Common Registration Assessment, Part 1
Infusion rate calculations in mL per hour
Three original questions on volume over time, with the minutes-to-hours step shown every time.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
Infusion rate questions ask for volume divided by time. The only trap is the time unit: pumps run in mL per hour, but the question often gives minutes.
Write the time in hours before you divide, then check the answer against a rough figure.
The method
- Convert the time to hours (minutes ÷ 60).
- Divide the volume by the time in hours.
- If asked for the dose rate, multiply the mL/h by the concentration.
- Check against a rough estimate.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
500 mL of fluid is to run over 4 hours. What is the rate in mL/h?
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Answer: 125 mL/h
Working: 500 ÷ 4 = 125 mL/h.
Question 2
750 mL is to be given over 90 minutes. What is the rate in mL/h?
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Answer: 500 mL/h
Working: 90 minutes = 1.5 hours. 750 ÷ 1.5 = 500 mL/h.
Question 3
An infusion of 250 mg in 50 mL is to run over 30 minutes. What is the rate in mL/h and how many mg are given each hour?
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Answer: 100 mL/h and 500 mg/h
Working: 30 minutes = 0.5 hours. 50 ÷ 0.5 = 100 mL/h. 250 ÷ 0.5 = 500 mg/h.
Context from the GPhC-focused series
The Pharmaceutical Journal's infusion article is part of a series written for the GPhC registration assessment. Its aims include using an infusion rate to find the total volume a patient receives, calculating the product a patient receives from a dose in mg/kg, and following instructions about adjusting rates and maximum rates that must not be exceeded. Read each stem for those instructions.
Where marks are lost
- Dividing by minutes when the answer is wanted per hour.
- Rounding 1.5 hours to 2.
- Forgetting that the concentration links mL to mg.
Frequently asked questions
How do I convert minutes to hours?
Divide by 60. 45 minutes is 0.75 hours, 90 minutes is 1.5 hours.
What if the question asks for a dose per minute?
Work out per hour first, then divide by 60, or follow the units the stem uses.
Sources
- The Pharmaceutical Journal, medication maths: infusion rates (4 April 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
Try 15 free questions with worked answers. No sign-up required.
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
- Drip rate calculations: drops per minute
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
See also the formula sheet, the eight sample questions and the approved calculator page.