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How to do GPhC pharmacy calculations

Part 1 of the GPhC registration assessment is all calculations, and it is where many trainees lose easy marks. This free guide breaks every calculation type into its own in-depth page, from simple unit conversions to the trickier displacement values, suppositories and alligation, with step-by-step worked examples throughout.

The four golden rules

  • 1. Convert to one unit first. Get everything into the same unit before you start.
  • 2. Write the formula down. It stops you mixing up what to multiply and what to divide.
  • 3. Estimate to sanity-check. Should the answer be bigger or smaller than what you started with?
  • 4. Round only at the end. Carry full figures through the working.

Choose a calculation type

1

Units and conversions

Move confidently between grams, milligrams, micrograms and millilitres.

2

Doses and dosage units

Doses, mg/kg dosing and how many tablets to dispense.

3

Liquid doses

Turn a dose into a volume using the concentration.

4

Quantities to supply

How much to dispense, and how long a pack lasts.

5

Expressing concentration

% w/v, w/w, v/v, 1-in-X ratios and parts per million.

6

Dilutions

Use C1V1 = C2V2 for stock solutions and adding water.

7

Alligation

Find the ratio to mix two strengths to a target.

8

Displacement values: injections

Reconstitute powders to the right concentration.

9

Suppositories and displacement

The four-step method for suppositories and pessaries.

10

Infusion rates

mL/hour, drops/min, weight-based and units/hour.

11

Body weight and surface area

Mosteller BSA, BSA-based doses and BMI.

12

Creatinine clearance

Estimate renal function for dose adjustment.

13

Moles and millimoles

Convert mass and percentages to millimoles.

14

Paediatric calculations

Weight-based doses and suspension volumes for children.

Exam technique and common mistakes

  • Mixing up micrograms and milligrams is the single most common error. Convert first.
  • Read whether the question wants a dose, a volume, a quantity, or a rate, then pick the matching formula.
  • Do not round in the middle of a calculation. Carry full figures and round once at the end.
  • For displacement, remember to subtract: less diluent for injections, less base for suppositories.
  • Sanity-check the size of the answer. A 4.2 mL/hour infusion is plausible; 420 mL/hour usually is not.

Frequently asked questions

What calculations come up in the GPhC exam?

Part 1 of the GPhC registration assessment is calculations. Expect unit conversions, doses and quantities to supply, concentrations and dilutions, alligation, displacement values for injections and suppositories, infusion rates, body weight and surface area dosing, creatinine clearance, and moles and millimoles.

Is a calculator allowed in the GPhC calculations paper?

Yes, an on-screen calculator is provided for Part 1. You still need to set up each calculation correctly and convert units, so understanding the method matters more than arithmetic speed.

What is a displacement value?

A displacement value tells you how much base a drug displaces. For suppositories it is the weight of drug that displaces one part by weight of base. For injections the displacement volume is the volume the dry powder occupies once dissolved.

Are these calculation guides free?

Yes. Every PassPharm guide is free for everyone, with no account needed. The full question bank and timed mock papers are available with PassPharm Pro.

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