Skip to content
RMIT University Library - Learning Lab

The flow rate formula


What does Flow Rate = Volume/time mean? Flow rate is determined by the volume of liquid that passes by (into a patient) within a certain time period. This is the fundamental formula for all IV problems.  

This short video uses practical examples to explain the formula for calculating the flow rate of a medication infusion.

In the simplest form, this is the formula for finding the flow rate of an infusion. You need to know the volume of the liquid flowing into the patient and divide this by the time it takes. The volume will be measured in units of some variations of litres, millilitres, or microlitres. And time will be measured in hours or minutes, or could theoretically be calculated in seconds... But this would not be a time unit that would be very practical in nursing. What is important to remember, however, is the units put into the one formula are compatible. That means that if the flow rate is measured in mls per hour, the volume must be measured in terms of mls and the time must be in terms of hours.

If the flow rate is in microlitres per second, the time will be in seconds, and the volume will be put into the formula in microlitres. If the flow rate is in terms of drops per minute, the volume would have to be in drops. Mls and drops are both ways to measure volume, but they are still different units. To have flow rates in drops per minute, you would have to convert the mls value to a value of drops before you put it in. And likewise, the time needs to be in minutes, so if you have time in hours, you would have to convert it before you put it into the formula.

The formula, as it is, gives you a way to calculate the flow rate if you already have the volume and the time. However, you can calculate any of the three factors as long as you have the other two. The formula can be rearranged so that you calculate either the volume or the time, if these are the missing quantities. The triangle on the left hand side of the screen may help you derive any other helpful formulae. We can see how flow rate can be achieved using volume on time.

If we want to calculate volume and we are given the flow rate and the time, we would multiply these together… And if we need to find the time after we are given the volume and flow rate, then time equals volume divided by flow rate. 

Let’s put this formula to use. Here we have a problem whereby we know the volume of the liquid to be infused into the patient and we know the flow rate. The missing factor here is the time, the volume is 100 mls, and the flow rate is 25 mls per hour, and the formula for time is volume divided by flow rate. Plugging those values in, 100 mls divided by 25 mls per hour, the mls will cancel.

Notice here we have a fraction within the fraction and in that the 25 ml per hour is in the denominator. So the denominator of the denominator, that is the hours here, gets flipped up and multiplies out the numerator. 100 divided by 25 is 4 and the unit now at the top is hours.

In our next problem, we need to find the time again, but this time we are given the volume in mls and the flow rate in drops per minute. What is more, we for the time in terms of hours. Unfortunately, we cannot have hours and minutes in the same formula, so we will have to get our answer for time in minutes first, and then at the very end we can turn the minutes into hours, if that’s what’s needed. Similarly, the mls and the drops cannot exist together in the same formula, so one of them will have to change. We could turn 100 mls into 2000 drops, or we could change the 10 drops into half a ml. Either of these will bring us to the same answer.

I will turn the 10 drops into mls, so we have mls in the formula. This allows for the mls to be cancelled. The minutes at the bottom of the denominator get flipped to the top again, and minutes is clearly the unit that our answer will be in. 100 divided by a half is 200, that is 200 minutes, and 200 minutes is three-and-a-third of a hour, or three hours and 20 minutes.