Common Registration Assessment, Part 1
Suppository displacement values
Three original questions on displacement values, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
A displacement value tells you how much suppository base, in grams, is displaced by 1 g of a drug. If the displacement value is 2, every 1 g of drug takes the place of 2 g of base in the mould.
The questions here use fictional drugs and practise only the arithmetic. The mould size is fixed, so the drug reduces the base you weigh out; it never adds to it.
The method
- Write down the mould size in grams and the drug mass per suppository.
- Divide the drug mass by the displacement value to get the base displaced.
- Subtract the displaced base from the mould size to get base per suppository.
- Multiply by the number of suppositories for the total base.
- Check the units: displacement value questions work in grams throughout.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional drug has a displacement value of 2 in a suppository base. You prepare 12 suppositories in a 2 g mould, each containing 300 mg of the drug. How much base do you need in total?
Show the answer
Answer: 22.2 g
Working: 0.3 / 2 = 0.15 g displaced per suppository. Base per suppository = 2 - 0.15 = 1.85 g. 12 x 1.85 = 22.2 g.
Question 2
A second fictional drug has a displacement value of 1.5. You make 6 suppositories in a 2 g mould, each with 150 mg of drug. What is the total base required?
Show the answer
Answer: 11.4 g
Working: 0.15 / 1.5 = 0.1 g displaced. Base per suppository = 2 - 0.1 = 1.9 g. 6 x 1.9 = 11.4 g.
Question 3
You prepare 20 suppositories in a 1 g mould. Each contains 400 mg of a fictional drug with a displacement value of 4. How much base is needed?
Show the answer
Answer: 18 g
Working: 0.4 / 4 = 0.1 g displaced. Base per suppository = 1 - 0.1 = 0.9 g. 20 x 0.9 = 18 g.
Subtract, then multiply
Base per suppository is the mould size minus the displaced amount. Get that figure right first, then scale up for the batch.
Where marks are lost
- Adding the displaced base instead of subtracting it from the mould size.
- Mixing mg and g: convert the drug mass to grams before dividing.
- Forgetting to multiply the per-suppository base by the batch size at the end.
Frequently asked questions
Does a higher displacement value mean more or less base?
Less. A higher displacement value means each gram of drug replaces more base, so you weigh out less base per suppository.
What if the question asks for base per suppository only?
Stop after the subtraction. The multiplication by batch size is only needed when the question asks for the total.
Sources
- The Pharmaceutical Journal, medication maths: displacement volumes and values
- 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
- 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
- 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.