Part 1 of the GPhC registration assessment is 40 numerical free-entry questions in 120 minutes. You type the number — no multiple choice, no safety net of elimination. In June 2025, 16% of candidates failed Part 1 alone. The calculation types are defined in the CRA framework. They are finite. They are predictable. And they are drillable — if you drill the right types.
This guide maps each calculation type from the CRA framework with worked examples, the common errors that cost candidates marks, and how to drill each one effectively using iatroX.
Type 1: Dose and Dose Regimen Calculations
What it tests: Calculating a single dose, a daily dose, or a course dose from a prescribed regimen. The CRA uses various dose expressions — definite frequency (500mg every 8 hours), total daily dose (150mg/kg/day in 3 divided doses), and dose per kilogram.
Worked example: Amoxicillin 500mg three times daily for 7 days. How many capsules should be supplied? 3 × 7 = 21 capsules.
Common error: Confusing "three times daily" with "every 8 hours" when they produce different total daily doses (they do not in this case, but dose expressions like "1g twice daily" vs "500mg four times daily" produce the same daily dose with different individual doses).
Type 2: Unit Conversions
What it tests: Converting between units — mg to mcg, g to mg, mL to L, mmol to mg.
Worked example: Convert 0.25mg to micrograms. 0.25 × 1000 = 250 micrograms.
Common error: The tenfold error in mcg-to-mg conversion. Digoxin 250 micrograms misread as 250mg would be a 1000-fold overdose. The exam tests whether you can navigate these conversions without error.
Type 3: IV Infusion Rates
What it tests: Calculating mL/hour, drops/minute, or infusion duration. The Board of Assessors feedback specifically notes that candidates should consider all relevant information — infusion time, concentration, and rate.
Worked example: 500mL normal saline to be infused over 4 hours. Rate = 500 ÷ 4 = 125 mL/hour. If the giving set delivers 20 drops/mL: drops/min = (125 × 20) ÷ 60 = 41.7 ≈ 42 drops/min.
Common error: Forgetting to convert hours to minutes when calculating drops/minute.
Type 4: Displacement Values
What it tests: Calculating the correct volume of diluent to add when reconstituting a powder for injection, accounting for the volume occupied by the powder itself.
Worked example: Amoxicillin 250mg injection. Displacement value: 0.2mL per 250mg. Required final volume: 5mL. Volume of water to add: 5 - 0.2 = 4.8mL.
Common error: Ignoring the displacement value entirely and adding the full final volume of diluent — resulting in a lower concentration than intended and an underdose.
Type 5: Concentration Expressions
What it tests: %w/v, %w/w, 1:x ratios, parts per million, and molarity.
Worked example: What is the concentration of adrenaline 1:1000 in mg/mL? 1:1000 = 1g in 1000mL = 1000mg in 1000mL = 1mg/mL.
Common error: Confusing 1:1000 (1mg/mL) with 1:10,000 (0.1mg/mL). In emergency medicine, this distinction is life-critical (IV adrenaline in cardiac arrest uses 1:10,000; IM adrenaline in anaphylaxis uses 1:1000).
Type 6: Creatinine Clearance and Renal Dose Adjustments
What it tests: Cockcroft-Gault equation to estimate CrCl, then applying the result to BNF dose adjustment thresholds.
Formula: CrCl = [(140 - age) × weight × constant] / serum creatinine. Constant: 1.23 (male), 1.04 (female). Creatinine in µmol/L.
Worked example: 72-year-old female, 58kg, creatinine 142 µmol/L. CrCl = [(140-72) × 58 × 1.04] / 142 = [68 × 58 × 1.04] / 142 = 4102 / 142 = 28.9 mL/min. BNF dose adjustments for drugs renally excreted would apply at this level.
Type 7: Paediatric Weight-Based Dosing
What it tests: Calculating doses using mg/kg, checking against maximum dose thresholds, and determining volumes from available preparations.
Worked example: Child weighs 15kg. Ibuprofen dose: 5mg/kg three times daily. Single dose: 75mg. Available as 100mg/5mL. Volume per dose: (75/100) × 5 = 3.75mL.
Common error: Not checking the calculated dose against the maximum single or daily dose in BNFc.
Type 8: Quantities to Supply
What it tests: Calculating the total quantity of medicine to supply for a given course — number of tablets, volume of liquid, number of injections. The Board of Assessors notes that rounding should occur for individual doses before calculating the final supply quantity.
Worked example: Metformin 500mg twice daily for 28 days. Quantity: 2 × 28 = 56 tablets.
Common error: Rounding errors when the calculation involves liquids — rounding the volume per dose to a measurable amount before multiplying by the number of doses.
Type 9: Dilutions
What it tests: Calculating the volumes needed to dilute a concentrated solution to a required concentration, or determining the final concentration after dilution.
Worked example: You have 10mL of 20% w/v chlorhexidine. How much water must you add to make a 2% w/v solution? C₁V₁ = C₂V₂. 20 × 10 = 2 × V₂. V₂ = 100mL. Water to add: 100 - 10 = 90mL.
How to Drill With iatroX
iatroX tracks your accuracy and speed across each of these 9 types independently. The adaptive engine identifies which types you are slowest or least accurate on and serves more of those — with difficulty scaling as you improve. The performance dashboard shows your proficiency by calculation type, not just an aggregate score.
Drill timed. Always. From day one. 40 questions in 120 minutes is 3 minutes per question — and that includes reading the question, extracting the data, performing the calculation, checking the answer, and entering it. Speed matters as much as method.
Start at iatrox.com/quiz-landing?exam=uk-gphc.