Common Registration Assessment, Part 1
Diluting creams and ointments by weight
Practise mixing a stronger cream with a base to make a weaker one, with three original worked questions.
Published 8 October 2026. Independent revision material, not GPhC questions, not for patient care.
Diluting a cream or ointment is a weight calculation. The amount of active ingredient does not change when base is added, so the total drug stays fixed while the total weight grows.
The strengths here are fictional w/w percentages. Work in grams of active ingredient, then convert back to a percentage.
The method
- Find grams of active ingredient in each component.
- Add the actives and add the weights.
- Divide total active by total weight and multiply by 100.
- For a target strength, work backwards from the required active amount.
- Check the final strength lies between the strongest and weakest component.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
30 g of a 2% w/w cream is mixed with 90 g of base. What is the final strength?
Show the answer
Answer: 0.5% w/w
Working: Active = 0.02 × 30 = 0.6 g. Total = 120 g. 0.6 ÷ 120 = 0.5%.
Question 2
A fictional prescription needs 200 g of a 1% w/w cream from a 5% stock and base. How much stock is needed?
Show the answer
Answer: 40 g
Working: Active needed = 2 g. Stock = 2 ÷ 0.05 = 40 g, so add 160 g of base.
Question 3
100 g of 1% w/w cream is mixed with 300 g of 3% w/w cream. What is the final strength?
Show the answer
Answer: 2.5% w/w
Working: Active = 1 + 9 = 10 g. Total = 400 g. 10 ÷ 400 = 2.5%.
Grams of active
Convert every strength into grams of active before combining. The answer then follows from one division.
Where marks are lost
- Averaging percentages without weighting by weight.
- Forgetting the base adds to the total weight.
- Giving a strength above the strongest component.
Frequently asked questions
Why is active constant?
Adding base adds no drug, so grams of active stay the same.
Is this the same as a liquid dilution?
The logic is the same, but creams are measured by weight, not volume.
Sources
- The Pharmaceutical Journal, medication maths: dilutions
- The Pharmaceutical Journal, medication maths: quantities to supply and using provided formulae
- 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
- Scaling a formula to a batch size
See also the formula sheet, the eight sample questions and the approved calculator page.