Infusion Duration and Completion Time: CBT Questions
NMC CBT practice on infusion durations: how long a bag will last, when it finishes, and clock arithmetic across shifts, with worked answers.
Duration questions are the pump-rate formula turned around. Instead of asking what rate delivers a bag in a given time, they hand you the rate and ask when the bag runs out.
Time in hours = volume ÷ rate in mL per hour.
A 1,000 mL bag at 125 mL/hour: 1,000 ÷ 125 = 8 hours. If the division feels familiar, it should; it’s the same three numbers from the pump chapter rearranged.
The exam adds difficulty in two layers.
Decimal hours are not clock minutes
1,000 mL at 80 mL/hour gives 12.5 hours. The .5 is half an hour, thirty minutes, and the finish time from 08:00 is 20:30. The wrong option will be 20:50, built from reading the decimal as minutes. Convert every decimal by multiplying by 60: 0.5 hours is 30 minutes, 0.25 is 15, 0.75 is 45. Awkward decimals like 3.01 hours just mean “about 3 hours” and the options will be spaced to match.
Clock arithmetic
Completion-time questions bolt simple time addition onto the division: started 14:30, runs 5 hours, finishes 19:30. Easy in isolation. Under exam pressure, with a duration like 12 hours 30 minutes crossing a shift boundary, it’s worth doing on the whiteboard in two steps: add the whole hours, then the minutes. Questions that cross midnight follow the same method; 22:00 plus 4 hours is 02:00, and the exam expects the 24-hour clock throughout.
The reverse skill matters on shift: a pump showing 250 mL remaining at 83 mL/hour alarms in about three hours, which is the difference between preparing the next bag now or after handover. The CBT phrases some questions exactly this way, as workload planning rather than abstract maths.
From drip rate back to time
Occasionally a question gives a gravity drip rate and asks for duration. Reverse through the drip factor: drops per minute ÷ drops per mL = mL per minute, then volume ÷ that. So 42 dpm on a standard 20 drops/mL set moves 2.1 mL/min, and a 500 mL bag lasts 238 minutes, call it 4 hours. It looks harder than it is; it’s two divisions, and it only appears when the stem mentions a giving set rather than a pump.
Sanity band: routine maintenance bags run 4 to 12 hours. A calculated duration of 45 minutes or 80 hours for a litre of saline means a slipped decimal in the rate.
Five questions below, including the decimal-hours trap twice, because it’s the one that costs real marks. Then the two chapters that pull all of Part A together: multi-step problems.
Sources & further reading
Frequently asked questions
How do I convert decimal hours to minutes?
What if the rate is in drops per minute rather than mL/hour?
Why do duration calculations matter clinically?
Check your understanding
Quick quiz: Infusion Duration and Completion Time: CBT Questions
5questions. Click an answer to see the explanation. Your score is saved on this device only.
- 1
A 1,000 mL bag runs at 125 mL/hour. How long will it last?
- 2
A 500 mL bag runs by gravity at 42 drops per minute on a standard 20 drops/mL set. Roughly how long will it last?
- 3
An infusion of 500 mL at 100 mL/hour starts at 14:30. When will it finish?
- 4
A pump shows 250 mL remaining, running at 83 mL/hour. Roughly when will it alarm?
- 5
A 1 litre bag starts at 08:00 running at 80 mL/hour. When does it finish?
Keep reading
Infusion Pump Rates: mL per Hour CBT Practice Questions
NMC CBT infusion pump questions: mL/hour from volume and time, dose-calibrated infusions like heparin, and worked answers with the common traps.
IV Drip Rate Calculations: Drops per Minute CBT Practice
NMC CBT drip rate practice questions: the drops-per-minute formula, drip set factors (20, 15, 60), and worked answers for gravity infusions.
Prioritisation and Time Management Questions
NMC CBT prioritisation practice: ranking by clinical urgency, the deteriorating patient first, delegating to free time, and worked scenario answers.