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Med Math (dosage calculations, drips, and formulas)

We literally get hundreds of nursing students each week emailing us and telling us that they struggle with med math and dosage calculations in their Nursing Pharmacology course.

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We’ve got tons of resources to help you learn nursing pharmacology and medication math.

The above video is a sample video from our MedMaster Pharmacology Course.  

Resources to Help You Learn MedMath:

Pharmacology is kinda our favorite subject at NRSNG because we believe it plays such a crucial role in your job as a nurse.  The problem is that it’s not very well taught or focused on in nursing school or on nursing floors.

MedMaster Course contains nearly 80 hours of lectures covering Nursing Pharmacology including med math, dosage calculations, the must know meds, audio and video lectures, and so much more!

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Podcast Transcription

Okay, so, what we’re gonna do with this lecture is we’re gonna talk about the most

commonly asked calculation type questions, okay. We’re not gonna necessarily go

over whole lot of calculations, and things like that. But, what we’re gonna do is try to

understand these formulas and I truly believe that Math for Meds is fairly over

complicated than nursing school. Each of us to get into a nursing school, we took

college Algebra, a lot of us probably took pre-calculus, possibly calculus or physics,

courses like that in high school. So, we’re good with math. But what happens in

nursing school, is when we get these word problems, we just, our minds kinda shot

off. And so, I think, understanding these calculations is going to make this all a lot

easier, okay?

So, I think the one that confuses nurses more than anything is gonna be drops per

minute. These subsidy calculations just become very confusing because we’ve never

really, we’ve never titrated drips, we’ve never use drip chambers, or anything like

that. And even to this day, in my nursing career, I haven’t really used roller clamps or

anything like that to titrate my drips because nowadays, our pumps kinda manage all

that. If the power were to go out, if we did not have anyway to give the medication

and knowing this calculation would be very helpful. Let’s talk very quickly through

this calculation. To calculate drops per minute, what’s gonna happen a lot of times

with this question is they’re gonna ask you what’s the drip rate of the medication,

okay? So, the formula or information that you need to know, is you need to know

your volume and you need to know your time. Now, you also need to know your drip

factor. But a lot of times, drip factor is gonna be provided in the question. They’re

gonna tell you the drip factor of this chamber set is 15 drips/minute, and whatever,

and so that information is gonna be provided for. What you really need to try to

come up with, you need to come up with your volume, we really wanna have that in

mL, and then you need your time, and your time needs to be in minutes. Okay. Once

you can extract that information, you know, let’s say, your volume is 60 mL, drip

factor is, you know, 10 gtt/mL and then your time is 60 minutes. Okay, so we can see

from this already that our mL are gonna cancel out, and so we’re gonna have 600

drips (gtt) over 60 minutes. Okay, so, 600 divided by 60, that’s gonna give us 10

gtt/min. Okay, that’s really all there is to the drops per minute. Once you can extract

your volume, hopefully you get that in mL, you got your time, and then your drip

factor should be provided. So, once you have all that, I mean these questions

become very simple.

Okay. Now, let’s talk about Milliliters per Hour. So, this question will come in handy

as a nurse because it becomes helpful to know about how fast you need to run a

medication, sometimes, you’ll be given, you’ll be sent doses from pharmacy and they

tell you to run it over a specific time. So, it’s important to be able to figure out how

fast to run it, okay. So, the information you really need to get from this, you need to

know your volume and you need to know how long it’s going to run for. Okay, and

that’s going to be in hours. Okay, because we run all of our pumps in mL/hour. Okay.

So, again, so what you’re gonna extract from your formula here or from your, what’s

you’re going to extract from your question is gonna be your volume in mL, so, let’s

say, it’s 500 mL, and it needs to run over 2 hours. Okay, now. The biggest thing when

you’re doing math meds and things, is your gonna wanna see, okay, do I have my

right units where they need to be. Okay, so, we’re trying to be mL/hour. So, I got my

mL over my hours, that’s gonna give me mL/hour, right? There’s my mL/hour right

there. So, I have 500 mL, I have to run it over 2 hours, so, what rate do I wanna set

my pump at? Okay, so that becomes very simple. You do 500 divided by 2, you gotta

write it 250 mL an hour, okay. So, you just need to go over your pump, set your

pump at 250 mL/hour, and you’re set. Okay, so, that’s really all you need to know

from this equations, okay. And then, if you, where I think some nurses get confused

too, is maybe they don’t give you in hours, but they give you, let’s say, let’s just re-

arrange this really quick. So, let’s say, the question talks about, it’s gonna run at 250

