Common Registration Assessment, Part 1
Metformin tablet and liquid strength calculations for the GPhC assessment
Three original practice questions, with the working shown. Read the stated prescription or plan before calculating.
Published 15 October 2026. Independent revision material, not GPhC questions, not for patient care.
NHS guidance describes metformin liquids containing 500 mg, 850 mg or 1,000 mg in each 5 mL. The same volume can therefore contain different doses. Tablets and sachets also have different strengths.
Standard-release and slow-release schedules are not interchangeable merely because the mg totals match. Dose changes, renal suitability and formulation switches need separate clinical checks.
Each question fixes the values needed for its arithmetic. Write the unit beside every step and distinguish a single dose from a daily total or course supply. All prescriptions and patients here are fictional; these are revision materials, not for patient care.
The method
- Identify the exact formulation, strength, dose and frequency supplied in the question.
- Convert an X mg/5 mL strength to mg/mL if a dose-to-volume conversion is needed.
- Calculate one dose first; for tablets or capsules divide prescribed mg by mg per unit.
- Scale to the stated frequency and duration only when a total quantity is requested.
- Apply whole-container or complete-day rounding only if the question specifies it.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
A fictional prescription specifies standard-release metformin 500 mg twice daily for 28 days, using 500 mg tablets. What quantity is required?
Show the answer
Answer: 56 tablets
Working: 500 ÷ 500 = 1 tablet per dose; 1 × 2 × 28 = 56 tablets.
Question 2
A prescribed dose is 850 mg. Liquid contains 850 mg/5 mL. What is one dose in mL?
Show the answer
Answer: 5 mL
Working: 850 ÷ 5 = 170 mg/mL; 850 ÷ 170 = 5 mL.
Question 3
A fictional prescription specifies 1,000 mg twice daily for 7 days, using 1,000 mg/5 mL liquid. What volume covers the course?
Show the answer
Answer: 70 mL
Working: 1,000 mg corresponds to 5 mL; 5 × 2 × 7 = 70 mL. This does not authorise a formulation switch.
NHS: How and when to take metformin
https://www.nhs.uk/medicines/metformin/how-and-when-to-take-metformin/
Where marks are lost
- Using a remembered strength or schedule instead of the stated product.
- Reporting a daily total when the question asks for one dose or the course.
- Treating arithmetic as evidence that a real prescription is clinically suitable.
Frequently asked questions
Are these official assessment questions?
No. They are original revision exercises, not official GPhC questions and not for patient care.
Can a quantity calculation justify a formulation switch?
No. Follow the formulation and prescription in the exercise. Real switches and dose changes need clinical checks.
How should I check the answer?
Check the requested unit, then work backwards from the quantity to the stated dose and duration.
Sources
The questions above are original and use fictional patients and prescriptions. 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
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- Quantity to supply calculations
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- Rounding doses to measurable volumes
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- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
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- Top-up doses from measured drug levels
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- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
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- Accumulation towards steady state
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- 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
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- Suppository displacement values
- Isotonicity by freezing point depression
- Energy content of parenteral nutrition
- Syringe driver rate calculations
- Area under the curve calculations
- Renal dose adjustments using provided bands
- Insulin units and infusion calculations
- Working backwards from a stated dose
- Doses and days from multi-dose devices
- Combined additives in one infusion
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- Fewest tablets to make a dose
- Clearance from half-life and volume of distribution
- Maintenance fluid requirements by weight
- Sodium load from effervescent tablets
- Ethanol content of liquid preparations
- Oxygen cylinder duration calculations
- Urine output and fluid balance calculations
- Quantities for titration packs
- Infusion line priming and dead space
- Alcohol unit calculations for the GPhC assessment
- Paracetamol syrup calculations for children
- Ibuprofen liquid calculations for children
- Vitamin D conversions for the GPhC assessment
- Ferrous sulfate drops and tablet calculations
- Amoxicillin liquid and capsule calculations
- Folic acid dose and quantity calculations
- Adult paracetamol and the daily maximum
- Calcium and iron intake calculations
- Salt limit calculations by age
- Fingertip unit calculations for the GPhC assessment
- Adrenaline dose calculations for anaphylaxis
- Breakthrough opioid dose calculations
- IV fluid bolus calculations in children
- Neonatal maintenance fluid calculations
- Enoxaparin dose calculations by weight
- Levothyroxine dose by weight calculations
- Methotrexate weekly dose calculations
- Sensitivity, specificity and predictive value calculations
- Hypertonic saline bolus calculations in children
- Prednisolone tablet and liquid calculations for the GPhC assessment
- Cetirizine liquid volume and supply calculations for the GPhC assessment
- Lactulose dose and bottle supply calculations for the GPhC assessment
- Flucloxacillin capsule and course supply calculations for the GPhC assessment
- Clarithromycin liquid strength and formulation calculations for the GPhC assessment
- Senna tablet and syrup quantity calculations for the GPhC assessment
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- Aciclovir five-times-daily course calculations for the GPhC assessment
- Oral rehydration fluid deficit calculations for the GPhC assessment
See also the formula sheet, the eight sample questions and the approved calculator page.