Recently I was hospitalized because of a strange illness that causes all my joints and muscles to flared up in pain. It happened slowly at first, just a sporadic pain here and there on my hands and foot. But the pain got more and more intense and spread all over my body in just matter of days.

By the time I can’t even muster enough strength to get myself up from bed, I knew it then and there that something was really wrong. This was not just a seasonal cold fever that occasionally happened in the rainy season. This was something else entirely. It scared me enough to push me to visit the hospital without giving it a second thought (I have never been hospitalized before).

My wife helped me get changed, but when I got undressed she let out a small scream and asked, “What are these rashes that covers your back? They weren’t there before!” I didn’t realize it at that time that one of a telltale sign of this disease, which I find out later as Chikungunya, is an outbreak of rash all over the skin. It looks like measles, but different.

TL;DR The main reason why I never consider to get treated in a hospital before is that, I really hate being tethered to an IV bag. But alas, since I no longer have the luxury of choice, I have to cooperate with the nurses and allow myself to be tethered to an IV bag. Then on the second night, I was awaken in the early hour to see the horror of tubes ran over with my own blood, instead of the IV solution. This triggered a series of sleepless nights for the two of us, as we kept watch over the IV drip bag in turn so that it will not run out unattended again.

And it piqued my interest: how do we know for sure when the IV bag will run out of juice?

Calculating the Dispense Rate of the IV (Intravenous) Bag

We asked several nurses about it, but they each gave different answers. One of them said that it would run out in 5 hours, while another said it would run out in 8 hours. However, it’s different when we examine it ourselves. The numbers didn’t match the reality! I tried to calculate it on my own, but I was unable to finish the calculation due to missing numbers.

Then I finally asked ChatGPT about it, and here’s my prompt:

How long will it take for a 500 ml IV bag to run out, if it runs at 1 second per drip? Here’s a picture of the said IV bag.

It returned my question with this:

To calculate the length of time a 500 mL IV bag (such as the one in the image) would take to empty at 1 drop per second, we need to know the number of drops in a ml. For IV fluids, the standard drip factor is typically:

• Macrodrip: 20 drops per mL (standard in adult IV sets)
• Microdrip: 60 drops per mL (used for pediatrics or precise medication)

Assuming that this bag I was tied to is a macrodrip set (20 drops/mL) (most common for general IV infusions), we’ll come up with:

Calculation for a Macrodrip Set:

• Total volume = 500 mL
• Drip factor = 20 drops/mL
• Total drops = 500 mL × 20 drops/mL = 10,000 drops
• Drip rate = 1 drop/second
• Total time = 10,000 seconds

Convert it to hours and minutes:

• 10,000 seconds ÷ 60 = 166.67 minutes
• 166.67 minutes ÷ 60 ≈ 2 hours 47 minutes

Therefore if the IV runs at 1 drop per second, it will take approximately 2 hours and 47 minutes for the 500 mL IV bag to be fully infused.

And for a microdrip IV set which typically delivers 60 drops per mL — used when more precise, slower infusions are needed (e.g., for children or sensitive medications):

Calculation for a Microdrip Set:

• Total Volume = 500 mL
• Drip Factor (microdrip) = 60 drops/mL
• Total Drops = 500 mL × 60 drops/mL = 30,000 drops
• Drip Rate = 1 drop/second
• Total Time = 30,000 seconds

Convert it to hours and minutes:

• 30,000 seconds ÷ 60 = 500 minutes
• 500 minutes ÷ 60 = 8 hours 20 minutes

So if the IV runs at 1 drop per second using a microdrip set, it will take 8 hours and 20 minutes to infuse 500 mL

This almost hit home since the answer is closest to the figure mentioned by one of the nurses, which is at around 8 hours.

But what if, I would like to know about how to calculate IV flow rates for specific time durations (like “infuse over 4 hours”)?

Here’s the IV Flow Rate Formula (for gravity drip):

Flow Rate (drops/min) = (Total Volume × Drop Factor) ÷ Time

Variables:

• Flow rate = drops per minute (in gtt/min)
• Total volume = fluid to be infused (in mL)
• Drop factor = IV set calibration (in gtt/mL), usually 10, 15, 20 for macrodrip, 60 for microdrip
• Time = total infusion time (in minutes)

Example:

You want to infuse 500 mL over 8 hours using a macrodrip set (20 gtt/mL).

• Total Volume = 500 mL
• Drop Factor = 20 gtt/mL
• Time = 8 hours × 60 minutes = 480 minutes

Let’s plug it into the formula:

Flow Rate (drops/min) = (Total Volume × Drop Factor) ÷ Time
Flow Rate = (500 mL × 20 gtt/mL) ÷ 480 min. = 21 drops/minute

To infuse 500 mL over 8 hours using a microdrip set, we need to set the IV to drip at around 21 drops per minute. Ain’t that neat?

I wonder if this is applicable to other drip devices such as a coffee cold dripper, but I guess that’s a story for another day.

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Psst…
Want to see me take it to the next level by turning the formula into a functional app with the help of ChatGPT and v0? Stay tuned :)