Common Registration Assessment · Part 1 practice

GPhC calculation practice questions with worked answers

Eight original questions covering the calculation types that come up most. Try each one, then open the working.

Updated 1 October 2026

Revision use only. These are original questions, not GPhC questions. Drug X, Y and Z are fictional. Do not use any figure here for patient care. For the official examples see the GPhC example questions guide.

Question 1 · Percentage strength

How many grams of solute are in 500 mL of a 2.5% w/v solution?

Show the working and answer
  1. 2.5% w/v means 2.5 g in 100 mL.
  2. 500 mL is 5 times 100 mL, so 2.5 g x 5 = 12.5 g.

Answer: 12.5 g

Question 2 · Dose by weight

Drug X (a fictional drug) is prescribed at 15 mg/kg/day in 3 equally divided doses. The patient weighs 72 kg. What is each dose?

Show the working and answer
  1. Daily dose = 15 mg x 72 = 1,080 mg.
  2. Each dose = 1,080 mg / 3 = 360 mg.

Answer: 360 mg per dose

Question 3 · Infusion rate

500 mg of Drug Y (fictional) in 250 mL of sodium chloride 0.9% is to run over 4 hours. What rate in mL/hour, and how many mg/hour does the patient receive?

Show the working and answer
  1. Rate = 250 mL / 4 hours = 62.5 mL/hour.
  2. Drug rate = 500 mg / 4 hours = 125 mg/hour.

Answer: 62.5 mL/hour, which is 125 mg/hour

Question 4 · Dilution

You need 200 mL of a 0.5% solution and have a 5% stock. How much stock and how much diluent?

Show the working and answer
  1. Dilution factor = 5 / 0.5 = 10, so stock needed = 200 mL / 10 = 20 mL.
  2. Diluent = 200 mL - 20 mL = 180 mL.

Answer: 20 mL stock plus 180 mL diluent

Question 5 · Moles and mmol

How many mmol of sodium chloride are in 1.5 g? (Relative molecular mass 58.5.)

Show the working and answer
  1. 1 mmol of NaCl weighs 58.5 mg.
  2. 1.5 g = 1,500 mg. 1,500 / 58.5 = 25.64 mmol.

Answer: 25.6 mmol (to 1 decimal place)

Question 6 · Creatinine clearance

A woman is 60 years old, weighs 62 kg (use this weight) and has a serum creatinine of 95 micromol/L. Estimate her creatinine clearance with the Cockcroft-Gault equation.

Show the working and answer
  1. Use the BNF form: CrCl = (140 - age) x weight x constant / serum creatinine, with constant 1.04 for women and creatinine in micromol/L.
  2. (140 - 60) = 80. 80 x 62 = 4,960. 4,960 x 1.04 = 5,158.4. 5,158.4 / 95 = 54.3 mL/min.

Answer: 54.3 mL/min

Question 7 · Displacement volume

A 500 mg vial of Drug Z (fictional) has a displacement volume of 0.3 mL. How much diluent makes a final volume of 5 mL, and what volume gives a 350 mg dose?

Show the working and answer
  1. Diluent = final volume - displacement volume = 5 - 0.3 = 4.7 mL.
  2. Concentration = 500 mg / 5 mL = 100 mg/mL.
  3. Volume for 350 mg = 350 / 100 = 3.5 mL.

Answer: 4.7 mL diluent, then 3.5 mL for the dose

Question 8 · Supply quantity

A prescription is for 2 tablets twice daily for 28 days. How many tablets are needed?

Show the working and answer
  1. Per day = 2 x 2 = 4 tablets.
  2. 28 days x 4 = 112 tablets.

Answer: 112 tablets

Next steps

Frequently asked questions

Are these real GPhC questions?

No. They are original questions written for revision. The GPhC publishes its own example questions, which you should also work through. Drug X, Y and Z are fictional so the numbers cannot be mistaken for dosing advice.

Which formula sources are used?

The creatinine clearance question uses the Cockcroft-Gault form given in the BNF guidance on prescribing in renal impairment. The displacement question uses the displacement volume method described in The Pharmaceutical Journal. The others are straight arithmetic.

How should I use them?

Try each question first with a calculator and about 3 minutes, then open the answer. Log every miss in the mistake log and redo it a week later.

Practise more calculations

Timed calculation questions with worked answers.

Try 15 free questions