Common Registration Assessment, Part 1
Folic acid dose and quantity calculations
Three original questions on folic acid doses in micrograms and milligrams, using the NHS doses.
Published 13 October 2026. Independent revision material, not GPhC questions, not for patient care.
The NHS page gives 400 micrograms once a day while trying to get pregnant and in the first 12 weeks of pregnancy. A higher dose of 5 mg once a day is advised if there is a higher chance of a neural tube defect, and 5 mg a day is also the usual dose to treat folate deficiency anaemia, sometimes raised to 15 mg.
The calculation is nearly always unit conversion: 1 mg is 1,000 micrograms. After that it is tablets per day and total quantity.
The method
- Write the conversion: 1 mg = 1,000 micrograms.
- Put the dose and the tablet strength in the same unit.
- Divide the dose by the tablet strength to get tablets per dose.
- Multiply by doses per day and by the number of days.
- Convert weeks or months to days using the figure the question gives.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
How many micrograms is 5 mg, and how many times bigger is it than a 400 microgram dose?
Show the answer
Answer: 5,000 micrograms, which is 12.5 times bigger
Working: 5 × 1,000 = 5,000 micrograms. 5,000 ÷ 400 = 12.5.
Question 2
A woman takes one 400 microgram tablet daily for the first 12 weeks of pregnancy. How many tablets does she need, and how many mg is that in total?
Show the answer
Answer: 84 tablets, 33.6 mg
Working: 12 × 7 = 84 days, so 84 tablets. 84 × 400 = 33,600 micrograms = 33.6 mg.
Question 3
A patient is told to take 15 mg a day using 5 mg tablets. How many tablets a day, and how many for 28 days?
Show the answer
Answer: 3 tablets a day, 84 tablets for 28 days
Working: 15 ÷ 5 = 3 tablets. 3 × 28 = 84 tablets.
Exam approach
Convert to one unit before dividing. A dose in micrograms and a tablet in mg is the usual trap.
Where marks are lost
- Treating 5 mg as 500 micrograms.
- Dividing a mg dose by a microgram tablet strength without converting.
- Counting months as 4 weeks each without checking what the question says.
- Forgetting that there are 7 days in each of the 12 weeks.
Frequently asked questions
Are there other doses on the NHS page?
Yes. To prevent anaemia the usual dose for adults and children aged 12 and over is 5 mg every 1 to 7 days, and with methotrexate the usual dose is 5 mg once a week on a different day.
How long is folic acid taken before pregnancy?
The NHS page says ideally for 3 months before trying for a baby and during the first 12 weeks of pregnancy.
Sources
- NHS, how and when to take folic acid
- 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
- Dose by weight calculations for the GPhC assessment
- 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
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- Ideal and adjusted body weight from formulae
- Diluting creams and ointments by weight
- Scaling a formula to a batch size
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- Tablet fractions and split doses
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- Percentage change and mark-up
- Molar solutions and mass per volume
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- Direct proportion and cross-multiplication
- Scientific notation and very small quantities
- Stock use and reorder quantities
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- Density and specific gravity conversions
- Doses per day and hours between doses
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- Area under the curve calculations
- Renal dose adjustments using provided bands
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- Working backwards from a stated dose
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- Clearance from half-life and volume of distribution
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- Alcohol unit calculations for the GPhC assessment
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- Calcium and iron intake calculations
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See also the formula sheet, the eight sample questions and the approved calculator page.