Common Registration Assessment, Part 1
Scaling a formula to a batch size
Practise scaling quantities in a master formula up or down, with three original worked questions.
Published 8 October 2026. Independent revision material, not GPhC questions, not for patient care.
A master formula is written for a set quantity, often 100 mL. To make a different quantity, multiply every ingredient by the same scaling factor.
The formulae here are fictional. Find the factor first, then apply it to each ingredient in turn.
The method
- Write the master formula and its batch size.
- Find the factor: required quantity ÷ master quantity.
- Multiply each ingredient by the factor.
- Check the final volume or weight matches the request.
- Convert units at the end if asked.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A fictional formula for 100 mL contains 2 g of A and 5 mL of B. How much of each is needed for 350 mL?
Show the answer
Answer: 7 g of A and 17.5 mL of B
Working: Factor = 350 ÷ 100 = 3.5. A = 2 × 3.5 = 7 g. B = 5 × 3.5 = 17.5 mL.
Question 2
Using the same formula, how much A is needed for 60 mL?
Show the answer
Answer: 1.2 g
Working: Factor = 0.6. 2 × 0.6 = 1.2 g.
Question 3
A fictional mixture holds 12 mg per 5 mL. A 28-day course is 10 mL twice daily. How much drug is needed in total?
Show the answer
Answer: 1344 mg
Working: Daily volume = 20 mL. Total = 560 mL. 560 ÷ 5 × 12 = 1344 mg.
One factor for everything
Compute the factor once and reuse it. This reduces the chance of using different factors on different lines.
Where marks are lost
- Scaling only some of the ingredients.
- Using the wrong factor direction.
- Forgetting the course length when finding total volume.
Frequently asked questions
What if the factor is below 1?
The same method applies. Every ingredient gets smaller by the same factor.
Do I scale the water too?
Only if it is given as a fixed quantity. Quantity sufficient (qs) is made up to the final volume.
Sources
- The Pharmaceutical Journal, medication maths: quantities to supply and using provided formulae
- The Pharmaceutical Journal, medication maths: using provided formula
- 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
See also the formula sheet, the eight sample questions and the approved calculator page.