Common Registration Assessment, Part 1
Doses and days from multi-dose devices
Three original questions on devices and days of cover, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
Device questions are really quantities-to-supply questions in disguise. The device holds a stated number of doses or a stated volume, the regimen uses a fixed number per day, and you divide one by the other.
The questions here are fictional and practise only the arithmetic. Count only complete days or complete doses unless the question says otherwise.
The method
- Write the total contents of the device in doses, units or drops.
- Work out the use per day from the regimen, including both eyes or both times of day.
- Divide the total by the daily use.
- Round down to complete days or doses when the question asks how long it lasts.
- State the unit of your answer clearly.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional inhaler contains 200 doses. The regimen is 2 puffs twice daily. How many days does one inhaler last?
Show the answer
Answer: 50 days
Working: 2 x 2 = 4 doses per day. 200 / 4 = 50 days.
Question 2
A fictional insulin pen contains 300 units. The patient injects 24 units once daily. How many full days does the pen cover?
Show the answer
Answer: 12 days
Working: 300 / 24 = 12.5, so 12 full days.
Question 3
A fictional 10 mL eye drop bottle gives 20 drops per mL. The regimen is 1 drop into both eyes twice daily. How many days does the bottle last?
Show the answer
Answer: 50 days
Working: Total = 10 x 20 = 200 drops. Use = 1 x 2 x 2 = 4 drops per day. 200 / 4 = 50 days.
Total over daily
Contents divided by daily use, rounded down. Write the regimen out in full first so nothing is double counted or missed.
Where marks are lost
- Forgetting that both eyes or twice daily doubles the daily count.
- Rounding up partial days instead of counting complete days.
- Mixing drops and mL: convert the bottle to drops before dividing.
Frequently asked questions
Do I include the first dose of the next day?
No. Days of cover means complete days only, so always round the division down.
What if a device has a stated expiry after opening?
Use the shorter of the two figures: days of contents or the stated in-use shelf life, whichever runs out first.
Sources
- The Pharmaceutical Journal, medication maths: using a resource or artefact when calculating doses and dose regimens
- 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
- Days supply from pack sizes
- 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
- Age-band doses from a provided table
- Tablet fractions and split doses
- Stones, pounds, inches and metric conversions
- Micrograms per minute from mg per hour
- Percentage change and mark-up
- Molar solutions and mass per volume
- Osmolarity from component concentrations
- Direct proportion and cross-multiplication
- Scientific notation and very small quantities
- Stock use and reorder quantities
- Parts per million and very dilute solutions
- Sodium chloride equivalents and isotonic solutions
- Density and specific gravity conversions
- Doses per day and hours between doses
- Finding the volume of diluent to add
- Capping a dose at a stated maximum
- Whole vials and ampoules to draw up
- Reading values from a provided graph
- Micrograms per kg per minute infusions
- Percentage composition of a compound preparation
- Suppository displacement values
- Isotonicity by freezing point depression
- Energy content of parenteral nutrition
- Syringe driver rate calculations
- Area under the curve calculations
- Renal dose adjustments using provided bands
- Insulin units and infusion calculations
- Working backwards from a stated dose
- Combined additives in one infusion
See also the formula sheet, the eight sample questions and the approved calculator page.