How do you calculate an IV rate in mL/h?
Divide the volume by the time in hours. The result tells the pump how many millilitres to give each hour.
Rate (mL/h) = Volume (mL) ÷ Time (h)
The time must be in hours. Convert any minutes first: minutes ÷ 60 gives the fraction of an hour. So 6 h 30 min is 6 + 30 ÷ 60 = 6.5 h, and 45 minutes on its own is 0.75 h.
How to use the calculator
- Enter the Volume to infuse in mL.
- Enter the whole hours in Time: hours and any remaining minutes in and minutes. For an infusion under an hour, leave the hours empty and type only the minutes.
- Read the Pump rate in mL/h. The rate Per minute sits underneath as a cross-check.
- Check the working underneath to see the time conversion and the division.
Worked examples
Example 1 – whole hours. 1,000 mL is to run over 8 hours. 1,000 ÷ 8 = 125 mL/h (about 2.08 mL/min).
Example 2 – minutes only. 100 mL is to run over 30 minutes. 30 min ÷ 60 = 0.5 h, then 100 ÷ 0.5 = 200 mL/h. Dividing by 30 instead gives 3.33, which is the rate per minute, not per hour.
Example 3 – hours and minutes. 500 mL is to run over 6 hours 30 minutes. 6 + 30 ÷ 60 = 6.5 h, then 500 ÷ 6.5 = 76.92, shown as 76.9 mL/h.
The dosage calculation methods guide works a rate problem like these by formula and by dimensional analysis.
IV rate quick reference
Pump rate in mL/h for common volumes and running times, to one decimal place as the calculator shows it.
| Volume | 1 h | 2 h | 4 h | 6 h | 8 h | 12 h |
|---|---|---|---|---|---|---|
| 100 mL | 100 | 50 | 25 | 16.7 | 12.5 | 8.3 |
| 250 mL | 250 | 125 | 62.5 | 41.7 | 31.3 | 20.8 |
| 500 mL | 500 | 250 | 125 | 83.3 | 62.5 | 41.7 |
| 1,000 mL | 1,000 | 500 | 250 | 166.7 | 125 | 83.3 |
mL/h, drops and finish time
A pump takes mL/h. A gravity infusion needs drops per minute instead, which depend on the giving set: convert with the drip rate calculator. To know when the bag will run out at a given rate, use the infusion time calculator. For drugs prescribed in mcg/kg/min, such as vasopressors, the mcg/kg/min infusion calculator gives the mL/h directly.
Common mistakes
- Dividing by minutes instead of hours. 100 mL over 30 min is 200 mL/h, not 3.3 mL/h. The answer is off by a factor of 60.
- Writing 6 h 30 min as 6.3 h. Thirty minutes is half an hour, so it is 6.5 h. Let the minutes box do the conversion.
- Using the bag size instead of the volume to infuse. If part of the bag has already run, or only part is prescribed, enter the volume that actually remains.
- Forgetting extra volume. Line priming, flushes and added drugs can change the total. It’s worth checking what your local policy counts.
A quick sense check
A longer time gives a slower rate, and twice the volume over the same time gives twice the rate. Most adult maintenance fluids run at tens to low hundreds of mL/h. A result in the thousands, or below 1 mL/h, usually means hours and minutes got mixed up. It’s advisable to check the final rate against the prescription.
Frequently asked questions
What is the formula for mL/h?
Divide the volume in mL by the time in hours. For example, 1,000 mL over 8 hours is 1,000 ÷ 8 = 125 mL/h. If the time includes minutes, turn it into hours first. The calculator does that conversion for you and shows every step, so you can check it against your own working.
How do I work out mL/h for an infusion shorter than an hour?
Turn the minutes into hours first, then divide. 100 mL over 30 minutes is 100 ÷ 0.5 = 200 mL/h, not 100 ÷ 30. In the calculator, leave the hours empty or at 0 and type the minutes in the second box. The conversion to hours appears in the working.
Can I enter hours and minutes together?
Yes. Put the whole hours in the first box and the remaining minutes in the second. The calculator adds them as hours, so 6 h 30 min becomes 6.5 h before the division. That avoids the common slip of writing 6 h 30 min as 6.3 h, which gives a rate that is too fast.
Should a pump rate be rounded?
Round once, at the end, to what your pump accepts. The calculator shows the rate to one decimal place, which many pumps accept, while some take whole numbers only. It’s best to follow your pump and local policy. Rounding the volume or the time before dividing is best avoided, because those small errors carry into the rate.
Is mL/h the same as drops per minute?
No. mL/h is a volume per hour, and it is what you programme into a pump. Drops per minute are counted for a gravity infusion and depend on the drop factor of the giving set. 125 mL/h is about 42 gtt/min with a 20 gtt/mL set, but 125 gtt/min with a microdrip set.
Why is the rate per minute shown?
It is a quick cross-check, and it helps when a protocol is written in mL/min. It is simply the hourly rate divided by 60: 125 mL/h is 2.08 mL/min. If the per-minute figure looks like the rate you expected per hour, you have probably divided by minutes instead of hours.