Body Surface Area Calculator

Body Surface Area (BSA) is the total external surface area of the human body, expressed in square metres (m²). This calculator estimates it from your height and weight using three established clinical formulas. BSA matters because, for many physiological purposes, it tracks metabolic mass, blood volume, and organ size far better than body weight on its own. That makes it a workhorse number in clinical medicine — and a useful one for students, nurses, pharmacists, and clinicians who need a quick, transparent estimate alongside their own protocols.

The formulas and the method

There is no single "true" BSA formula; each was fitted to a different study population, so this tool reports three and an average. The most widely used in oncology is the Mosteller formula because it is simple enough to do on a calculator: BSA = √(height × weight ÷ 3600), with height in centimetres and weight in kilograms. The Du Bois & Du Bois formula (1916) is the historical standard: BSA = 0.007184 × height^0.725 × weight^0.425. The Haycock formula, BSA = 0.024265 × height^0.3964 × weight^0.5378, was derived with infants and children in the sample and is often preferred for paediatric estimates. The three usually agree to within a few percent for typical adults, and the displayed average gives a sensible middle value.

Why BSA instead of body weight?

Weight-based dosing can over- or under-treat people with unusual body composition, because the body's capacity to metabolise and clear a drug scales more closely with surface area than with mass. BSA approximates that physiological capacity, which is why it underpins:

• Chemotherapy dosing — most cytotoxic agents are prescribed in mg/m².
• Burn fluid resuscitation — the Rule of Nines and the Parkland formula use the percentage of total body surface area burned.
• Cardiac index — cardiac output normalised to BSA (Cardiac Index = CO ÷ BSA) lets clinicians compare patients of different sizes.
• Renal clearance — glomerular filtration rate is conventionally normalised to a standard 1.73 m² of body surface.

The average adult BSA is roughly 1.7–1.9 m²; the "standard" reference value of 1.73 m² comes from population averages and is what GFR results are normalised against.

Worked example

Consider an adult who is 170 cm tall and weighs 70 kg. Using Mosteller: height × weight = 170 × 70 = 11,900; divide by 3600 to get 3.306; the square root is about 1.82 m². Running the Du Bois formula on the same person gives a very close figure, and the three-formula average lands in the same region — which is exactly the reassurance a clinician wants before applying a mg/m² dose. Change either input and you can see how sensitive the result is: BSA rises with both height and weight, but less steeply than weight alone, which is the whole point of using it.

Tips

• Use accurate, recently measured height and weight — estimated or self-reported values are the biggest source of error in any BSA calculation.
• For children, the Haycock estimate is generally the most appropriate of the three.
• If a treatment protocol names a specific formula, use that one; consistency within a protocol matters more than which formula is "best".

Common mistakes

• Mixing up units. All three formulas here expect centimetres and kilograms; feeding in inches or pounds gives a meaningless answer.
• Assuming one formula is universally most accurate. Du Bois can underestimate BSA in obese patients, while Mosteller and Haycock behave differently at the extremes — the right choice depends on the patient and the protocol.
• Confusing BSA with BMI. BMI screens weight-for-height at population level; BSA is a physiological scaling quantity for dosing and haemodynamics. Two people with the same BMI but different heights can have different BSAs.

Important: this calculator is an educational estimate, not a clinical decision tool. Actual drug dosing, fluid resuscitation, and any medical use must be performed and verified by a qualified healthcare professional following the relevant clinical protocol.