Common Registration Assessment, Part 1
Diluting a stock to a lower strength
A shortage, a missing strength and a stronger stock on the shelf. Three original questions on how much stock to measure.
Published 5 October 2026. Independent revision material, not GPhC questions, not for patient care.
The Pharmaceutical Journal's tutorial on concentrations and dilutions uses two typical situations: a paediatric liquid out of stock where you dilute the stronger version, and a suspension formula that calls for a strength you do not hold. In both you work out how much stock gives the right amount of drug.
Two methods give the same answer. Compare the strengths as a ratio, or use C1 × V1 = C2 × V2. When a formula is for a different volume, scale it first and dilute second.
The method
- Write the strength you have and the strength you need in the same unit.
- Scale any formula to the final volume you are making.
- Find the stock volume with the ratio of strengths or C1 × V1 = C2 × V2.
- Make up to the final volume with the diluent; do not add that volume on top.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A prescription is for 120 mL of a liquid at 2.5 mg/5 mL. Only a stock at 10 mg/5 mL is available. What volume of stock do you measure before making up to 120 mL?
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Answer: 30 mL
Working: The stock is 4 times stronger than needed, so you need one quarter of the final volume. 120 ÷ 4 = 30 mL of stock, made up to 120 mL with a suitable diluent.
Question 2
You are asked for 200 mL of a 0.5% w/v solution and you hold 4% w/v. What volume of the 4% solution is used?
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Answer: 25 mL
Working: C1 × V1 = C2 × V2, so 4 × V1 = 0.5 × 200 and V1 = 25 mL of the 4% solution, made up to 200 mL.
Question 3
A formula for 20 mL needs 8 mL of a 2% w/v solution. You are making 250 mL, and you only hold the solution as 5% w/v. What volume of the 5% solution do you need?
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Answer: 40 mL
Working: Scale the formula: 250 ÷ 20 = 12.5, so 12.5 × 8 = 100 mL of 2% w/v. Convert to 5% w/v: 5 × V1 = 2 × 100, so V1 = 40 mL.
The journal's two situations
In the journal's tutorial, a paediatric pholcodine liquid is prepared by diluting a stronger linctus, and a carbamazepine suspension is made from a 3.75% w/v stock when the formula needs 2% w/v. Both start from the amount of drug required and work back to the stock volume. The questions here use different drugs and numbers.
Where marks are lost
- Adding the stock volume to the final volume instead of making up to it.
- Scaling the formula after diluting, so the ratio is applied twice.
- Mixing mg/5 mL with mg/mL.
Frequently asked questions
Do I add the diluent volume or make up to volume?
Make up to the final volume. The diluent is the final volume minus the stock.
Which method should I trust?
Either, as long as you check the answer with the other. A stock 4 times stronger needs one quarter of the volume.
Sources
- The Pharmaceutical Journal, medication maths: concentrations, dilutions and health economics (19 July 2021)
- GPhC, Common Registration Assessment specification and permitted items for 2026
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
- Medicine cost and switch calculations
See also the formula sheet, the eight sample questions and the approved calculator page.