Pharmaceutical and Healthcare Calculations: What the GPhC Part 1 Tests

Scope and time budget

Part 1 allows 120 minutes for 40 problems, three minutes each. Part 2 can also contain a calculation inside a clinical scenario, with an on-screen calculator on questions that need one. That means calculation skill is useful across the paper, not only in Part 1.

The real Part 1 requires you to type a number with no options. Our items offer five options built from common errors, so use them to learn the traps, and then cover the options and type your own answer. The calculations paper guide sets out the routine in full.

What is tested

• Doses and regimens. Daily and divided doses, mg/kg dosing, per-dose versus per-day figures, and how to turn a prescription into a number of doses.
• Paediatric dosing. Weight-based and body-surface-area calculations, with a sense-check against the adult dose.
• Unit conversions. Grams, milligrams, micrograms and nanograms, plus millimoles and units. BNF style writes microgram and nanogram in full.
• Kidney function. Estimating clearance from age, weight, sex and serum creatinine, then applying a renal dose band. The stem usually gives the formula.
• Displacement. Displacement volumes for reconstituted powders and displacement values for suppositories.
• Concentrations. Percentage w/v, w/w and v/v, 1 in x ratios and mg/mL.
• Dilutions and mixing. C1V1 = C2V2, alligation and mixing strengths of creams or liquids.
• Molecular weight. Moles, millimoles and milliequivalents, including salts.
• Infusion rates. mL/hour, drops per minute, mg/hour and units/hour from a dose and a time.
• Unit-based dosing. Insulin, heparin and similar, where the answer stays in units.
• Pharmacokinetics. Half-life, steady state, loading dose and clearance, with the equation given.
• Health economics. Cost per quality-adjusted life year, number needed to treat and cost comparisons.
• Quantities to supply. Dose, frequency and duration converted to a quantity and a number of packs.
• Preparations. Fingertip units for creams, ointment mixing and liquid preparations.
• Fluids and nutrition. Electrolyte, fluid and parenteral nutrition calculations.
• Plasma levels. Interpreting levels against a target range and acting on the result.
• Practical measurement. Rounding rules, significant figures and what a syringe can actually measure.

UK guideline anchors

• SI units only. Weights in kg, doses in mg, microgram or units, concentrations in %, mg/mL, 1 in x or mmol/L, eGFR in mL/min/1.73 m2, creatinine in micromol/L, glucose in mmol/L, HbA1c in mmol/mol. Expect no mg/dL and no degrees Fahrenheit.
• Cockcroft-Gault. Uses actual body weight in mL/min in BNF-style questions. Learn the formula and the sex constants, and expect it to be supplied.
• Rounding. GPhC feedback says candidates often miss rounding instructions. Follow the stem exactly.
• BNF and BNFc. Dose ranges and age bands are given in the stem where a candidate would otherwise need the book.

Conversions to know without thinking

These relationships are used in every topic from dilutions to infusions. If any of them takes you more than a second, drill them until it does not. Speed on the basics is what buys time for the multi-step problems.

How the topics combine

Real questions rarely test one topic in isolation. An infusion problem may begin with a weight-based dose, require a unit conversion and end with a rate. A reconstitution question may need a displacement value and then a dilution. A renal question may ask you to estimate clearance and then choose a dose from a band. When you revise, practise chains of two and three topics, because that is where the three-minute limit bites.

A useful habit is to label each line of working with the step it performs: convert, dose, volume, rate, round. If you cannot label a line, you probably do not need it, or you have skipped a step. Labelling also makes your own review faster when you return to a flagged question.

Calculations that sit inside Part 2

Part 2 can also contain calculations in a clinical setting. These tend to be shorter, with the data in the stem and a calculator available on the questions that need one. They reward the same unit discipline as Part 1, but you have only about 75 seconds. Practise the quick ones: a daily dose from a regimen, the number of tablets to supply, or a volume from a concentration. These are the ones to do quickly so that time remains for the harder items.

Three typical traps

1. The unit trap. A microgram answered in milligrams is a thousand-fold error. Convert first and keep units on every line.
2. The extra-step trap. Multi-step problems, such as dose to volume to rate, lose marks when one line is skipped. Write each step.
3. The rounding trap. An answer rounded to the wrong decimal place, or to a figure that cannot be measured, is wrong even if the working is right.

How to revise it

1. Do a short set every day. Twenty to thirty minutes, rotating through the twelve topics.
2. Write units on every line. It catches most conversion errors before they reach the answer.
3. Sense-check before typing. Compare the answer with the usual dose or the size of the bag.
4. Practise without options. Cover the choices, type your answer, then check.
5. Keep an error log by cause. Conversion, method, arithmetic, rounding, misread. Fix the commonest cause first.

Next, read the calculations paper guide and place daily practice in your study plan. Return to the hub for the full blueprint.