Updated: Oct 4
HEY-HEY everyone. It’s been a minute since I wrote a blog so I thought I would drop one on you all about drop factor calculations… (Giggles to self 🤭) ... We’re going to be discussing all things DROP FACTOR, including tubing and formula calculations! This is your warning. This is a MATH BLOG. 💧🧮
Drop factors can be intimidating if we don’t have a good understanding of how med math works or what drop tubing is. That being said, today we will cover what drop factor tubing is. What in the world does “gtt” means, and how to set up med math formulas to solve for drop factor calculations.
First things first, let us get one argument out of the way and off the table. Infusion pumps are safer. They do have the ability to deliver precise volumes at programmed rates or intervals helping to alleviate or minimize the human error factor. Everyone should have them, but the fact is that not everyone does. Because not everyone does, it is important to be able to keep up on learning and education of the current things that we do utilize… while still advocating for safer. So I digress, let's get on with drop factor calculations.
Lettuce 🥬 start by reviewing or learning what drop set tubing is. Drop set tubing has a couple of different names, such as “drip tubing” or “drip set” with the most common one I’ve heard it referred to as gravity tubing. For consistency, we will reference the tubing as “drop factor” or “drop set” for this blog.
Drop factor, simply stated, is how many drops are in 1 mL of a solution. It is expressed as gtts/mL and can be found on the tubing packaging. The abbreviation for drop is expressed as “gtt.” (2) Drop factor tubing can be classified into two types of drop sets. Macrodrip sets, with drop factors of 10, 15 and 20 drops per mL. Microdrip sets, with a drop factor of 60 drops per mL. A Microdrip set is utilized for precise IV fluid infusion for children, infants and critical medications. Macrodrip sets are typically utilized to infuse large volumes or to infuse fluids quickly. (1) It is very important that we know our drop factor of the tubing that we are using as it will be a key “ingredient” in figuring out our drop factor formulas. 👩🍳
When utilizing drop set tubing, we are calculating our ordered rate in drops (gtts) per minute. In simple terms, how many drops do we want dripped (dropped) into our fluid chamber each minute. The manual roller clamp sets how many drops we want in a minute. Adjust the roller to increase or decrease how fast a drop of solution drips out into the chamber. The height of the IV bag can also affect the flow rate. To explain this further, if I adjust my roller clamp to the ordered rate we want and THEN raise or lower my IV pole, it changes the gravity displaced on the IV bag/solution which can increase or decrease how fast our drops will flow out. So, any time we change the height of the bag, recheck the drip rate.
Med Math Formula:
There are honestly a couple of ways to calculate drop factor calculations. We can utilize formulas that we memorize, or we can utilize dimensional analysis as a way to consistently work through almost any med math problem, including drop factor calculations. This blog will heavily discuss drop factor calculations using dimensional analysis as my target audience is to help ones who are struggling or just not quite getting it (that’s ok! ❤️✌️). I think dimensional analysis really helps anyone struggling to see the correlation between what to multiply or divide and how we obtain or cancel unit labels.
Let’s start with a quick overview of dimensional analysis. Dimensional analysis also known as the factor-label method, as it pertains to med math, is the problem-solving method used to help easily convert one unit to another unit using known conversion factors. (3) The goal is to add conversion factors that cancel all the units except for the ones needed…. It will make sense; here is an example of converting pounds to kilograms.
In order to set up an appropriate “formula” for drop factor calculations, we need to know a couple of things: common conversions and what is required to solve the problem. This brings me to my next point, common conversions. As a medical professional, common conversions are as follows. These are a few MUST know.
1 hr = 60 min
1L = 1000mL
1 mg = 1000mcg
1 g = 1000mg
1 kg = 2.2lb
I absolutely cannot stress how important it is to know our conversion factors (memorize them) and how important it is to be working in the same unit. Knowing conversion factors will help with efficiency and how to set up our med math formula.
Requirements for the drop factor formula include:
1. What are we solving for
2. Drop factor set (tubing)
3. Rate to Infuse
NOTE: obtaining the rate could technically make calculating drop factor rates a two-step process because we sometimes have to calculate the rate as it is not always given to us.
We can also use rates that are given in mLs/min. It boils down to knowing when and how to utilize conversion factors.
