In November 2024, 42% of GPhC CRA candidates failed. Part 1 calculations is where the majority of those failures originate. Forty questions in two hours — three minutes per question — with the BNF open. And yet the pass rate for Part 1 alone hovers around 84% at best, meaning roughly 1 in 6 candidates fail before their Part 2 performance even matters.
The candidates who fail Part 1 are not candidates who cannot do mathematics. They are candidates who have not drilled the specific calculation types under timed conditions with the BNF open. They know the method when they see it in a worked example. They cannot execute it in three minutes when the question stem is unfamiliar and the clock is running.
This is what you will face, what goes wrong, and how to fix it.
The 8 Calculation Types
1. IV Infusion Rates
What it looks like: A patient requires 1g vancomycin in 250mL sodium chloride 0.9% infused over 100 minutes. Calculate the infusion rate in mL/hour.
The method: Rate (mL/hr) = Volume (mL) ÷ Time (hours). 250 ÷ (100/60) = 250 ÷ 1.667 = 150 mL/hour.
The most common error: Failing to convert minutes to hours. Candidates divide 250 by 100 and get 2.5 — which is mL per minute, not mL per hour. Under time pressure, the unit mismatch is not caught.
2. Weight-Based Dosing
What it looks like: A child weighs 18kg. Dose: gentamicin 7mg/kg once daily. Calculate the dose in mg.
The method: 7 × 18 = 126mg.
The most common error: Not checking against the BNFc maximum dose. A heavier child might produce a calculated dose that exceeds the ceiling — and the exam tests whether you catch this. The second error: confusing mg/kg per dose with mg/kg per day when the regimen involves multiple doses.
3. Creatinine Clearance and Dose Adjustment
What it looks like: A 74-year-old woman, 52kg, serum creatinine 148µmol/L. Calculate CrCl using Cockcroft-Gault. The BNF recommends dose reduction when CrCl <30mL/min.
The method: CrCl = [(140-74) × 52 × 1.04] / 148 = [66 × 52 × 1.04] / 148 = 3569 / 148 = 24.1 mL/min. CrCl <30 — dose reduction required.
The most common error: Using the male constant (1.23) for a female patient. Or using eGFR from the lab report instead of calculating CrCl — they are not interchangeable for drug dosing purposes.
4. Displacement Values
What it looks like: Ceftriaxone 1g powder for injection. Displacement value: 0.4mL per 1g. Required final volume: 10mL. Calculate the volume of water for injection to add.
The method: 10 - 0.4 = 9.6mL.
The most common error: Ignoring displacement entirely and adding 10mL — producing a lower concentration than intended. The Board of Assessors feedback flags this specifically and repeatedly.
5. Concentration and Dilution
What it looks like: You have 500mL of 10% w/v glucose solution. How many grams of glucose does this contain?
The method: 10% w/v = 10g per 100mL. In 500mL: 50g.
The most common error: Confusing %w/v (g per 100mL) with %w/w (g per 100g). For aqueous solutions, w/v is standard. For creams and ointments, w/w applies. The exam exploits this ambiguity.
6. Unit Conversions
What it looks like: A prescription states fentanyl 75 micrograms. Express this dose in milligrams.
The method: 75 ÷ 1000 = 0.075mg.
The most common error: Moving the decimal point in the wrong direction. 75 micrograms becomes 7.5mg instead of 0.075mg — a 100-fold overdose. This specific error kills patients in practice and kills scores in exams.
7. Liquid Formulations and Quantities to Supply
What it looks like: Amoxicillin 125mg/5mL suspension. Dose: 250mg three times daily for 5 days. Calculate the total volume to supply.
The method: Volume per dose: (250/125) × 5 = 10mL. Daily: 30mL. Course: 150mL.
The most common error: Rounding per-dose volumes before multiplying by frequency. The Board of Assessors specifically notes that rounding should occur at the individual dose level when the preparation has measurable graduations — but the final supply should be calculated from the unrounded dose in multi-step calculations unless otherwise instructed.
