How to calculate infusion time in hours and minutes

Infusion time (hr) = volume (mL) ÷ rate (mL/hr). The milliliters cancel and the answer is in hours.

WisTech's dosage text defines infusion time as the length of time it takes for an infusion to complete, and Open RN notes that nurses work it out so they know when to stop an infusion or hang the next bag.

In dimensional analysis, start from the volume and multiply by 1 hr over the milliliters the pump gives in that hour, so milliliters cancel and hours remain; Open RN's completion-time example uses the same two factors. In the formula method it reads as the volume to infuse over the volume the pump gives in one hour, times that hour.

The first worked example in this guide, a heparin bag on a pump, is set out in all three notations. It asks only how long the bag lasts: DoseReady does not cover titration, including heparin rate changes by aPTT.

Divide a volume by a rate and the result is often not a whole number of hours. Then the rounding rule, not the arithmetic, decides what you write. Mercer County Community College's guidelines say every question should specify its rounding requirement, and mark a question incorrect when the answer is not given to the requested decimal place.

The dextrose bag in the second worked example in this guide never comes out even: the exact time is 40/3 hr, a decimal that repeats without end. The example gives the answer under its stated rule, nearest tenth with half rounding up, then what the same working gives to the nearest hundredth and to the nearest whole number. One division, three answers; only the rule differs.

Round once, at the end. Open RN's rule is “do not round until calculating the final answer.” The rounding rules guide compares the rules programs publish.

Open RN's rule for times is that anything after the decimal point is a portion of an hour and has to be converted to minutes. Keep the whole hours, then multiply the decimal part by 60 min per hr.

Work from the exact time, not from the rounded answer. The dextrose bag's exact time is 13 hr 20 min: the whole hours, then the leftover fraction of an hour times 60. Converting the answer rounded to the tenth, 13.3 hr, gives 13 hr 18 min instead, because the rounding has already moved the time.

The part after the decimal point is never a count of minutes on its own; read it as minutes and the time is wrong even though the division was right. Some questions ask for the hours rounded under a stated rule and others for hours and minutes, so read which one the question wants.

Some problems give a gravity infusion's drip rate and the tubing's drop factor instead of a pump rate; WisTech's infusion time exercises include several. Turn the drip rate into milliliters per hour first, then divide the volume by that rate.

Milliliters per hour = drops per minute × 60 ÷ drop factor. That is the drip rate formula run backward: multiply the drops per minute by 60 min per hr to get drops per hour, then divide by the drops in each milliliter, which Open RN notes is printed on the tubing package. With 60 gtt/mL microdrip tubing the two sixties cancel, so the drip rate in drops per minute is already the rate in milliliters per hour.

The IV drip rate guide works the formula forward, from a volume and a time to drops per minute.

Open RN takes the time one step further: add the infusion time to the start time, in 24-hour (military) time, to find when the infusion will be complete. Add the whole hours first, then the minutes. An infusion that runs past midnight finishes the next day; WisTech's dosage text works an example that ends the following afternoon.

The worked examples in this guide stop at the duration. More infusion time problems, each with its worked solution and the answer under a stated rule, are on the infusion time practice page.

1. 1

   Rounded to the wrong precision: The division is right and the answer is rounded to a different place than the question asks for, or left unrounded. Check the requested precision before you write the time down.

2. 2

   Conversion factor inverted: The rate goes in upside down, so the volume is multiplied by the rate instead of divided by it. The units no longer come out as hours, which is the quickest way to catch it.

3. 3

   Conversion skipped: The rate never enters the chain, so the volume in milliliters is written down as a number of hours. Check that the hours came from dividing by the rate.

Infusion time practice problems→

Each problem opens to its worked solution, the answer under a stated rounding rule and the sources it is built from.

How do you calculate infusion time?

Divide the volume to infuse, in milliliters, by the rate in milliliters per hour. The result is the infusion time in hours; round it under your program's rule, or turn the decimal part into minutes by multiplying it by 60.

How do you convert a decimal of an hour into minutes?

Keep the whole hours and multiply the decimal part of the hour by 60. Use the unrounded time, because converting a time already rounded to the tenth of an hour can shift the minutes.

How do you find infusion time from drops per minute?

Turn the drip rate into milliliters per hour first: drops per minute times 60, divided by the tubing's drop factor. Then divide the volume by that rate to get the infusion time in hours.

Do you round infusion time?

Infusion time is rounded to whatever place the question or your program's rules state, and some questions ask for hours and minutes instead. Either way, work from the unrounded result and round once, at the end.

1. [1]WisTech Open, Nursing Dosage Calculations, Introduction to IV Flow Rates and Infusion Time (2024)wtcs.pressbooks.pub
2. [2]Open RN, Nursing Skills 2e, 5.16 IV Completion Time (WisTech Open, 2023)wtcs.pressbooks.pub
3. [3]Mercer County Community College Nursing, Medication Calculation Guidelines (updated September 2020)mccc.edu
4. [4]Open RN, Nursing Skills 2e, 5.5 Rounding (WisTech Open, 2023)wtcs.pressbooks.pub
5. [5]WisTech Open, Nursing Dosage Calculations, IV Flow Rates and Infusion Time Practice (2024)wtcs.pressbooks.pub
6. [6]Open RN, Nursing Skills 2e, 5.14 IV Infusion by Gravity (WisTech Open, 2023)wtcs.pressbooks.pub

No licensed clinician has reviewed this guide. How the guides are made. Exam practice, never for patient care. Follow your program's rules.

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