Common Registration Assessment, Part 1
Percentage composition of a compound preparation
Practise finding the percentage strength of each ingredient in a compounded mixture, with three original worked questions.
Published 10 October 2026. Independent revision material, not GPhC questions, not for patient care.
A compound preparation lists several ingredients and a total weight. The percentage strength of each is its weight divided by the total, multiplied by 100.
The trap is the total: it includes every ingredient, including the base, and phrases like water to 100 g mean the final total is 100 g. The examples are fictional and practise the division and percentage work.
The method
- Find the total weight of the preparation, including the base.
- For each ingredient, divide its weight by the total.
- Multiply by 100 to get the percentage w/w.
- If the formula says an ingredient to a final weight, use that final weight as the total.
- Check the percentages are plausible: each below 100, and actives usually small.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional ointment contains 2 g of drug in a total weight of 50 g. What is the percentage strength of the drug?
Show the answer
Answer: 4% w/w
Working: 2 / 50 x 100 = 4% w/w.
Question 2
A fictional cream contains 5 g of active ingredient and 15 g of emulsifying wax, with water to a final weight of 100 g. What are the percentages of active and of wax?
Show the answer
Answer: 5% w/w active and 15% w/w wax
Working: Active = 5 / 100 x 100 = 5%. Wax = 15 / 100 x 100 = 15%. Water to 100 g means the total is 100 g.
Question 3
A fictional mixture contains 10 g of ingredient A and 5 g of ingredient B, with water to a final weight of 200 g. What is the combined percentage of the two solids?
Show the answer
Answer: 7.5% w/w
Working: Combined solids = 10 + 5 = 15 g. 15 / 200 x 100 = 7.5% w/w.
Total first, always
Write the total weight at the top of your working before any division. If the total is wrong, every percentage after it is wrong.
Where marks are lost
- Dividing by the weight of one ingredient instead of the total weight.
- Misreading water to 200 g as water 200 g, which changes the total.
- Forgetting to multiply by 100, leaving the answer as a fraction.
Frequently asked questions
What does w/w mean here?
Weight in weight: grams of ingredient per 100 g of the total preparation. Semi-solid and powdered preparations are expressed this way.
How is this different from a dilution question?
A dilution changes the strength of one active by adding base. Composition questions give you finished amounts and ask you to state each strength.
Sources
- The Pharmaceutical Journal, medication maths: dilutions
- The Pharmaceutical Journal, preparing for the GPhC pharmaceutical calculations assessment
- 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
See also the formula sheet, the eight sample questions and the approved calculator page.