Common Registration Assessment, Part 1
Days supply from pack sizes
Work out how many days a pack lasts and how many packs are needed, with three original worked questions.
Published 8 October 2026. Independent revision material, not GPhC questions, not for patient care.
Supply questions ask how long a pack lasts or how many packs cover a period. The core step is daily use: quantity per dose multiplied by doses per day.
These examples are fictional. They practise the arithmetic and the rule that you supply whole packs and count only whole days when the question asks for full days.
The method
- Work out the daily quantity: amount per dose × doses per day.
- Divide the pack size by the daily quantity for days supplied.
- For packs needed, find the total quantity then divide by pack size.
- Round up when you must cover the full period with whole packs.
- Round down when the question asks for full days.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional pack has 28 tablets. The dose is 2 tablets a day. How many days does it last?
Show the answer
Answer: 14 days
Working: 28 ÷ 2 = 14 days.
Question 2
A fictional course is 1 tablet twice daily for 56 days. Packs hold 28 tablets. How many packs are needed?
Show the answer
Answer: 4 packs
Working: Total = 2 × 56 = 112 tablets. 112 ÷ 28 = 4 packs.
Question 3
A fictional 150 mL bottle is used at 7.5 mL three times a day. How many full days does it last?
Show the answer
Answer: 6 full days
Working: Daily use = 22.5 mL. 150 ÷ 22.5 = 6.67, so 6 full days.
Daily amount first
Once you know the daily quantity, every supply question becomes a single division.
Where marks are lost
- Dividing by the dose instead of the daily amount.
- Rounding up when the question asks for full days.
- Rounding down when whole packs are needed to cover a period.
Frequently asked questions
Should I round up or down?
Read the question. Packs to cover a period round up. Full days from a pack round down.
What if doses vary by day?
Total each day separately, add them, then divide by the pack size.
Sources
- The Pharmaceutical Journal, medication maths: quantities to supply and using provided formulae
- The Pharmaceutical Journal, preparing for the GPhC pharmaceutical calculations assessment
- GPhC, 2026 assessment specification and permitted items
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
Try 15 free questions with worked answers. No sign-up required.
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
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Electrolyte content over time
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- Loading dose and volume of distribution
- Top-up doses from measured drug levels
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
- Bioavailability and equivalent oral doses
- Oral maintenance dose and dosing interval
- Elimination rate constant and half-life
- Accumulation towards steady state
- Ratio strengths and one in X
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
- Unit conversions for mass and volume
- Units and units per mL
- Reconstituting powders and final concentration
- Millimoles and milliequivalents
- Infusion time and volume remaining
- Percentage w/v and w/w conversions
- Ideal and adjusted body weight from formulae
- Diluting creams and ointments by weight
- Scaling a formula to a batch size
See also the formula sheet, the eight sample questions and the approved calculator page.