How do you calculate infusion time?
Divide the volume by the rate. It is the IV rate formula rearranged to solve for time.
Time (h) = Volume (mL) ÷ Rate (mL/h)
The answer comes out in decimal hours, which are easy to misread on a clock. The calculator turns the decimal part into minutes (decimal × 60). If you give a start time, it adds the duration to it. To go the other way, from a set duration to the rate, use the IV rate calculator.
How to use the calculator
- Enter the Volume left to infuse in mL.
- Enter the Rate in mL/h, as set on the pump or worked out from a drip count.
- If you want a finish time, enter the Start time (optional) in 24-hour format.
- Read the Infusion time in hours and minutes and, if you entered a start time, the clock time under Finishes at.
Worked examples
Example 1 – a whole number of hours. 1,000 mL at 125 mL/h, started at 08:00. 1,000 ÷ 125 = 8 h, so the infusion time is 8 h 0 min and it finishes at 16:00.
Example 2 – across midnight. 500 mL at 80 mL/h, started at 22:30. 500 ÷ 80 = 6.25 h. 0.25 × 60 = 15 min, so 6 h 15 min. 22:30 + 6 h 15 min = 04:45 (+1 day).
Example 3 – a bag already running. 350 mL is left at 100 mL/h, and it is now 14:10. 350 ÷ 100 = 3.5 h = 3 h 30 min, finishing at 17:40.
Infusion time chart
Infusion time for common volumes and rates, as the calculator shows it (to the nearest minute).
| Volume | 50 mL/h | 80 mL/h | 100 mL/h | 125 mL/h | 150 mL/h | 250 mL/h |
|---|---|---|---|---|---|---|
| 100 mL | 2 h 0 min | 1 h 15 min | 1 h 0 min | 0 h 48 min | 0 h 40 min | 0 h 24 min |
| 250 mL | 5 h 0 min | 3 h 8 min | 2 h 30 min | 2 h 0 min | 1 h 40 min | 1 h 0 min |
| 500 mL | 10 h 0 min | 6 h 15 min | 5 h 0 min | 4 h 0 min | 3 h 20 min | 2 h 0 min |
| 1,000 mL | 20 h 0 min | 12 h 30 min | 10 h 0 min | 8 h 0 min | 6 h 40 min | 4 h 0 min |
For a gravity infusion, get the real rate first with the drops per minute to mL/h calculator. To keep track of the total given over a shift, the fluid balance calculator adds up intake and output.
Common mistakes
- Reading decimal hours as minutes. 3.5 h is 3 h 30 min and 6.25 h is 6 h 15 min. Reading the digits after the point as minutes gives the wrong finish time.
- Using the full bag volume for a running infusion. Only the volume that is left matters for the finish time.
- Mixing up 12-hour and 24-hour times. Enter start times in 24-hour format to avoid a twelve-hour error.
- Ignoring interruptions. Every pause, line change or rate change pushes the finish time back. It’s worth recalculating after any change.
A quick sense check
At a higher rate the same volume finishes sooner, and halving the rate doubles the time. If the result says a litre will run in under an hour, or a 100 mL bag will take all day, it’s worth checking the rate and the units. The estimate should match the duration in the prescription.
Frequently asked questions
How do I calculate how long an infusion will take?
Divide the volume by the rate. 500 mL at 125 mL/h takes 500 ÷ 125 = 4 hours. The answer comes out in decimal hours, so convert the part after the point into minutes before you read it on a clock. The calculator does both steps and shows them in the working.
How do I turn decimal hours into hours and minutes?
Keep the whole hours and multiply the decimal part by 60. 6.25 h is 6 hours plus 0.25 × 60 = 15 minutes, so 6 h 15 min, not 6 h 25 min. Likewise 3.5 h is 3 h 30 min. Reading the digits after the point as minutes is a common source of wrong finish times.
How is the finish time worked out?
The calculator adds the duration, rounded to the nearest minute, to the start time you enter. If the infusion runs past midnight it shows +1 day. For example, 500 mL at 80 mL/h started at 22:30 takes 6 h 15 min and finishes at 04:45 the next day.
Should I enter the full bag or what is left?
Enter the volume still to be infused. For a bag that is already running, read the remaining volume from the pump or the bag. If you enter the full bag, the finish time comes out too late. With 350 mL left at 100 mL/h at 14:10, the bag finishes at 17:40.
Does this work for gravity infusions?
Yes, as long as you know the rate in mL/h. Count the drops first and convert them with the drops per minute to mL/h calculator, then use that rate here. Gravity rates drift as the bag empties, so it’s worth recounting the drops and recalculating if the finish time matters.
Why does the pump show a slightly different time?
Pumps count down the volume actually delivered, and pauses, bolus doses or rate changes all move the finish time. Some pumps also round the rate or the volume. The result is best treated as a planning estimate, and it’s worth recalculating after any interruption or change of rate.