The 9 GPhC Calculation Types, With a Worked Example for Each
Part 1 of the GPhC registration assessment is 40 calculations in 2 hours, so about 3 minutes each, and there are no answer options: you type the number. The questions come from a small number of predictable types, and you can practise every one of them.
By Umar Majid ·
Part 1 of the GPhC exam is 40 calculations in 2 hours, with no options to choose from, but the questions come from a small number of predictable types. Below are the nine you need to know, a worked example of each, and the mistakes that cost trainees marks year after year.
Use the same 4 steps every time
- Read the whole question. Underline what it's asking for and in which units.
- Write the formula or set up the ratio before you touch the calculator.
- Keep the units at every step. Most errors are unit errors.
- Sense-check the answer. Would a patient really take 47 tablets a day?
1. Dose and dose regimen
A patient weighing 76 kg needs a phenytoin loading dose of 20 mg/kg. What dose in mg?
- Dose = 20 mg/kg × 76 kg = 1,520 mg
Check for a maximum dose stated in the question, and whether the dose is per dose or per day.
2. Unit conversions
A patient needs digoxin 0.25 mg. You stock 62.5 microgram tablets. How many tablets per dose?
- 0.25 mg = 250 micrograms
- 250 ÷ 62.5 = 4 tablets
Micrograms, milligrams and nanograms are easy to mix up. Convert everything to the same unit first.
3. IV infusion rates
1 litre of sodium chloride 0.9% is to be given over 8 hours using a giving set that delivers 20 drops/mL. What's the rate in drops per minute?
- Total drops = 1,000 mL × 20 drops/mL = 20,000 drops
- Total minutes = 8 × 60 = 480 minutes
- 20,000 ÷ 480 = 41.67, so 42 drops per minute
In mL per hour, that's 1,000 ÷ 8 = 125 mL/hour.
The Board of Assessors has flagged infusion rates as a weak area. Watch hours vs minutes, and use every piece of information together: volume, time, concentration and rate.
4. Displacement values
You need to make 10 suppositories, each containing 200 mg of a drug with a displacement value of 1.5. The mould holds 1 g of base. How much base do you need?
- Total drug = 10 × 200 mg = 2 g
- Base displaced by the drug = 2 g ÷ 1.5 = 1.333 g
- Base needed = (10 × 1 g) − 1.333 g = 8.67 g
Divide by the displacement value, don't multiply. And look out for any instruction to make extra to allow for losses.
5. Concentration expressions
Chlorhexidine mouthwash is 0.2% w/v. How many mg are in a 10 mL dose?
- 0.2% w/v = 0.2 g in 100 mL = 200 mg in 100 mL = 2 mg/mL
- 2 mg/mL × 10 mL = 20 mg
Take care with w/v, w/w and v/v, and with ratio strengths such as 1 in 10,000. Before you start, write out what the expression means in grams and millilitres.
6. Creatinine clearance and renal dose adjustment
A 78-year-old woman weighs 58 kg and has a serum creatinine of 110 micromol/L. Use the Cockcroft-Gault equation to estimate her creatinine clearance.
CrCl (mL/min) = (140 − age) × weight (kg) × 1.04 (female) ÷ serum creatinine (micromol/L)
- (140 − 78) = 62
- 62 × 58 = 3,596
- 3,596 × 1.04 = 3,739.84
- 3,739.84 ÷ 110 = 34 mL/min
The constant is 1.23 for men and 1.04 for women.
The question will tell you which weight to use, or give you what you need to work out ideal body weight. Then apply the result to the dosing table you're given.
7. Paediatric weight-based dosing
A child weighing 14 kg is prescribed paracetamol 15 mg/kg. You have oral suspension 120 mg/5 mL. What volume per dose?
- Dose = 15 mg/kg × 14 kg = 210 mg
- Volume = 210 mg ÷ 120 mg × 5 mL = 8.75 mL
There are several paracetamol strengths, so check which liquid you're working with. Follow any instruction to round to a volume that can be measured accurately with an oral syringe.
8. Quantities to supply
Prednisolone 40 mg once daily for 5 days, then reduce by 5 mg every 3 days until stopped. How many 5 mg tablets should you supply?
| Stage | Daily dose | Days | Total |
|---|---|---|---|
| Start | 40 mg | 5 | 200 mg |
| Reduce | 35 mg | 3 | 105 mg |
| 30 mg | 3 | 90 mg | |
| 25 mg | 3 | 75 mg | |
| 20 mg | 3 | 60 mg | |
| 15 mg | 3 | 45 mg | |
| 10 mg | 3 | 30 mg | |
| 5 mg | 3 | 15 mg | |
| Total | 620 mg |
620 mg ÷ 5 mg = 124 tablets
This is one of the most commonly dropped areas. Account for dose, frequency, formulation and duration, and check whether a dose is written as a total daily amount. Remember that capsules can't be split and ampoules are single use.
9. Dilutions
How much 10% w/v stock solution do you need to make 500 mL of a 0.5% w/v solution?
Use C1 × V1 = C2 × V2:
- 10% × V1 = 0.5% × 500 mL
- V1 = 250 ÷ 10 = 25 mL, made up to 500 mL with diluent
Set the equation up the right way round. The stronger solution always needs the smaller volume.
Where do trainees lose the most marks?
The GPhC Board of Assessors publishes feedback after each sitting, and for Part 1 the same themes keep coming up.
The first is not reading carefully: missing a detail, or giving the answer in the wrong units. The second is rounding at the wrong stage. Round individual doses before you work out a total supply, and otherwise round at the end. Then there's unrealistic rounding, so think about what can really be measured or supplied, and follow any rounding instruction in the question. Finally, people ignore dosage form rules. Capsules aren't divisible, and ampoules are single use.
How should I practise?
- Do a few every day. 15 minutes daily beats a 3-hour session once a week.
- Practise by type first, then mix them up, so you have to recognise the type yourself.
- Once you're accurate, time yourself. Aim for 3 minutes per question.
- Keep an error log of every mistake and why it happened. Most trainees make the same three or four errors over and over.
- Use the GPhC's own example questions alongside your practice.
Common questions
How many calculations are in Part 1? 40 questions in 2 hours, with free-entry numerical answers.
Can I use a calculator? Yes, either a GPhC-approved model or the on-screen calculator.
What was the Part 1 pass mark? It changes every sitting. In June 2025 it was 24 out of 40.
Do I need to memorise formulae? Some, yes. You should know common ones such as body mass index and body surface area, while others, like Cockcroft-Gault, may be given in the question. Check the current assessment framework.
Get confident with calculations
Calculations get easier once you understand the method rather than memorising steps. Our Calculations Course works through every type with worked examples, and our question bank is organised by the same nine calculation types so you can drill your weak areas.
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