4. Conversion factors
Only if needed.
But how do I know I need them? Ask yourself: do I have any units that are not needed and therefor need canceled? OR Do I have the correct unit on the top and bottom for what I want to solve for?
Step 1: Set up
After reading the problem, I ask: What am I solving for? That is the first thing I write down, followed by the setup of the rest of the problem. The reason for this is, it is a great visual to see what unit will go on top and what unit will go on bottom. This way, I know where the rest of my information needs to go.
Step 2: Cancel Units.
Once our problem is set up, we can cross-cancel units. The goal is to only have the unit we want on top remaining, and only unit we want on the bottom remaining. If we do not have the correct units remaining on top and bottom, or we have an extra unit that we have not been able to cancel out, we most likely did not set our problem up correctly OR we need to add an additional conversion factor.
Step 3: SOLVE
The final step is to solve. This is done by Solving the top (multiplying all the numbers) and solving the bottom (also, multiplying all of the numbers) and then dividing the top by the bottom. We have our answer, cake. 🍰
Below is an overview of the three steps we just discussed. Really focus on what units need to go on the top and what units need to go on the bottom. This heavily affects the cross-cancellation of units as well as ending with the appropriate unit on top and bottom for what we want to solve for. 🔬
If that wasn’t enough, I do have an easier way to solve for drop factor calculations. But this is a fair WARNING**** if we do not know why we are doing something or how we are doing something, that can cause med errors. The reason dimensional analysis was used was to show the relationship of units with conversion factors with one another. Using the “shortcut” or the “quick way” will NOT help if we run into a scenario that calls for different units and we don’t know how to accommodate them. That being said, once we have a solid foundation or a firm grasp on “why” we need certain numbers and units by all means, make it easier on yourself and utilize other formulas.
Drop factor calculations are a thing. As much as I will advocate for infusion pumps to be utilized over drop factor tubing, the fact remains that we are still utilizing drop tubing in many settings. Hopefully with the tools and practice that this blog offers, we can be better equipped in our drop factor calculations. Med math in general is not easy and not fun. Not only do we need to understand how to calculate medication, we need to continuously practice and practice to gain accuracy, efficiency, and confidence in our calculations to help eliminate medication errors. I have attached some practice questions below. Practice calculating the drip rate for each. The answers will be located at the end of the blog.
Brittany Grandfield, Flight Nurse.
(Honestly the only real credential: 2005 Lorimor Watermelon Queen.)
How about some practice questions?
You can calculate these out on your own and then check our answer at the end of the blog.
1. My 40kg pediatric patient needs a maintained drip of D5NS. Her rate is 80 mLs/hr using the 4-2-1 rule. My drop factor tubing is 60 gtts/mL. What will our rate in gtts/min be?
2. Our septic patient needs a fluid bolus of 1 liter over 1 hour. Our drop factor tubing is 10 gtts/mL. What will our gtts/min be?
3. We want to administer TXA 1g diluted in 100mLs over 10 mins. Our drop factor tubing is 10gtts/mL. What will our rate in gtts/min be?
4. My hypotensive septic patient needs an additional pressor. We want to start an Epinephrine drip. Our concentration is 1mg/100mLs. We want to initiate the drip at 2mcg/min. Our drop factor tubing is 60gtts/mL.
If you need help with the rate calculated in mLs/hr refer to my previous math blog.
A. What will our rate be in mLs/hr:
B. What will our drip rate be in gtts/min:
1. “IV Drip Rate Calculation Formula - Nursing Review (Video).” Mometrix, 18 Aug. 2023,
2. “Medical Definition of GTT.” RxList, RxList, 29 Mar. 2021,
3. “Drug Calculations: How to Use Dimensional Analysis.” NursingCenter,
www.nursingcenter.com/ncblog/august-2021/dimensional-analysis. Accessed 29 Sept.
Medication Math Answers:
1. Answer: 80 gtts/min.
2. Answer: 166 gtts/min.
3. Answer: 100 gtts/min.
4. Answer: Two step answer. Calculations for rate in mLs/hr first to be able to calculate
gtts/min for the final answer.
a. 12 mLs/hr
b. 12 gtts/min