Common Registration Assessment, Part 1
Infusion time and volume remaining
Practise finding how long an infusion lasts, the rate for a set time, and the volume left, using three original questions.
Published 8 October 2026. Independent revision material, not GPhC questions, not for patient care.
Rate, volume and time are linked: volume = rate × time. Questions on infusions ask for any one of the three when you know the other two, and often add a clock time.
The examples are fictional and arithmetic only. Keep time in hours when the rate is per hour, and convert minutes to hours by dividing by 60.
The method
- Write down volume, rate and time, marking the unknown.
- Match the time unit to the rate unit.
- Use time = volume ÷ rate, or rate = volume ÷ time.
- For volume remaining, subtract volume given from the starting volume.
- Round only at the end, as the question instructs.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A 500 mL bag runs at 125 mL/hour. How long does it take?
Show the answer
Answer: 4 hours
Working: 500 ÷ 125 = 4 hours.
Question 2
A fictional 1 litre infusion starts at 09:30 and runs at 80 mL/hour. How much remains at 14:00?
Show the answer
Answer: 640 mL
Working: 09:30 to 14:00 is 4.5 hours. Given = 80 × 4.5 = 360 mL. Remaining = 1000 − 360 = 640 mL.
Question 3
A fictional 250 mL infusion must run over 45 minutes. What rate in mL/hour, to the nearest whole number?
Show the answer
Answer: 333 mL/hour
Working: 45 minutes = 0.75 hours. 250 ÷ 0.75 = 333.3, so 333 mL/hour.
Sense check
If the rate is higher than the volume per hour you expect, the time must be shorter. Compare the answer with a rough estimate.
Where marks are lost
- Leaving minutes in a per-hour calculation.
- Counting the start and end times wrongly when finding elapsed time.
- Forgetting to convert litres to mL.
Frequently asked questions
How do I find elapsed time?
Subtract the start from the end on the 24-hour clock, then convert to hours and fractions of an hour.
When should I round?
Only at the final step and to the level the question asks for.
Sources
- The Pharmaceutical Journal, medication maths: quantities to supply and using provided formulae
- The Pharmaceutical Journal, preparing for the GPhC pharmaceutical calculations assessment
- GPhC, 2026 assessment specification and permitted items
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
- 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
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- Loading dose and volume of distribution
- Top-up doses from measured drug levels
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
- Bioavailability and equivalent oral doses
- Oral maintenance dose and dosing interval
- Elimination rate constant and half-life
- Accumulation towards steady state
- Ratio strengths and one in X
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
- Unit conversions for mass and volume
- Units and units per mL
- Reconstituting powders and final concentration
- Millimoles and milliequivalents
- Days supply from pack sizes
- Percentage w/v and w/w conversions
- Ideal and adjusted body weight from formulae
- Diluting creams and ointments by weight
- Scaling a formula to a batch size
See also the formula sheet, the eight sample questions and the approved calculator page.