Common Registration Assessment, Part 1
Using a provided formula in calculations
When the formula is handed to you, the marks are in following it exactly. Three original questions with the working shown.
Published 5 October 2026. Independent revision material, not GPhC questions, not for patient care.
The Pharmaceutical Journal's article on provided formulae says pharmacists rely on equations, conversion charts, algorithms and formulae to set doses, concentrations and infusion rates. In the assessment you are given the tool and asked to apply it to a patient.
The skill is not recall. It is reading the algorithm line by line, picking out the numbers that belong to this patient, doing the steps in order and rounding the way the question asks. The invented algorithm in question 2 is for practice only, built to mimic that structure.
The method
- Read the whole formula or algorithm before you touch the numbers.
- List the patient values you need and check each unit against the formula.
- Work through the steps in the order printed, writing the running result.
- Apply the rounding rule at the end and check any limits on pack or patch sizes.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
Use loading dose (mg) = Css (mg/L) × Vd (L). A child weighing 24 kg needs a target steady-state concentration of 10 mg/L. The volume of distribution is 0.6 L/kg. What is the loading dose?
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Answer: 144 mg
Working: Vd = 0.6 × 24 = 14.4 L. Dose = 10 × 14.4 = 144 mg. Note that 10 microgram/mL is the same as 10 mg/L, so check the unit before you multiply.
Question 2
A practice algorithm (made up for revision, not for clinical use) says: (1) daily dose A = dose × times per day; (2) if an enhancer is taken, multiply A by 1.3; (3) add 20 × the daily mg of drug R; (4) multiply the total by 0.5. A patient takes 100 mg three times a day with the enhancer, and 2 mg of drug R three times a day. What is the result of step 4?
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Answer: 255 mg
Working: Step 1: 100 × 3 = 300 mg. Step 2: 300 × 1.3 = 390 mg. Step 3: drug R is 2 × 3 = 6 mg a day, and 20 × 6 = 120 mg, so 390 + 120 = 510 mg. Step 4: 510 × 0.5 = 255 mg. Follow the steps in the order given; the order changes the answer.
Question 3
The same algorithm ends with: patch strength (mg) = result ÷ 20, rounded to the nearest 2 mg, and patches come as 1, 2, 3, 4, 6 and 8 mg. A result of 255 mg is reached. What daily patch dose do you supply, and what is the unrounded figure?
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Answer: 12 mg (unrounded 12.75 mg)
Working: 255 ÷ 20 = 12.75 mg. The nearest multiple of 2 mg is 12 mg, which can be made from an 8 mg and a 4 mg patch, because more than one patch may be used when the question allows it.
A worked example from the journal
The Pharmaceutical Journal's article walks through a Parkinson's disease question: it calculates a daily levodopa dose, adjusts it for a COMT inhibitor, adds a levodopa equivalent for a dopamine agonist, applies a final factor and rounds the patch strength to the nearest 2 mg. The lesson is the order of operations. Question 2 here follows that shape with invented numbers.
Where marks are lost
- Skipping a branch of the algorithm because it looks like it does not apply.
- Applying an adjustment before a step that should come first.
- Rounding at every step instead of only where the question says to.
Frequently asked questions
Do I need to memorise the formulae?
The article says to practise with them so nothing is a surprise, but the questions provide the resource. Your job is to apply it carefully.
Is the algorithm on this page clinical?
No. It is invented for revision and must not be used to dose a patient.
Sources
- The Pharmaceutical Journal, medication maths: using provided formulae (13 May 2025)
- The Pharmaceutical Journal, medication maths: how to use provided formulae and solve quantity to supply calculations (16 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
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
See also the formula sheet, the eight sample questions and the approved calculator page.