Common Registration Assessment, Part 1
Dose by weight calculations for the GPhC assessment
Three original questions on mg/kg dosing, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
Weight-based dosing looks simple, and most marks are lost on the wording rather than the arithmetic. The question tells you whether the rate is per dose or per day, whether it is split, and sometimes whether a maximum applies.
Read it twice before you touch the calculator. Underline the weight, the rate and its time unit, then decide what the answer has to be: a daily total, a single dose or a volume.
The method
- Write the rate with its unit, for example 6 mg/kg/day.
- Multiply by the weight in kg to get the total for that time period.
- Divide by the number of doses if the total is split.
- Convert to a volume if the question asks for one, using the strength.
- Compare with any maximum dose in the stem.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A child weighing 18.5 kg is prescribed Drug X at 6 mg/kg per day in 3 equal doses. What is each dose in mg?
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Answer: 37 mg
Working: 6 × 18.5 = 111 mg per day. 111 ÷ 3 = 37 mg per dose.
Question 2
A patient weighing 64 kg needs a single dose of Drug Y at 25 micrograms/kg. The ampoule contains 500 micrograms in 1 mL. What volume do you draw up?
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Answer: 3.2 mL
Working: 25 × 64 = 1,600 micrograms. 1,600 ÷ 500 = 3.2 mL.
Question 3
Drug Z is 12.5 mg/kg per dose, twice daily, with a maximum of 750 mg per dose. The patient weighs 70 kg. What single dose do you supply?
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Answer: 750 mg
Working: 12.5 × 70 = 875 mg, which is above the 750 mg maximum, so the dose is capped at 750 mg.
What the BNF says about children's doses
The BNF's guidance on prescribing in children says many children's doses are standardised by weight, so you multiply by the body weight in kilograms, and that this method should be used rather than scaling down an adult dose. It also notes that in an overweight child, calculating by body weight can give much higher doses than needed, and the dose should then be calculated from an ideal weight related to height and age. If a question says which weight to use, use that one.
Where marks are lost
- Multiplying a per-day rate when the question asks for a single dose.
- Mixing micrograms and milligrams: 1 mg is 1,000 micrograms.
- Forgetting a stated maximum dose and giving the raw calculated figure.
Frequently asked questions
Is the weight always in kilograms?
Usually, but check. A question may give weight in grams for an infant or in stones and pounds in a patient history. Convert before you multiply.
What if the calculated dose is above the maximum?
Where the stem gives a maximum, the dose you would supply is the maximum, not the calculated figure. Read what the question asks for.
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
- 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
- Body surface area calculations
- 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.