Body Surface Area (BSA) and Dose Calculator

What body surface area is and why oncologists use it

Body surface area (BSA) is an estimate of the outside area of your body in square metres, calculated from height and weight. Since the 1950s most cytotoxic chemotherapy drugs have been dosed per square metre rather than per kilogram, because BSA tracks blood volume, kidney and liver blood flow, and therefore how quickly a drug is cleared, better than weight alone does.

A typical adult has a BSA between 1.5 and 2.0 m². Your protocol lists each drug as a strength in mg/m²; the pharmacy multiplies that by your BSA to prepare your dose.

Du Bois or Mosteller: does the formula matter?

Du Bois and Du Bois published their formula in 1916 from measurements of nine people, and it remains the reference in many older protocols. Mosteller simplified it in 1987 to a square root that can be done on a pocket calculator, and many European hospitals and prescribing systems now use it.

For adults of average build the two agree within about 2 to 3 percent, which is smaller than the rounding pharmacies apply. If your treatment plan shows a slightly different BSA from this calculator, the formula, the rounding of your height and weight, or a cap explained below is usually the reason.

From mg/m² to milligrams: a worked example

Someone 165 cm tall weighing 60 kg has a BSA of about 1.66 m² (Du Bois). A drug prescribed at 75 mg/m² gives 75 × 1.66 = 124.5 mg, which the pharmacy will round according to the vial sizes and the protocol’s rounding rules.

Not every cancer drug works this way. Carboplatin is dosed from kidney function with the Calvert formula (see our creatinine clearance calculator), many antibody and targeted treatments are dosed per kilogram or as a flat dose for everyone, and oral drugs often come in fixed tablet strengths.

Why your dose may not match the number on this page

• Caps: some protocols and hospitals cap BSA at 2.0 or 2.2 m² for larger patients. The American Society of Clinical Oncology advises against routine capping in obese adults, so practice varies between countries and centres.
• Blood counts and organ function: low neutrophils or platelets, or reduced kidney or liver function, lead to dose reductions written into the protocol.
• Earlier side effects: a difficult cycle often means the next dose is lowered by a set percentage.
• Age and general fitness: performance status and other illnesses influence the starting dose.
• Weight changes: many centres recalculate BSA when weight changes by around 10 percent during treatment, so your dose may be adjusted mid-course.

Questions to ask your team

Bring the result with you and ask:

• Which BSA formula and which height and weight did you use for my dose?
• Is my dose capped, rounded or reduced, and why?
• Will my dose change if my weight changes, and how often will you check it?
• Which of my drugs are dosed by BSA, by weight or by kidney function?