Common Registration Assessment, Part 1
Equivalent doses from provided conversion tables
Read a fictional conversion table, combine daily totals and apply a stated adjustment without importing real drug ratios.
Published 7 October 2026. Independent revision material, not GPhC questions, not for patient care.
A conversion question gives a relationship, not permission to rely on a remembered clinical ratio. Find the total daily dose first. Then read which direction the table runs and whether an adjustment applies before or after conversion.
The Pharmaceutical Journal describes conversion tables and algorithms as reference-based calculation tasks. The table in each exercise here is invented for Drug A and Drug B. It must not be used for real opioids, steroids, benzodiazepines or other medicines.
The method
- Identify the units and time period in the table.
- Calculate the source daily total.
- Apply the equivalence ratio in the correct direction.
- Apply any adjustment at the specified step.
- Divide into doses after obtaining the adjusted daily total.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional table states 10 mg Drug A equals 4 mg Drug B. Drug A is taken at 15 mg three times daily. What is the equivalent daily amount of Drug B?
Show the answer
Answer: 18 mg/day
Working: Drug A total = 45 mg/day. Drug B = 45 × 4 ÷ 10 = 18 mg/day.
Question 2
A fictional table states 20 mg Drug A equals 5 mg Drug B. Source daily amount is 80 mg Drug A. Reduce the calculated Drug B amount by 25%. Find the adjusted daily amount.
Show the answer
Answer: 15 mg/day
Working: Unadjusted Drug B = 80 × 5 ÷ 20 = 20 mg/day. Retain 75%: 20 × 0.75 = 15 mg/day.
Question 3
A fictional table states 6 mg Drug A equals 15 mg Drug B. Source dose is 9 mg Drug A twice daily. Divide the equivalent Drug B daily total into 3 equal doses. Find each dose.
Show the answer
Answer: 15 mg per dose
Working: Drug A = 18 mg/day. Drug B = 18 × 15 ÷ 6 = 45 mg/day. Each dose = 45 ÷ 3 = 15 mg.
Keep the ratio visible
Write the table’s equivalence beside your working. If 10 mg A equals 4 mg B, doubling A to 20 mg doubles B to 8 mg in this fictional table. That check catches a reversed ratio.
Where marks are lost
- Converting one dose when a daily total is required.
- Inverting the ratio.
- Keeping 25% after a 25% reduction.
Frequently asked questions
Can I use a ratio I remember instead?
No. Use the fictional table’s values, even if you know a clinical conversion for a real medicine.
Should I invent a safety adjustment?
Not in the arithmetic answer. Apply the specified adjustment. Real prescribing requires a separate clinical assessment.
Sources
- The Pharmaceutical Journal, medication maths: using provided formula
- 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
- Thresholds and dose-banding algorithms
See also the formula sheet, the eight sample questions and the approved calculator page.