Common Registration Assessment, Part 1
Doses per day and hours between doses
Practise turning a dosing frequency into doses per day, hours between doses and totals over a course, with three original worked questions.
Published 10 October 2026. Independent revision material, not GPhC questions, not for patient care.
Frequency wording carries the arithmetic. Every 8 hours means 3 doses a day; twice daily means 2. Course totals come from multiplying doses per day by days, and single doses come from dividing a daily total.
These fictional questions practise the counting and dividing. The traps are counting an extra dose at the start of a course and mixing up per dose with per day.
The method
- Convert the frequency to doses per day: 24 divided by hours between doses.
- Or convert doses per day to hours between doses: 24 divided by the number.
- Multiply doses per day by the number of days for a course total.
- Divide a daily total by doses per day for a single dose.
- Convert between mg and tablets or volume only after the counting is right.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional medicine is taken every 8 hours for 3 days. How many doses is that in total?
Show the answer
Answer: 9 doses
Working: 24 / 8 = 3 doses per day. 3 x 3 = 9 doses.
Question 2
A fictional daily dose is 1.5 g split into 3 equal doses. Tablets are 250 mg. How many tablets are taken each day?
Show the answer
Answer: 6 tablets
Working: Each dose = 1500 / 3 = 500 mg = 2 tablets. Per day: 2 x 3 = 6 tablets.
Question 3
A fictional medicine is taken twice daily, 12 hours apart, starting at 8 am on Monday with the last dose at 8 pm on Thursday. How many doses are taken?
Show the answer
Answer: 8 doses
Working: 2 doses on each of Monday, Tuesday, Wednesday and Thursday = 8 doses.
Count on paper
Draw a short timeline for any question with start and end times. Thirty seconds of drawing prevents the commonest lost mark in this topic.
Where marks are lost
- Counting a first-day extra dose when the course is described by start and end times.
- Using 12 hours for twice daily when the question says every 8 hours.
- Dividing the course total by days when the question asks for a single dose.
Frequently asked questions
Does four times daily mean every 6 hours?
In exam arithmetic, yes: 24 divided by 4. In practice doses are spaced over waking hours, but calculations questions use the even division unless told otherwise.
How do I avoid the fence-post error with start and end times?
List the days and mark each dose on them before counting. Two doses a day from Monday morning to Thursday evening is 8, not 9.
Sources
- The Pharmaceutical Journal, medication maths: quantities to supply calculations
- 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
- 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
- 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
See also the formula sheet, the eight sample questions and the approved calculator page.