Common Registration Assessment, Part 1
Reconstituting powders and final concentration
Work out the strength and the volume to draw up after a powder is made up, with three original worked questions.
Published 8 October 2026. Independent revision material, not GPhC questions, not for patient care.
A powder for reconstitution gives a final volume that can be larger than the diluent added, because the powder takes up space. When a question states the final volume, use it for the concentration.
The examples are fictional. They practise the sequence: find the final concentration, then find the volume that holds the required dose.
The method
- Identify the total drug in the container and the final volume after reconstitution.
- Divide total drug by final volume for the concentration.
- Divide the required dose by the concentration for the volume.
- If a displacement value is given, add it to the diluent to check the final volume.
- Check that the answer is less than the final volume available.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional 1 g powder is made up with 9.2 mL of water. The powder displaces 0.8 mL. The dose is 650 mg. What volume is drawn up?
Show the answer
Answer: 6.5 mL
Working: Final volume = 9.2 + 0.8 = 10 mL. Concentration = 1000 ÷ 10 = 100 mg/mL. 650 ÷ 100 = 6.5 mL.
Question 2
A fictional 500 mg powder is made up to a final volume of 5 mL. What volume gives 125 mg?
Show the answer
Answer: 1.25 mL
Working: Concentration = 500 ÷ 5 = 100 mg/mL. 125 ÷ 100 = 1.25 mL.
Question 3
A fictional suspension is 250 mg per 5 mL. A bottle is made up to 120 mL. How many grams of drug does it contain?
Show the answer
Answer: 6 g
Working: Concentration = 250 ÷ 5 = 50 mg/mL. 50 × 120 = 6000 mg = 6 g.
Total first, then per mL
Total drug divided by final volume gives a clean concentration, and every later step becomes a single division.
Where marks are lost
- Using the diluent volume as the final volume when displacement is given.
- Forgetting to convert mg to g at the end.
- Giving a volume larger than the container holds.
Frequently asked questions
What is displacement?
It is the volume the powder itself occupies. Diluent plus displacement gives the final volume.
What if no displacement is stated?
Use the final volume the question gives. Do not invent a displacement value.
Sources
- The Pharmaceutical Journal, medication maths: quantities to supply and using provided formulae
- The Pharmaceutical Journal, medication maths: dilutions
- 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
- 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
See also the formula sheet, the eight sample questions and the approved calculator page.