GPhC calculations: 8 question types with worked examples

6 November 2026 · 4 min read · By the PharmacySkills team

Calculations are the part of the registration assessment people fear most, and also the part where practice pays off most reliably. There are only a handful of question types. Once you recognise which type you are looking at, the method follows.

Here are the eight types we see most often, each with a worked example. Cover the answer, try it yourself, then check.

1. Dose by body weight

The most common question of all. Work out the dose, then convert it into a volume.

A child weighing 22 kg is prescribed a medicine at 7.5 mg/kg twice daily. The suspension contains 125 mg in 5 mL. What volume is each dose?

Dose = 7.5 mg × 22 kg = 165 mg.

There are 125 mg in 5 mL, so 165 mg is in 165 ÷ 125 × 5 = 6.6 mL.

Watch out for: "per day in divided doses" versus "per dose". Read the question twice.

2. Percentage strength (w/v)

"% w/v" means grams per 100 mL. That is all you need to remember.

How many grams of sodium chloride are in a 500 mL bag of sodium chloride 0.9% w/v?

0.9% w/v = 0.9 g in 100 mL. So 500 mL contains 0.9 × 5 = 4.5 g.

3. Dilutions

Use C1 × V1 = C2 × V2: the amount of drug stays the same, only the volume changes.

You have a 20% w/v stock solution. How much do you need to make 250 mL of a 2% w/v solution?

V1 = (2 × 250) ÷ 20 = 25 mL of stock, made up to 250 mL with diluent.

Sense check: you are diluting tenfold (20% to 2%), so you need one tenth of the final volume. 25 mL is one tenth of 250 mL.

4. Ratio strengths

"1 in 1,000" means 1 g in 1,000 mL. Convert it to mg/mL and the question becomes easy.

Adrenaline is available as 1 in 1,000. How many milligrams are in 0.5 mL?

1 g in 1,000 mL = 1,000 mg in 1,000 mL = 1 mg/mL. So 0.5 mL contains 0.5 mg.

Remember that 1 in 10,000 is ten times weaker: 0.1 mg/mL.

5. Infusion rates

You may be asked for mL per hour or drops per minute.

1 litre of fluid is to be given over 8 hours using a giving set that delivers 20 drops/mL. What is the rate in mL/hour and in drops/minute?

mL/hour = 1,000 ÷ 8 = 125 mL/hour.

Drops/minute = (1,000 mL × 20 drops/mL) ÷ (8 × 60 minutes) = 20,000 ÷ 480 = 41.7, so 42 drops/minute.

6. Millimoles

Convert grams to millimoles using the molecular mass: mmol = mg ÷ molecular mass.

How many millimoles of sodium are in 500 mL of sodium chloride 0.9%? (Na = 23, Cl = 35.5)

From example 2, 500 mL contains 4.5 g = 4,500 mg of sodium chloride. Molecular mass of NaCl = 23 + 35.5 = 58.5.

4,500 ÷ 58.5 = 76.9 mmol of sodium chloride, and one sodium per molecule, so about 77 mmol of sodium.

7. Renal function (Cockcroft–Gault)

Many doses in the BNF are adjusted by creatinine clearance, which you estimate with the Cockcroft–Gault equation:

CrCl (mL/min) = (140 − age) × weight (kg) × constant ÷ serum creatinine (micromol/L)
constant = 1.23 for men, 1.04 for women

A 70-year-old man weighs 80 kg. His serum creatinine is 120 micromol/L. Estimate his creatinine clearance.

(140 − 70) × 80 × 1.23 ÷ 120 = 6,888 ÷ 120 = 57.4 mL/min.

Watch out for: whether the question wants actual or ideal body weight. It will tell you.

8. Unit conversions and days' supply

Simple, but they catch tired people at the end of the paper.

A patient takes 62.5 micrograms of a drug each morning. Tablets are 125 micrograms and scored. How many tablets for 28 days?

62.5 micrograms is half a tablet. 0.5 × 28 = 14 tablets.

Remember: 1 mg = 1,000 micrograms, and 1 microgram = 1,000 nanograms. Write micrograms in full in practice; "mcg" and "µg" cause errors.

How to practise

• Ten a day. Little and often beats occasional marathons.
• Always sense check. Before moving on, ask "is this answer plausible?" A 600 mL dose of oral suspension for a toddler is a sign you have slipped a decimal point.
• Write every step. On the day you can retrace and find a slip in seconds.
• Time yourself. Part 1 gives you about three minutes per question, so practise at that pace once your accuracy is good.

For a full revision plan, see how to pass the GPhC registration assessment.

Sources: BNF: Prescription writing; BNF: Prescribing in renal impairment; GPhC registration assessment framework.

This article is for revision and education. Always check the current BNF, NICE guidance and your local policies before applying it in practice.