Common Registration Assessment, Part 1
Amoxicillin liquid and capsule calculations
Three original questions on amoxicillin liquid volumes and capsule quantities, using the NHS strengths.
Published 13 October 2026. Independent revision material, not GPhC questions, not for patient care.
The NHS page lists amoxicillin capsules as usually 250 mg to 500 mg three times a day, and amoxicillin liquid in two strengths: 125 mg in 5 mL and 250 mg in 5 mL.
The doses in these questions are stated in the question, as a prescription would state them. The work is to turn a dose in mg into mL, and then into a whole course supply.
The method
- Find mg per mL for the strength on the bottle: 125 ÷ 5 = 25 mg per mL, 250 ÷ 5 = 50 mg per mL.
- Divide the prescribed dose in mg by the mg per mL to get the volume per dose.
- Multiply by doses per day and then by the days of the course.
- For capsules, divide the dose by the capsule strength to get capsules per dose.
- Round the total volume up to a quantity that covers the whole course.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
A dose of 250 mg is prescribed as liquid. What volume is needed with the 125 mg in 5 mL strength, and with the 250 mg in 5 mL strength?
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Answer: 10 mL and 5 mL
Working: 125 ÷ 5 = 25 mg per mL, so 250 ÷ 25 = 10 mL. 250 ÷ 5 = 50 mg per mL, so 250 ÷ 50 = 5 mL.
Question 2
An adult is prescribed 500 mg three times a day for 5 days using 250 mg capsules. How many capsules are needed?
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Answer: 30 capsules
Working: 500 ÷ 250 = 2 capsules per dose. 2 × 3 = 6 a day. 6 × 5 = 30 capsules.
Question 3
A child is prescribed 150 mg three times a day for 7 days using the 125 mg in 5 mL liquid. What volume is needed in total?
Show the answer
Answer: 126 mL
Working: 150 ÷ 25 = 6 mL per dose. 6 × 3 = 18 mL a day. 18 × 7 = 126 mL.
Exam approach
Write mg per mL for the bottle first. The total supply is volume per dose times doses per day times days.
Where marks are lost
- Mixing up the two liquid strengths.
- Forgetting to multiply by 3 doses a day.
- Giving the volume of one dose as the total supply.
- Rounding the total down so the course is short.
Frequently asked questions
How long should the course last?
The NHS page says to carry on until the course is finished, even if the person feels better.
How should the liquid be measured?
The NHS page says to use the oral syringe or the measuring spoon that comes with the medicine, and not a kitchen teaspoon or other household spoon. The liquid is usually made up by the pharmacist.
Sources
- NHS, how and when to take amoxicillin
- 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
Try 15 free questions with worked answers. No sign-up required.
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See also the formula sheet, the eight sample questions and the approved calculator page.