Common Registration Assessment, Part 1
Vitamin D conversions for the GPhC assessment
Three original questions on converting vitamin D between micrograms and international units, using the NHS figures.
Published 13 October 2026. Independent revision material, not GPhC questions, not for patient care.
Vitamin D products are labelled in micrograms (mcg) or international units (IU). The NHS page gives the conversion: 1 microgram is 40 IU, so 10 micrograms is 400 IU.
Questions give a label in one unit and a limit in the other, so the first job is to put both in the same unit before you compare.
The method
- Write the conversion: 1 microgram = 40 IU.
- To go from micrograms to IU, multiply by 40.
- To go from IU to micrograms, divide by 40.
- Add up products in the same unit before comparing with a limit.
- State the answer with its unit and say whether it is under or over the limit.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
A capsule contains 1,000 IU of vitamin D. How many micrograms is that?
Show the answer
Answer: 25 micrograms
Working: 1,000 ÷ 40 = 25 micrograms.
Question 2
A 6 year old takes one 1,000 IU capsule a day. The NHS upper limit for children aged 1 to 10 is 50 micrograms. How many such capsules would reach the limit?
Show the answer
Answer: 2 capsules
Working: 1,000 IU = 25 micrograms. 50 ÷ 25 = 2 capsules, which is the limit and not under it.
Question 3
An adult takes one product with 2,000 IU and another with 25 micrograms. What is the total in micrograms and in IU, and is it under the NHS limit of 100 micrograms (4,000 IU)?
Show the answer
Answer: 75 micrograms or 3,000 IU, under the limit
Working: 2,000 ÷ 40 = 50 micrograms. 50 + 25 = 75 micrograms. In IU: 25 × 40 = 1,000, and 2,000 + 1,000 = 3,000 IU. Both are under 100 micrograms and 4,000 IU.
Exam approach
Put both numbers in micrograms before you compare. Then check the answer against the NHS pairs: 10 micrograms is 400 IU, 25 is 1,000 IU, 50 is 2,000 IU.
Where marks are lost
- Multiplying by 40 when the conversion needs a division, or the reverse.
- Comparing a total in IU with a limit in micrograms.
- Mixing micrograms with milligrams: the NHS page says a microgram is 1,000 times smaller than a milligram.
- Forgetting to add the second product.
Frequently asked questions
What does the NHS recommend as a daily amount?
Children from 1 year and adults need 10 micrograms a day, and babies up to 1 year need 8.5 to 10 micrograms.
What are the upper limits?
The NHS page says not to take more than 100 micrograms (4,000 IU) a day for adults and children aged 11 to 17, 50 micrograms (2,000 IU) for children aged 1 to 10, and 25 micrograms (1,000 IU) for infants under 12 months.
Sources
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.