Dexcom algorithm for calculating IOB and meal bolus?

I find the meal bolus calculator confusing so I never use it. However, my Doc wants me to try. Anybody able to explain how they calculate your meal bolus according to IOB? Anybody ever use it? How reliable is it?

1 Like

Actually could you explain in slightly more detail what you mean? Which device would you be using?

The way we do it is trivial such that you would not ask hence you have to be asking something different.

1 Like

I use Omnipod pump. But, it doesn’t deliver a flat 2 units per 15 grams of carb. It adjusts the dosage according to the ‘Insulin On Board’, or some such thing. So, if I eat a sandwich, it might sometimes deliver 4 units, and other times 2 units. I don’t know how it works so I don’t trust it and I don’t use it. But, its possible that it has never been set properly because I don’t know how it works. My nurse educator wants me to use it. Does that make sense? I don’t know how it calculates IOB, either. I once tried to find out and someone told me that I would never find out because it was proprietary. That doesn’t seem likely.

1 Like

AFAIK, the configurable setting that most impacts the auto calculated value for “Insulin on Board” (IOB) is the “Duration of Insulin Action” (DIA). This can be called slightly differently on different pumps but I looked it up and believe this is the proper term for the Omnipod.

Changing this can have a pretty big impact - sometimes larger than you would imagine.

I have heard people setting this anywhere from 2 hrs to 6 hrs. When changing this, I would suggest to change it by only half an hour at a time and give the new setting a few days to see how it works. Changing this value very well may mean you then need to change other values in particular the insulin to carb (I:C) ratio.

I believe we have this set to 3.5 hrs.

One symptom you might see that would indicate the DIA needs to be increased to a longer time value would be if you are (well if using the “wizard” or whatever your pump calls the calculator) seeing LOW blood sugars when eating a “second” meal only a few hours after eating the “first meal”. This would be from stacking insulin but not having the pump take that into account. On the “second” meal, you might really still have insulin working in your system bringing your BG numbers down, but if the DIA is of too small a value then the pump might think you have little to no IOB and therefore tell you to dose more insulin to bring your BG down with your meal. Whereas with a longer DIA time value, the pump wizard would calculate still some insulin remaining from your first meal still working on your BG and would thereby subtract some insulin from what it tells you to dose.

Wordy - I know.

Anyway - sorry if that was what you already knew and I missed the point of the question?

1 Like

Oh boy. I don’t know about this. Thank you, that gives me a starting point. I don’t trust the wizard.

1 Like

Nice pic. lol
In both the Animas and the Tandem pump when using the “wizard” it will show the detail. So it shows how much for the food, how much for the BG, how much for the IOB. Which is good because then you get to decide if maybe you don’t agree with a part of it then you can make a change.
Like for the BG maybe it says 2 units but you know not only you are a bit high but your CGM shows a steep rise so you might think bump the 2 to a 2.5 or whatever.
But yeah - I would not like just a “magic” number. I like to see the details and then decide if I agree with the bottom line. I would hope you have that option on your pump (which we have never used).

@Eric2 uses the Omnipod and I have no doubt he can tell you exactly how this works. I am only guessing in the dark. lol

Wow, that is a real selling point for Animas and Tandem. Thanks a bunch.

Well, the omnipod bolus calculator uses your current BG, your target BG, your correction factor, your current IOB, and the carbs you are going to eat.

IOB is calculated with a very primitive linear reduction calculation - straight depletion of total units by the insulin duration. Despite the fact that insulin does not deplete linearly!

But I don’t use their calculator!

Because my calculation figures in all those things their calculator does, plus some things their calculator has no clue about:

  • Which direction is my BG going
  • How quickly is my BG going up or down
  • How active have I been
  • How active will I be
  • What time is it
  • What kind of food am I about to eat
  • Is dessert planned
  • How hungry am I
  • How well has my body been responding to insulin today
  • How long before I eat
  • Is there easy access to some sugar later if I am low
  • Am I alone or do I have a safety net

To me, pump calculations (on any pump) are just not that smart. So I don’t bother with them.


@Eric2 - Does the Omnipod allow any of the details of the calculation (at all) to be viewed or does it simply give the one number as the final result?

1 Like

It doesn’t show the calculation. It shows the BG you just entered and the carbs you just entered and how much you should take. All the calculations are done behind the curtain!

The calculations are simple. But the whole thing is very simple, and to me it is almost worthless. I tried it a little bit, but I was always changing the number. It didn’t take me long to come to the conclusion that if I change the number anyway, there is no reason to ask it.

It’s like when my kids ask me something, and then don’t listen to the answer. What’s the point of asking?!?!

Both the Animas and the Tandem show details of the calculations and I had (erroneously) assumed that was common across all pumps.

I like the (Animas and Tandem) pump calculator as a good starting point. We have the pump configured for the various ratios for the times of day along with the target BG for time of day. The pump calculator then pulls these configured ratios for the time of day into the calculation. This makes the starting point easier.

On top of the pump calculations (which show the details) we then make adjustments.

I wish my 530g could adjust at half-hour intervals. I’m pretty sure my value is 3.5 hours, but I have to choose 3 or 4.