mL/hr, you have a volume of 500 mL, how many hours? Okay. So, all that becomes,

just becomes, you’re canceling this out, alright, you’re just gonna cancel this out and

you’re gonna put this. So, you’re gonna divide the 500 by the 250 and so, you’d end

up with hours over here, hours is gonna equal 500 mL, divided by 250 mL/hr. Again,

we’re canceling out our mL, and that’s gonna give us 2 hours. Okay, and that’s

correct, right? Now, the thing you wanna also keep in mind here with NCLEX

question, is every NCLEX question, you’re gonna have calculator available, you’re

also gonna have scratch paper and these are multiple choice questions, so, worst

comes to worst, start going through your options and plug in the numbers in to your

formula, and see, okay, does this makes sense? Is this the right answer? Am I getting

the right answer that I need. Okay, but, mL/hour is really just a very basic division

that you’re very used to from your college algebra, from your high school algebra. I

mean, this basic division here is something that we did in 7th grade, you know, we

just have divided by, cancel out units and that’s gonna give us our answers.

Okay, now, infusion time, this is really just what we did on this equation here, except

if you look at it, we’re gonna do volume over mL/hr. Volume over mL per hour over

infusion time. Okay, so, infusion time equals volume, infusion time equals volume

over mL per hour. Okay, isn’t that the way we just worked out this equation, right?

So, time, infusion time, equals volume over mL per hour. Okay? And that’s what is

this equation is, it is that other equation re-arranged. So, once you know one of the

equations, honestly, you don’t need to memorize two equations here, you need to

memorize one and just know how to re-work the equation to get what you need to

know. Okay, so, a lot of times, this volume to be infused would be written a VTBI.

That will be written, you know, as you’re working in the hospital, as well as possibly

in equations. So, your time, is going to be VTBI over mL/hour. Okay. And again, this

volume to be infused is gonna be in mL, so that’s obviously gonna cancel out there,

we’re left with the unit of time. We’re trying to find time. So, that’s how we’re gonna

get it. Okay, so, very easily, just work to those equations and that’s gonna give you

what’s you need to know.

Last is we’ll talk about dosage. Dosage calculations can be confusing to people but I

want to tell you how easy they can be. First of all, the one thing you need to keep in

mind is you may have to convert like liters to milliliters to get everything in the right

unit, okay. So, you want units to be the same, units must equal units, okay. You gotta

be doing the same units here. And, then, when you’re dealing with oral medication,

your quantity is going to equal 1, okay. So, for example, the physician writes an order

to give a, let’s say, the physician writes an order to give tylenol 650 mg, you have

tablets that are 325 mg, okay, it’s oral, so it’s one. So, that calculation that

calculation very easily becomes, so, what’s the dose? How much you’re gonna get?

How many tablets you’re gonna get, so, we have tablets that are 650 mg, our dose is

650 mg, what we have available is 325 mg tablets, so, that comes out to 2. So, we’re

gonna give them 2 tablets. Okay, so, it becomes that easy, really, you take your

desired, what do you want them to get and what do you have available to give them.

And that just becomes an incredibly simple division problem, right? So, let’s say,

you’re desired dose is 2, what you have available is 4, okay, so, 2 divided by 4 equals

0.5 tablets mg, whatever we’re working with here. So, that’s really how simple as it


Okay, you don’t need to overthink this, using the very simple work through exactly

what’s being asked. And just kinda, and just think about it too. Okay, let’s say you

accidentally forget, you flipped these things, okay? The dosage, so the physician

wants you to give 650, give tablets of 325. Let’s say, you forget the way to do it and

you flipped the equation and this would give you 0.5. Now, the, you kinda just think

through it, okay? The physician wanted you to give 650, you had 325, does it makes

sense that you’re only gonna give him half of the 325 to get 650? Well, that doesn’t

makes sense. So, you know, you might have done the equation wrong, you just flip

the numbers until you get something that makes sense. So, every time, that’s the

other kinda tip I’m gonna give you. Every time you get an answer, just kind think

through it, does it actually makes sense. Okay, and I want you to download the little

cheat sheet with the equations and watch this video a couple of times and then,

we’ll get another video up here about exactly how, how to do a couple of equations,

we’ll have a couple of examples.

Date Published - Sep 3, 2016
Date Modified - Sep 3, 2016

Jon Haws

Written by Jon Haws

Jon RN CCRN is a critical care nurse at a Level I Trauma center in Dallas, TX. His passions include learning about anatomy and physiology and teaching. When he isn't busting out content for NRSNG.com he loves spending time with his family.