Learning Lab

Flow rate keeps a measure of how much liquid is flowing by for each unit of time. We need to know an amount of liquid, be it mls or drops, and the time unit it takes to flow, be it hours or minutes. mls per hour and drops per minute are the units that you will commonly work with. They can express the same amount and you can and should be able to convert between them.


The strength of expressing your flow rate in mls per hour is that you get a clearer idea of how much liquid volume is being added to the patient's system. The beauty of having flow rate in drops per minute is that you can more easily calibrate how much medication is going in and how quickly. And also you can judge that visually by counting and timing the drops.


It is important to be able to make very fine calibrations like this, especially when dealing with children. In addition, your instrumentation will determine what unit that you must use and so that you need to be able to convert between the units and set your instruments according to their needs.


If you want to convert flow rate in mls per hour to flow rate in drops per minute you will need a couple of conversion factors to do this. Firstly, we want to turn mls into drops, and we know that there are 20 drops per ml.


We want to get rid of the mls so we should put that at the bottom and the drops value can go on the top. We also want to turn the hours into minutes. One hour equals 60 minutes. So we will put the hour on the top, so we can cancel it and the minutes that we want can go on the denominator of our conversion factor. So, we notice that after the cancelling we have multiplied the numerator by 20 and the denominator by 60. Let’s merge those together and 20 over 60 simplifies down to one-third.


So if you have a flow rate in mls per hour and you multiply it by third you will get a flow rate in drops per minute, and that’s the case for whatever the original flow rate may be, but remember that is only true when 20 drops is equivalent to 1 ml, and that this is when it’s for an IV for an adult size patient. 

So from this we know that flow rate in mls per hour, can be converted to drops per minute by multiplying the value by one-third. If you want to go from drops per minute back to mls per hour, you do the opposite, which is to divide by a third. But that is effectively the same as multiplying by 3… But remember, this is only true for macro drops, which are the drops used by adult infusions.


Let’s recall our understanding of macro drops and micro drops. Macro drops for adult infusions are of other size, such as there are 20 drops per mil. For paediatric infusions on the other hand - drops are much smaller, there are 60 drops making up that one mil, and the drop factor is 60 drops per ml

So, if we are dealing with micro drops and we want to turn flow rate into mls per hour into microdrops per minute, we can follow the same sort of calculation we did before. But we do not use 20 drops-per-mil we use 60-drops-per-mil. The difference is though, this time we are multiplying the numerator by 60 drops, and the denominator by 60 minutes… Once again we can merge them together, and 60 on 60 is one.