Common Registration Assessment, Part 1
Calcium and iron intake calculations
Three original questions on daily calcium and iron intake, using the NHS amounts.
Published 13 October 2026. Independent revision material, not GPhC questions, not for patient care.
The NHS page says adults aged 19 to 64 need 700 mg of calcium a day, and that more than 1,500 mg a day could cause stomach pain and diarrhoea. For iron it gives 8.7 mg a day for men aged 19 and over, 14.8 mg for women aged 19 to 49, and 8.7 mg for women aged 50 and over.
The NHS also says high doses of iron, over 20 mg, can cause side effects, and that 17 mg or less a day of iron supplements is unlikely to cause harm. Questions add diet and supplements and compare with these amounts.
The method
- List the amounts: calcium 700 mg, high above 1,500 mg; iron 8.7 mg or 14.8 mg, 17 mg supplement guide, 20 mg side effects.
- Add the diet and each supplement in the same unit.
- Subtract from the target to find the shortfall.
- Compare the total with the upper figure.
- State which side of the figure the answer falls on.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
An adult aged 30 gets 450 mg of calcium from food. How much is the shortfall to 700 mg, and how many 250 mg calcium tablets cover it?
Show the answer
Answer: 250 mg, which is 1 tablet
Working: 700 - 450 = 250 mg. 250 ÷ 250 = 1 tablet.
Question 2
A woman aged 30 takes two tablets of 10 mg iron a day as a supplement. What is the total, and how does it compare with the 17 mg and 20 mg figures?
Show the answer
Answer: 20 mg, which is 3 mg above 17 mg and not over 20 mg
Working: 2 × 10 = 20 mg. 20 - 17 = 3 mg above the 17 mg figure. 20 is not over 20 mg.
Question 3
An adult gets 500 mg of calcium from food and takes two 600 mg calcium supplements a day. What is the total, and how far is it above 1,500 mg?
Show the answer
Answer: 1,700 mg, which is 200 mg above 1,500 mg
Working: 2 × 600 = 1,200 mg. 1,200 + 500 = 1,700 mg. 1,700 - 1,500 = 200 mg.
Exam approach
Add every source first, then compare once. Check the age and sex for the iron figure before using it.
Where marks are lost
- Using the iron figure for the wrong age group.
- Forgetting to include food as well as supplements for calcium.
- Treating exactly 20 mg as over 20 mg.
- Mixing mg of calcium with mg of calcium carbonate.
Frequently asked questions
Should these amounts come from supplements?
The NHS page says most people should be able to get the calcium and iron they need from a varied diet.
Who needs more iron?
The NHS page says women who lose a lot of blood during their periods are at higher risk of iron deficiency anaemia and may need supplements, and to speak to a GP or dietitian.
Sources
- NHS, calcium
- NHS, iron
- 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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More calculation topics
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- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
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See also the formula sheet, the eight sample questions and the approved calculator page.