Common Registration Assessment, Part 1
Rounding doses to measurable volumes
A child's dose rarely comes out as a neat volume. Three original questions on rounding each dose before working out the course.
Published 5 October 2026. Independent revision material, not GPhC questions, not for patient care.
The Pharmaceutical Journal's rounding guide includes a clobazam question where each dose has to be rounded to the nearest whole mL to help the parents give it, and the two weeks' supply is built from the rounded dose. Its dose and dosage tutorial adds a mg/kg/day dose divided three times a day.
The rule is to round the single dose when the scenario says so, then use the rounded dose for every later step. Round up the number of bottles only at the end.
The method
- Work out the total dose in mg, then the single dose.
- Convert the single dose to mL using mg/mL.
- Round the single dose as the scenario instructs.
- Use the rounded dose for the day, then the course, and round packs up last.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A child weighing 18 kg is prescribed 250 micrograms/kg twice daily. The suspension is 10 mg in 5 mL and each dose is rounded to the nearest 0.5 mL for an oral syringe. How many mL are needed for 14 days?
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Answer: 70 mL
Working: 18 × 250 = 4,500 micrograms = 4.5 mg. 4.5 ÷ 2 mg/mL = 2.25 mL, which rounds to 2.5 mL. 2.5 × 2 × 14 = 70 mL.
Question 2
A child weighing 11 kg is prescribed 7.5 mg/kg/day in 3 divided doses. The suspension is 250 mg in 5 mL and each dose is rounded to the nearest 0.1 mL. What total volume is given in 7 days?
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Answer: 12.6 mL
Working: 11 × 7.5 = 82.5 mg a day. ÷ 3 = 27.5 mg a dose. 250 mg in 5 mL is 50 mg/mL, so 27.5 ÷ 50 = 0.55 mL, which rounds up to 0.6 mL. 0.6 × 3 × 7 = 12.6 mL.
Question 3
A child takes 4.5 mL twice a day for 14 days. The suspension is supplied in 100 mL bottles. How many bottles are needed?
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Answer: 2 bottles
Working: 4.5 × 2 × 14 = 126 mL. 126 ÷ 100 = 1.26, so 2 bottles are needed because part of a bottle cannot be supplied.
What the sources say
The rounding guide's clobazam example rounds 2.375 mL to 2 mL per dose before totalling 56 mL over two weeks. The dose tutorial's antibiotic example divides a mg/kg/day dose into three doses and asks for the volume per dose to two decimal places. The numbers on this page are different.
Where marks are lost
- Multiplying the unrounded volume by days, then rounding the total.
- Rounding the wrong direction at an exact half.
- Rounding down the number of bottles.
Frequently asked questions
Why does rounding the dose first change the total?
The patient can only be given the rounded volume each time, so every dose in the course is that volume.
How is 0.55 mL rounded to one decimal place?
Up to 0.6 mL, because the digit is 5.
Sources
- The Pharmaceutical Journal, a guide to rounding for pharmacy students and trainees (27 October 2021)
- The Pharmaceutical Journal, medication maths: how to solve dose and dosage calculations (video tutorial, updated 25 March 2025)
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional drugs. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
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More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Electrolyte content over time
- Infusion dose per kg per hour
See also the formula sheet, the eight sample questions and the approved calculator page.