Common Registration Assessment, Part 1
Working backwards from a stated dose
Three original questions on reverse calculations, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
Not every question runs forwards. Sometimes the assessment states the dose the patient receives and asks you to unpick the volume, the number of tablets or the strength that produced it.
The questions here are fictional and practise only the arithmetic. The skill is to invert each step: where a forwards question multiplies, you divide, in the reverse order.
The method
- Write what is actually asked for and underline it.
- Write the stated dose with its unit.
- Invert the usual chain one step at a time, dividing where you would multiply.
- Check the answer by running the calculation forwards again.
- Give the answer in the unit the question requests.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A patient takes 7.5 mL twice daily of a fictional suspension containing 250 mg per 5 mL. What total daily dose in mg does this deliver?
Show the answer
Answer: 750 mg
Working: 7.5 mL is 1.5 x 5 mL, so 1.5 x 250 = 375 mg per dose. Twice daily: 375 x 2 = 750 mg per day.
Question 2
A fictional prescription needs a 500 mg dose from 125 mg tablets, taken twice daily for 14 days. How many tablets are supplied in total?
Show the answer
Answer: 112 tablets
Working: 500 / 125 = 4 tablets per dose. 4 x 2 x 14 = 112 tablets.
Question 3
A fictional infusion delivered 360 mg over 6 hours at a steady 3 mL per hour. What was the concentration of the infusion in mg per mL?
Show the answer
Answer: 20 mg/mL
Working: Volume delivered = 3 x 6 = 18 mL. 360 / 18 = 20 mg/mL.
Invert, then verify
Reverse the chain one step at a time, then run it forwards to prove you get the stated dose back.
Where marks are lost
- Multiplying where the reverse direction needs a division.
- Forgetting the frequency when turning a single dose into a daily or course total.
- Skipping the forwards check, which catches most inverted arithmetic.
Frequently asked questions
How do I spot a reverse question?
The dose or total is given and a component is missing. If the question hands you the answer you would normally calculate, you are working backwards.
Is checking forwards really needed?
It takes seconds. Re-run the numbers in the normal direction and confirm you land back on the stated dose.
Sources
- The Pharmaceutical Journal, medication maths: how to solve dose and dosage calculations (video)
- 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
- Doses and days from multi-dose devices
- Combined additives in one infusion
See also the formula sheet, the eight sample questions and the approved calculator page.