IV Infusion Time Calculation: How Long a Bag Will Last and When It Finishes
Divide the volume by the rate to get the time in hours: 1,000 mL at 125 mL/hr runs for 8 hours. Turn any decimal part into minutes by multiplying by 60, then add the time to the start time to find when it finishes.
The Formula
Completion time = start time + infusion time, in 24-hour clock.
How to work it out
- 1 Divide the volume by the rate.
- 2 Separate the whole hours from the decimal. 6.25 hours is 6 hours plus 0.25 × 60 = 15 minutes.
- 3 Add to the start time. Started at 0900, 6 hr 15 min finishes at 1515.
- 4 If the question asks for hours as a decimal, give the decimal; if it asks for a clock time, give the clock time. Read which.
Worked Examples
1,000 mL is running at 125 mL/hr. How long will it take to infuse?
- 1,000 mL ÷ 125 mL/hr = 8 hr.
Answer: 8 hr
500 mL is started at 1000 at 80 mL/hr. When will it finish?
- 500 ÷ 80 = 6.25 hr.
- 0.25 hr × 60 = 15 min, so 6 hr 15 min.
- 1000 + 6 hr 15 min = 1615.
Answer: 1615, after 6 hr 15 min
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Common Mistakes
- Reading 6.25 hours as 6 hours 25 minutes
The decimal part is a fraction of an hour. 0.25 hr is 15 minutes, not 25.
- Dividing rate by volume
Time is volume divided by rate. A tiny answer for a big bag means the division is upside down.
- Forgetting to add the minutes
When the question asks for a completion time, carry the minutes across. 0900 plus 6.25 hr is 1515, not 1500.
Questions Students Ask
How do I handle a rate in drops per minute?
Convert to mL/hr first: gtt/min ÷ drop factor × 60. Then divide the volume by that rate.
Why does the exam use 24-hour time?
Because hospitals do. 1515 cannot be mistaken for 3:15 in the morning.
Keep practicing in lesson 11: Infusion Time and Completion Time
Fresh numbers every time. Ten correct answers finish the lesson. The practice round and the first five problems of lesson 1 are free.
Open lesson 11