Common Registration Assessment, Part 1
Quantity to supply calculations
Three original questions on how much to supply: a course by body surface area, a tapering regimen and packs for a set number of days.
Published 5 October 2026. Independent revision material, not GPhC questions, not for patient care.
Quantity to supply questions ask how much medicine goes into the bag. The Pharmaceutical Journal's tutorial on the topic lists three skills the assessment tests here: working out the right quantity from a prescription, using a provided formula, and handling units and rounding correctly.
Most of the work is bookkeeping. Write each phase of the regimen on its own line, count the days in each phase, and only then add them up. Rounding happens at the point where the real world forces it, usually a whole tablet or a whole pack.
The method
- Write the dose and how often it is taken in each phase of the regimen.
- Count the days in each phase and check they add up to the total.
- Multiply doses per day by days for each phase, then add the phases.
- Round at the point a whole tablet, capsule or pack is needed, and say the unit in your answer.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A patient is 160 cm tall and weighs 64 kg. Drug A is prescribed at 150 mg/m² once daily for 5 days, as 50 mg capsules. Use BSA (m²) = √(height in cm × weight in kg ÷ 3600) and round the daily dose to the nearest whole capsule. How many capsules are needed for the course?
Show the answer
Answer: 25 capsules
Working: BSA = √(160 × 64 ÷ 3600) = √2.844 = 1.69 m². Daily dose = 150 × 1.69 = 253.5 mg, which is about 5.07 capsules, so 5 capsules a day. 5 × 5 days = 25 capsules. Rounding the daily dose first matters: you cannot dispense part of a capsule.
Question 2
Drug B 100 mg tablets are prescribed as 2 tablets three times a day for 5 days, then 2 tablets twice a day for 7 days, then 1 tablet daily until the end of 28 days. How many tablets are needed for the 28 days?
Show the answer
Answer: 74 tablets
Working: Days 1 to 5: 6 tablets a day × 5 = 30. Days 6 to 12: 4 tablets a day × 7 = 28. Days 13 to 28 is 16 days: 1 tablet a day × 16 = 16. Total = 30 + 28 + 16 = 74 tablets.
Question 3
A patient takes 1 tablet in the morning and 2 tablets at night. Tablets come in packs of 28. How many packs are needed for 56 days?
Show the answer
Answer: 6 packs
Working: 3 tablets a day × 56 days = 168 tablets. 168 ÷ 28 = 6 packs exactly, so no rounding up is needed. Check the other way: one pack lasts 28 ÷ 3 = 9.3 days, which is why you work in tablets, not days.
What the tutorial covers
The Pharmaceutical Journal's tutorial says that deciding the right quantity to supply from a prescription, using formulae, and understanding units and rounding are the skills being assessed. The three questions on this page practise those one at a time. They are original and are not taken from the tutorial.
Where marks are lost
- Multiplying the unrounded dose by the number of days and rounding at the end, when the patient must take whole capsules each day.
- Giving a tapering regimen the wrong number of days in the last phase.
- Rounding down packs when the question needs enough supply for the whole course.
Frequently asked questions
When do I round up?
When the quantity is a count of whole items needed to cover the course, such as packs. Round the dose first if it cannot be split.
Do I use calendar days or doses?
Count doses per day for each phase, then multiply by that phase's days. Calendar days only set how long each phase runs.
Sources
- The Pharmaceutical Journal, medication maths: how to use provided formulae and solve quantity to supply calculations (16 July 2021)
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional drugs. 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
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
See also the formula sheet, the eight sample questions and the approved calculator page.