Common Registration Assessment, Part 1
Body surface area calculations
Three original questions using the Mosteller equation, with a dose per m² step in two of them.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
Some doses, particularly in oncology and paediatrics, are given per square metre of body surface area. The Mosteller equation is a widely used way to estimate BSA from height and weight.
Whether you have to remember the formula depends on the question. Read what it provides, and use that formula or value.
The method
- Multiply height (cm) by weight (kg).
- Divide by 3,600.
- Take the square root to get BSA in m².
- Multiply by the dose per m² if a dose is asked for.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A patient is 160 cm tall and weighs 60 kg. Calculate the BSA using the Mosteller formula.
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Answer: 1.63 m²
Working: 160 × 60 = 9,600. 9,600 ÷ 3,600 = 2.6667. √2.6667 = 1.633 m², or 1.63 m² to 2 decimal places.
Question 2
A patient is 175 cm and 80 kg. The dose is 75 mg/m². What dose in mg?
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Answer: 147.9 mg
Working: 175 × 80 = 14,000. 14,000 ÷ 3,600 = 3.889. √3.889 = 1.972 m². 1.972 × 75 = 147.9 mg.
Question 3
A child is 110 cm and 19 kg. The dose is 25 mg/m². What dose in mg?
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Answer: 19 mg
Working: 110 × 19 = 2,090. 2,090 ÷ 3,600 = 0.5806. √0.5806 = 0.762 m². × 25 = 19 mg.
What the BNF says about BSA
The BNF's guidance on prescribing in children says doses are occasionally standardised by body surface area in m2, and that BSA estimates are sometimes preferable to body weight because many physiological phenomena correlate better with surface area. It says BSA can be estimated from weight and refers readers to the BNFC for the method.
Where marks are lost
- Forgetting the square root.
- Using metres for height when the formula uses cm.
- Rounding BSA early and shifting the final dose.
Frequently asked questions
Do I need to memorise the Mosteller formula?
Learn it, but read the question: some provide a BSA, a nomogram value or a different equation, and you use what you are given.
Should I round BSA before multiplying?
Keep the full calculator value until the end, unless the question tells you to round BSA first.
Sources
- BNF, prescribing in children
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional drugs. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
Try 15 free questions with worked answers. No sign-up required.
More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
See also the formula sheet, the eight sample questions and the approved calculator page.