Part 1 of the General Pharmaceutical Council (GPhC) Registration Assessment is 40 calculations in 2 hours. Knowing the main calculation categories and which areas carry the most weight lets you target your practice where it matters most.
This guide breaks down the main calculation categories you should expect, based on the assessment framework.
Doses and dosing
This is one of the broadest categories in the framework. You need to be comfortable with:
- Single-dose calculations — given a dose in mg/kg and a patient weight, calculate the dose.
- Daily dose and divided doses — working out total daily dose and then splitting it into the correct number of administrations.
- Dose adjustments for renal impairment — calculating adjusted doses based on creatinine clearance or eGFR. You may need to use the Cockcroft-Gault equation.
- Paediatric doses — doses based on weight or body surface area. Pay close attention to whether the dose is per kg per dose or per kg per day.
- Loading doses and maintenance doses — understanding the difference and calculating each.
The key pitfall here is unit errors. A dose might be given in micrograms but the available formulation is in milligrams. If you do not convert correctly, you will be out by a factor of a thousand.
Concentrations and dilutions
These questions test whether you can work with solutions of different strengths:
- Percentage concentrations — w/v (grams per 100 mL), w/w (grams per 100 grams), v/v (mL per 100 mL). Know the definitions cold.
- Ratio strengths — 1 in 1000, 1 in 10,000, etc. You must be able to convert between ratio strength and percentage or mg/mL without hesitation.
- Dilutions — using C1V1 = C2V2 or equivalent reasoning to calculate how much of a concentrated solution is needed to make a diluted one.
- Serial dilutions — worth understanding if your practice material includes multi-step dilution questions.
Adrenaline (epinephrine) is the classic example tested here. Adrenaline 1 in 1,000 contains 1 mg per mL. Adrenaline 1 in 10,000 contains 100 micrograms per mL. Mixing these up in clinical practice is dangerous, and mixing them up in the exam loses you marks.
Infusion rates
Infusion rate calculations are a core Part 1 skill:
- Volume over time — given a volume to infuse and a time period, calculate the rate in mL/hour.
- Dose-based rates — given a required dose in mg/hour or micrograms/kg/min and a known concentration, calculate the infusion rate.
- Drop rates — converting mL/hour to drops per minute using a giving set factor (commonly 20 drops/mL for standard sets, 60 drops/mL for microdrop sets).
The trap in infusion questions is usually the units. A question might give you the dose in micrograms/kg/min, the concentration in mg/mL, and the patient weight in kg. You need to chain several conversion steps without error.
Moles and millimoles
These questions test your understanding of amount-of-substance calculations:
- Moles to grams — using molecular weight to convert between mass and moles.
- Millimoles in a given volume — if a solution contains a known concentration in mmol/L, calculating the number of millimoles in a specific volume.
- Electrolyte calculations — particularly sodium, potassium, calcium. Working out how many millimoles of an electrolyte are in a given infusion bag or oral supplement.
Make sure you know the molecular weights of commonly tested electrolytes and compounds. The question may or may not provide the molecular weight — if it does not, you are expected to know or calculate it from atomic masses.
Displacement values
Displacement values come up in reconstitution calculations for injectable medicines:
- A powder occupies space when dissolved, so adding the stated volume of diluent does not give you the stated final volume.
- You need to calculate the actual volume after reconstitution using the displacement value, then work out the correct volume to draw up for the required dose.
These questions can catch candidates out because the method is less familiar. Once you understand the principle, they are straightforward — but you need to have done enough practice that the method is automatic.
Unit conversions
Unit conversion underpins almost every other calculation type, but some questions test it directly:
- Milligrams to micrograms (multiply by 1,000)
- Grams to milligrams (multiply by 1,000)
- Litres to millilitres (multiply by 1,000)
- Micrograms to nanograms (multiply by 1,000)
- Parts per million (ppm) — 1 ppm = 1 mg per litre for aqueous solutions
Get these wrong and everything built on top of them is wrong too. Write out your conversions explicitly every time rather than trying to do them in your head.
Quantity to supply
These are dispensing-focused calculations:
- How many tablets or capsules to supply for a given course. Dose frequency multiplied by duration, accounting for any loading doses.
- Volume of liquid to supply — similar logic but converting doses to volumes using the concentration of the liquid formulation.
- Divided supplies — if supplying an instalment prescription (common for Schedule 2 CDs), calculating the correct amount per instalment.
These feel simpler than clinical calculations, but careless errors are easy. Always check whether the question asks for whole packs or exact quantities, and whether you need to round up or down.
How to structure your calculation practice
Do not just practise your strong categories. Use this list to identify which types you are least confident with, and allocate more practice time there.
A balanced weekly approach might look like:
- Daily: 10 to 15 mixed calculations under timed conditions (3 minutes per question mirrors the exam pace)
- Twice a week: A focused session on one specific category — especially whichever type is generating the most errors
- Weekly: Review your error log to see which categories are causing the most mistakes
Quick FAQs
- Which calculation type is tested most often? There is not much value in trying to predict a single "most common" type. The safer approach is to prioritise the core framework areas: dosing, concentrations and dilutions, infusion rates, unit conversions, quantity to supply, and enough practice with moles and displacement values that the method is familiar.
- Do I get a calculator in the exam? Yes. An on-screen calculator is provided for Part 1. You do not need to do mental arithmetic, but you do need to set up the calculation correctly before using the calculator. Most errors are in the setup, not the arithmetic.
- Are formulas provided? Some may be provided in the question stem, but you should not rely on this. Know the standard formulas — Cockcroft-Gault, infusion rate, C1V1=C2V2, percentage concentrations, moles calculations — so that you can apply them without prompting.
- How fast do I need to be? You have 2 hours for 40 questions, which is 3 minutes per question. Some calculations are quicker than others, so aim to finish straightforward ones in under 2 minutes to bank time for more complex multi-step problems.