The Four GPhC Calculations the June 2026 Cohort Found Hardest

After every sitting, the GPhC's Board of Assessors publishes a short report on where candidates struggled. For Part 1 of the June 2026 assessment it named four calculation types: displacement volumes, infusion rates, medical statistics and pharmacokinetics. It added three general points about quantities to supply, units and rounding.

The report is careful to say these insights do not mean the topics are more likely to come up next time. But every Part 1 paper includes at least one of each of the nine calculation types, so these four will be there in November. They are worth getting right.

Each worked example below uses invented values to show the method. They are not past questions.

1. Displacement volumes

What the report said: apply pharmaceutical principles to preparing medicines, for example using displacement volumes.

When you reconstitute a powder, the powder itself takes up space. If you add 5 mL of diluent to a vial, you end up with more than 5 mL. Questions usually give you the displacement volume and ask how much diluent to add, or what volume holds a given dose.

Worked example. A vial contains 500 mg of an antibiotic powder with a displacement volume of 0.4 mL. You need a final concentration of 100 mg/mL. How much diluent do you add, and what volume gives a 350 mg dose?

• Final volume needed: 500 mg ÷ 100 mg/mL = 5 mL
• Diluent to add: 5 mL − 0.4 mL = 4.6 mL
• Volume for 350 mg: 350 mg ÷ 100 mg/mL = 3.5 mL

The trap: adding 5 mL of diluent. That gives a final volume of 5.4 mL and a concentration of about 92.6 mg/mL, so every dose drawn up from it is too small.

2. Infusion rates

What the report said: use all the relevant information in the scenario when calculating infusion rates, such as the infusion time, the concentration and the rate.

Infusion questions often contain more numbers than you need, and the one you skip is usually the one that matters.

Worked example. A drug is prescribed at 5 micrograms/kg/minute for a 70 kg patient. The infusion is 250 mg in 50 mL. What rate is that in mL/hour?

• Dose per minute: 5 × 70 = 350 micrograms/minute = 0.35 mg/minute
• Dose per hour: 0.35 × 60 = 21 mg/hour
• Concentration: 250 mg ÷ 50 mL = 5 mg/mL
• Rate: 21 ÷ 5 = 4.2 mL/hour

A second shape to know. One litre is to run over 8 hours through a giving set delivering 20 drops/mL. How many drops per minute, to the nearest whole number?

• Total drops: 1,000 mL × 20 = 20,000 drops
• Minutes: 8 × 60 = 480
• Rate: 20,000 ÷ 480 = 41.67, so 42 drops per minute

The traps: micrograms against milligrams, per minute against per hour, and using the weight when the dose is not per kilogram.

3. Medical statistics

What the report said: correctly extract and apply evidence-based medicine data to evaluate treatment options and make decisions about patient care.

These questions supply trial results, often in a table, and ask for a figure such as the absolute risk reduction or the number needed to treat. The calculation is short; reading the table correctly is the hard part.

Worked example. In a trial, 12% of patients on placebo had the event, against 8% on the treatment. Find the absolute risk reduction, the relative risk reduction and the number needed to treat.

• Absolute risk reduction: 12% − 8% = 4% (0.04)
• Relative risk reduction: 4 ÷ 12 = 33%
• Number needed to treat: 1 ÷ 0.04 = 25

The traps: dividing by the treatment rate instead of the control rate for the relative risk reduction, and forgetting to round the number needed to treat up to a whole patient when it is not a whole number. The number needed to harm works the same way, using the increase in an adverse event.

4. Pharmacokinetics

What the report said: apply the principles of pharmacokinetics to an individual patient, including the effect of half-life and creatinine clearance on dosing decisions.

Worked example: half-life. A drug has a half-life of 6 hours. The plasma concentration is 40 mg/L at 08:00. What is it at 20:00?

• Time elapsed: 12 hours = 2 half-lives
• 40 → 20 → 10 mg/L

Worked example: creatinine clearance. Simple formulae you use every day, such as body mass index, are not supplied. More complex ones are, so a question that needs the Cockcroft–Gault equation will give it to you. Take a 70-year-old man weighing 60 kg with a serum creatinine of 120 micromol/L, using the version supplied as:

CrCl (mL/min) = (140 − age) × weight (kg) × 1.23 ÷ serum creatinine (micromol/L)

• (140 − 70) × 60 × 1.23 = 70 × 60 × 1.23 = 5,166
• 5,166 ÷ 120 = 43.05, so about 43 mL/min

The question then usually supplies a dosing table, and the mark depends on reading the right band for 43 mL/min.

The traps: using the wrong weight when the question specifies one, and rounding too early.

Three more things the report flagged

Quantities to supply. Consider the dose, frequency, formulation and duration together. Doses are written in a variety of ways. For a reducing course of two tablets twice a day for five days, then one twice a day for five days, the total is 20 + 10 = 30 tablets.

Units and realism. Check the units of your answer and ask whether it is practical. An answer of 3.7 tablets, or an infusion meant to run over a day coming out at 400 mL/hour, should make you check your working.

Rounding. Know when to round during a calculation and when not to. Where the question wants a particular precision, it says so, for example "give your answer to one decimal place". Round only at the end unless the scenario makes you round earlier, such as whole tablets or whole vials.

Practise them under time pressure

You have about three minutes per calculation in Part 1. Working these out slowly is not enough; you need to do them accurately at speed. Every GPhC mock paper has a full 40-question Part 1 with all nine types, marked by calculation type so you can see which ones cost you marks. For a quick start, the free GPhC practice questions open with five calculations. Our wider guide, GPhC calculations: how to prepare, covers all nine types.

From the GPhC's Key insights from the June 2026 Common Registration Assessment and its framework for sittings in 2026, read on 18 October 2026. Worked examples use invented values. Dose Up is independent and is not endorsed by the GPhC.