Common Registration Assessment, Part 1
Ibuprofen liquid calculations for children
Three original questions on children's ibuprofen liquid, using the NHS age-based doses and dosing gaps.
Published 13 October 2026. Independent revision material, not GPhC questions, not for patient care.
The NHS describes ibuprofen liquid for children from 3 months as 100 mg in 5 mL. The dose is set by age, and the gap between doses depends on whether it is given 3 or 4 times in 24 hours.
Questions turn the volume into mg, find the earliest time for the next dose, or work out how many doses a bottle holds. Always read the label and the leaflet, as the NHS page says.
The method
- Work out mg per mL: 100 mg ÷ 5 mL = 20 mg per mL.
- Multiply the volume to give by 20 to get the dose in mg.
- For timing, 4 doses in 24 hours need at least 4 hours between doses, and 3 doses need at least 6 hours.
- For a bottle, divide the bottle volume by the volume per dose.
- Multiply by doses per day to find how long the bottle lasts.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
A 5 year old is given 7.5 mL of ibuprofen 100 mg in 5 mL. What is the dose in mg, and the most they can have in 24 hours at 3 doses a day?
Show the answer
Answer: 150 mg per dose, and 450 mg in 24 hours
Working: 100 ÷ 5 = 20 mg per mL. 20 × 7.5 = 150 mg. 150 × 3 = 450 mg.
Question 2
A 9 month old has a first dose at 08:00. What are the earliest times of the other doses if the child has 4 doses in 24 hours, and if the child has 3 doses?
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Answer: 4 doses: 12:00, 16:00, 20:00. 3 doses: 14:00, 20:00
Working: 4 doses a day need at least 4 hours between doses: 08:00, 12:00, 16:00, 20:00. 3 doses a day need at least 6 hours: 08:00, 14:00, 20:00.
Question 3
A 2 year old is given 5 mL of 100 mg in 5 mL liquid 3 times a day. How many doses are in a 100 mL bottle, and how many days does it last?
Show the answer
Answer: 20 doses, which is 6 full days with 2 doses left
Working: 100 ÷ 5 = 20 doses. 20 ÷ 3 = 6.67, so 6 full days of 3 doses and 2 doses left over.
Exam approach
Write 20 mg per mL next to the strength, then do the dose. For timing questions list the clock times so the 4 hour and 6 hour gaps are easy to check.
Where marks are lost
- Using the gap for 4 doses (4 hours) when the child is on 3 doses (6 hours).
- Not converting mL to mg before checking the dose.
- Not checking whether the bottle is 100 mg in 5 mL or the stronger 200 mg in 5 mL.
- Counting part-days as full days.
Frequently asked questions
Is there a stronger liquid?
Yes. The NHS page says a stronger liquid of 200 mg in 5 mL is available for children aged 7 and over, and to check the leaflet or ask a pharmacist before using it.
What should be given with it?
The NHS page says it is best given with food or milk, so the child does not get indigestion.
Sources
- NHS, how and when to give ibuprofen for children
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional patients and prescriptions. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
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See also the formula sheet, the eight sample questions and the approved calculator page.