Body surface area calculator (Mosteller and Du Bois)

Body surface area by the Mosteller formula is the square root of height in centimetres multiplied by weight in kilograms, divided by 3600. Mosteller is the method most protocols standardise on, and mixing it with Du Bois between prescriptions is a recognised source of dosing error.

When to use the body surface area calculator

Use for cytotoxic chemotherapy and the smaller group of other drugs dosed per square metre. Surface area correlates better with metabolic rate and blood volume than weight alone, which matters most for agents with a narrow therapeutic index.

The body surface area formula

BSA in m² = √[(height in cm × weight in kg) ÷ 3600]. This is the Mosteller formula, chosen over older equations because it is simple enough to check by hand.

Worked example

A patient is 170 cm and 70 kg. BSA = √[(170 × 70) ÷ 3600] = √(11900 ÷ 3600) = √3.31 = 1.82 m².

The mistake that costs marks

Surface-area dosing does not make a cytotoxic drug safe, it only reduces variation between patients of different sizes. Questions that offer BSA dosing as a way to avoid toxicity are testing whether you understand it as a normalisation step rather than a safety measure.

Study aid, not a clinical tool

This calculator exists so you can learn the method and check your working for an examination. It is not medical advice and not clinical decision support, and it must not be used to make a decision about a real patient. Verify any result against your local formulary, the product literature and a qualified pharmacist. Our medical disclaimer sets out the full position.

Calculators you will usually need alongside this

Most questions that reach for one of these reach for another a step later, so these are the three that most often pair with it.

• mg/kg dose calculator — Weight-based dose
• Creatinine clearance calculator (Cockcroft-Gault) — Creatinine clearance
• Ideal and adjusted body weight calculator — Ideal body weight

Keep going

Calculations are only half of what examinations test. The other half is knowing why a drug does what it does, which is what decides whether a number you calculated is even the right number to be calculating. Start with Drug name stems, decoded. The drug class pages cover the mechanism behind the number, or browse the rest of the free calculators and try the practice questions.