That curtain is easily pulled open. At the “Suggested bolus” screen, you can press the “?” and the hocus-pocus will be revealed. The screen then shows:

Meal (carbs/IC ratio) = x units
Correction (current BG +/- target BG = +/- y units
Meal IOB = +/- z units
Correction IOB = +/- zz units
Total = suggested bolus

Under that it explains: “Meal IOB will reduce only correction bolus. Correction IOB will first reduce correction bolus, then meal bolus.”

When I had my pump training, the trainer told me, “Trust the pump” with regards to the bolus calculation. Initially I ignored her, thinking, “That’s not enough (or way too much) insulin,” and made my own calculations and was invariably WRONG WRONG WRONG. Once I trusted the pump and said instead, “If you say so” (with sometimes but not always various adjustments such as @Eric2 outlines above), everything went perfectly (relatively speaking).


Waaaaay back when, OmniPod didn’t have IOB in any shape or form. That was frustrating, but Caleb and I would figure out a ballpark estimate by always considering when his last bolus was. We got used to that method of considering IOB.

Then OmniPod updated their PDM and software and voila - we had IOB!

This was phenomenal for caregivers at school. IOB was BY FAR the most difficult concept to help people understand and what I spend the most time on, on an ongoing basis, trying to explain.

While it was helpful to keep Caleb safe at school because there was some automatic consideration of IOB and even a display of it on the home screen, the specifics of the calculation usually did not work for Caleb as we found it to be too conservative for his dosing. Even with a DIA set to 2 hours, we found it subtracted more insulin than was necessary for Caleb.

For the most part, we do not defer to the PDM bolus calculation, however, we do use the information to make decisions. Like @Eric2 mentions, we consider factors that the calculator cannot - direction of BG and Caleb’s trends within the last 48 hours being the most significant. Given those considerations, we will decide to give a “full bolus,” which is a straight correction or carb bolus without consideration of IOB (he manually adds back any IOB subtracted), or a partial bolus or a “suggested bolus,” which is the PDM calculation.

There is a screen that shows the calculation. When you hit the ? somewhere along the way, you get a breakdown of the suggested bolus - amount for carbs, amount for correction and any reductions for IOB. That’s where Caleb easily identifies the IOB amount if he wants to add it or a portion of it back in. Or maybe users are hoping for more detail than that and I’m not understanding the question?

I’ve never fully understood Insulet’s method of IOB calculation even with the breakdown given in the detailed screen bc the treatment of meal IOB is different than the treatment of correction IOB. There’s some nuance to it that was explained to me, but lost on me. I haven’t pursued it further bc we’ve managed around it and rarely utilize the suggested bolus, but maybe if I took the time to understand I might use it more…I don’t think it’s proprietary information though.

I get aggrivated with things like that. Seems no reason other than somebody was lazy. Really - why not program the parmater to accept a decimal rather than a whole number? I seriously doubt that would have impacted a clinical trial or an FDA approval or anything. Blarg.

Although I did like our previous pump, the Animas Ping, one thing that was annoying was similar in that it only allowed whole numbers for the Insulin Carb (I:C) ratio. No big deal if you are changing from 1:15 to 1:14. Just a small tweak. See how it works. But what if you are considering going from 1:5 to 1:4? HUGE CHANGE!!!

Our new Tandem t:slim X2 does have some improvements over the older Animas Ping in some of these areas - which is great and seems like only common sense.

Below are some allowable ranges for configurations that can actually make a pump much easier or harder for a given person depending on their insulin needs but potentially some of those things that people don’t think to look at until after they already have the pump?

Quoted below (including misspellings - lol) from the Tandem t:slim X2 User Manual:

Basal Rate Increment: 0.001 at programed rates equal to or greater than 0.1 units/hr
Insulin-to-Carb (IC) Ratio: 16 time segments per 24-hour period; Ratio: 1 unit of insulin per x grams of carbs; 1:1 to 1:300 - (can be set by 0.1 below 10)
Temp Basal Rate: 15 minutes to 72 hours with 1 minute resolution with a range of 0% to 250%
Duration of Insulin Action: 1 time segment; 2 to 8 hours in 1-minute increments (default is 5 hrs)
Bolus Increment: 0.01 at volumes greater than 0.05 units

One workaround that worked fairly successfully for me was to add 0.5U to my breakfast bolus (IC ratio 1:5). My CDE was using some mathematical formula that was lost on me at the time (as is most math). I later ended up similarly adding 1U to the dinner bolus because what I really needed was something in between 1:7 and 1:8.

Addendum: It turns out there was no magic formula, just experience and common sense. 30 g carb with a ratio of 1:5 would be a bolus of 6U. With a ratio of 1:4, it’s 7.5U. For me, one wasn’t enough and the other was too much, and since I only needed a small amount extra, adding 0.5U to the bolus (to get 6.5) achieved basically what a ratio of 1:4.5 would have (6.6).

Right, I forgot about the “?” giving details. Thanks!

1 Like

Page 20 of: Omnipod Patient Resource guide

Found it. Highlighted in purple. I don’t think that I have ever seen that screen before.

1 Like


Readers of this thread might find Tim Street’s discussion informative: Why we are regularly wrong in the duration of insulin action (DIA) times we use, and why it matters. After reading this blog I changed my pump DIA to five hours.