8. Drip Rates
What it looks like: 1000mL sodium chloride 0.9% over 8 hours. Giving set: 20 drops/mL. Calculate the drip rate in drops/minute.
The method: Volume per hour: 1000/8 = 125mL/hr. Volume per minute: 125/60 = 2.083mL/min. Drops per minute: 2.083 × 20 = 41.7 ≈ 42 drops/min.
The most common error: Using the wrong giving set factor (15 vs 20 vs 60 drops/mL) because the candidate did not read the question stem carefully. Under time pressure, this is the first thing that slips.
Why the BNF Open Is Not the Safety Net You Think
Candidates hear "BNF open" and relax. They should not. Having the BNF open means you can look up a dose, a maximum, a formulation strength, or a renal threshold. It does not mean you can afford to spend 45 seconds finding it.
Three minutes per question includes reading the stem, extracting the relevant numbers, performing the calculation, checking the answer, and entering it. If you spend 45 seconds navigating the BNF to find the paediatric dose of amoxicillin — scrolling past the adult section, finding the correct age band, confirming the frequency — you have two minutes and fifteen seconds left for the calculation itself. That is tight for a multi-step problem.
The fix: practise with the BNF open from day one. Know the BNF layout well enough that you can find any dose in under 20 seconds. Practise looking up paediatric doses, creatinine clearance thresholds, and formulation strengths under time pressure. The BNF is a tool — and like any tool, it requires practice to use efficiently.
The specific BNF sections you need to navigate quickly: drug monographs (dose by indication, dose by age/weight for paediatrics, dose adjustments in renal impairment), the interactions section (for questions asking about co-prescribed medications), and the formulation information (available strengths, available preparations — essential for quantity-to-supply calculations). Practise finding each of these for common drugs — amoxicillin, metformin, ramipril, prednisolone, co-amoxiclav — until navigation is automatic.
The Timed Drilling Protocol That Actually Works
Static practice — doing 20 untimed calculations on a Saturday afternoon — builds method familiarity but not exam readiness. The exam tests method under time pressure, and the two are different skills.
The protocol: from week 1 of your calculations revision, every single practice session should be timed. Set a timer. Three minutes per question. If you cannot finish the question in three minutes, flag it and move on — just as you would in the real exam. After the session, go back to the flagged questions and identify why they took too long. Was it a BNF lookup that was slow? A multi-step calculation where you lost track? A unit conversion where you second-guessed yourself?
The diagnostic value is in the flagged questions, not the ones you got right. Those are the specific types and sub-skills that need additional drilling. An adaptive engine like iatroX automates this — it tracks both accuracy and speed by calculation type, identifies the types where you are slowest, and serves more of those. But even without an adaptive platform, the timed-and-flag protocol is more effective than untimed practice by a significant margin.
Aim for: 3 timed sessions per week of 20 questions each during weeks 2-3 of your revision plan. By week 4, you should be completing 40 questions in 120 minutes (full Part 1 simulation) at least once per week.
How iatroX Replicates Exam Conditions
iatroX provides Part 1 calculation questions in the exact exam format — numerical free-entry, timed, across all 8 types. But it goes further than replication.
The adaptive engine tracks your accuracy and speed across each calculation type independently. If your IV infusion rate questions are accurate and fast but your displacement value questions are slow and error-prone, the engine concentrates practice on displacement — not a rotation through all types equally. Difficulty scales within each type as your performance improves.
After each question, the explanation walks through the method step by step — not just "the answer is 42 drops/min" but "here is why, here is where the unit conversion happens, here is the step where most candidates make the error, and here is how to catch it." This is the difference between a correct/incorrect flag and a worked explanation that actually teaches.
Try 10 free calculation questions at iatrox.com/quiz-landing?exam=uk-gphc.