dividing by the adult figure would make it look dangerously low.

For infants, Boyd's formula is often preferred because it was derived from a wider range of body types, including young children. In practice, most paediatric units standardise on one formula so that trends over time stay comparable.

The four common BSA formulas side by side

All four take height (cm) and weight (kg) and return square metres.

Two practical points. First, the formulas disagree most at the extremes, small children and large adults, and least in the middle of the range. Second, the choice of formula is usually a protocol decision made by a hospital or a trial, not something an individual should pick to get a preferred number. Consistency matters more than which one you use.

One more thing worth knowing: BSA formulas were derived largely from white European and North American populations in the early twentieth century. Modern studies have found small systematic differences by ancestry and body composition, which is one of several reasons some oncology protocols are moving toward dosing based on pharmacokinetics rather than surface area.

Where BSA shows up: dosing, cardiac index and kidney function

Chemotherapy. Most cytotoxic drugs are dosed per m². The point is to give a similar exposure to a small and a large patient. Because BSA is only a proxy for how a body handles a drug, doses are usually capped, often at a BSA of 2.0 m², so that large patients do not receive an unsafe absolute dose.

Cardiac index. Cardiac output alone tells you little without knowing the size of the person. Dividing by BSA gives litres per minute per square metre, with a normal range of roughly 2.5 to 4.0. A cardiac output of 4.0 L/min is normal in a small adult and low in a large one.

Kidney function. Estimated GFR is reported per 1.73 m² so results can be compared between patients. That 1.73 figure is a standard adult BSA, roughly a 5-foot-9, 160-pound person.

Burns and fluid resuscitation. Burn surface area is expressed as %TBSA, a percentage of the body, and formulas like Parkland use it with body weight. Confusing BSA in m² with %TBSA is a common mix-up, but they are different quantities.

Nutrition and drug clearance studies. Metabolic rate, heat loss and many physiological parameters are indexed to surface area in research, which is why BSA shows up far beyond the pharmacy.

Limitations and why clinicians still use it

Body surface area is an approximation with well-known flaws. It does not distinguish fat from muscle, and adipose tissue handles drugs differently from lean tissue. Two people at 1.8 m², one a lean athlete, one with obesity, can have markedly different drug clearance. It also does not account for age-related changes in kidney and liver function, which is a major driver of how long a drug stays in the body.

Despite that, BSA survives because it is simple, reproducible, and broadly better than weight alone for scaling doses. The formula needs a tape measure and a scale, not a laboratory.

Where BSA gets used, it is almost always alongside other information: blood counts, kidney and liver panels, previous tolerance, and the specific protocol. A number in isolation is not a dose.

This calculator is a reference estimate, not medical advice. It applies published formulas to the height and weight you enter. It does not know your medical history, your labs, your medications or your diagnosis. Never adjust a medication dose based on this page, that is a conversation with your prescriber or pharmacist.