Tslim Control IQ - How Do They Calculate Insulin On Board

If you do give tslim a call and get a handle on it, please do share.

I, too, find the IOB on the main screen to be confusing. It seems to include ALL the insulin CIQ gives plus my meals boluses. The figure must include a variety of durations because it is cumulative.

I am running on 24/7 Sleep Mode, which has a tighter range but does not include bolusing for highs. The automatic bolus for highs in the regular CIQ is at 60% when one reaches 180. It is way above when I would correct. However, my manual corrections in Sleep Mode, have consistently been too much throwing me into seesaw situations that include some extreme lows. I have also noticed that my IOB has been much, much higher than ever before CIQ.

Something else I have noticed is that direct boluses without specifying carb intake are classified differently by the algorithm. I wonder if that skews the way it works.

I would like to know how exactly IOB is calculated and why it appears on the main screen if it is irrelevant to dosing.

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I am going to call Tandem technical support tomorrow as I think it is crazy that they actually put this on your home screen but nobody (from what I’ve seen here) seems to be clear on exactly what it is.

I have had the Control-IQ for about six weeks now and for me it is a God send for sleeping. I used to have to give myself bolus or take carbs two or three times a night. From day one this has kept me in range while sleeping.

I did notice that no matter what the IOB is in your home screen, if you shut off Control-IQ the IOB will immediately change to unabsorbed bolus insulin which I was use to on Medtronic. I think because Control IQ makes hundreds of changes to you basal it tries to figure out how much less undelivered basal insulin you have left from your profile setting and then adds or subtracts that from the unabsorbed bolus insulin. I want to know how to take this into consideration if wanted to make a manual change.

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Great! I’ve tried to get to Tandem’s Tech Support but the wait time has been too long for me to hang on.

Please clue us in when you find out what they say.

For starters, to me (and I DO use C-IQ) IOB is probably the third most important thing that I look at: First is current glucose level, second is trend arrow (or the 3-hr plot), but number three is IOB. If my glucose level is 130 mg/dl but dropping, I take very different actions if I have IOB = 0 (where I probably just let C-IQ dial back my basal) versus IOB = 2 units where I will definitely eat something.

Also, aside from the missing remaining time, IOB tells me the same thing regardless of whether I run C-IQ or not. In either case it is now much remaining active insulin do I have to bring down BG levels.

Why doesn’t C-IQ show the remaining time for IOB any more? I think that is because with automatic increases in basal rate, it is hard to define the end point. For starters, with a simple bolus of, for example, 5 units it is easy. Note: I am assuming that Tandem linearly tapers IOB over time. They may use a different curve, but if they do, the principle is the same, the math is just trickier.

So, under the linear decay of IOB assumption, 5 unit bolus would 5 U of IOB at bolus completion, 4 U of IOB after 1 hour, 3 U of IOB after 2 hours, 2 U of IOB after 3 hours, 1 U of IOB after 4 hours and 0 U of IOB after 5 hours. We all agree on that (assuming linear decay), correct?

Now, suppose in addition to that 5 U bolus at time = 0, C-IQ increases the basal by 0.1 U at time = 1 hours, another 0.1 U at 1 hr 30 minutes, a third 0.1 U at 2 hours, a fourth 0.1 U at 2 hr 30 minutes, and a fifth 0.1 U at 3 hours. Each of those additional 0.1 U will begin to “decay” by 0.02 U/hr (assuming linear decay) but all starting at different points in time.
I put that into an Excel spread sheet and plotted IOB versus time.

Here is what it looks like:


You can see that it has the initial steep decay due to the original 5 U bolus, but a long, gradual “tail” as each of the C-IQ added bits decay and timeout. So, after 5 hours the IOB is down to 0.2 U, but it doesn’t actually reach zero until 8 hours (5 hours after the last 0.1 increased bolus). So, while we can probably agree that the IOB ends somewhere between 5 and 8 hours after the initial bolus in this example, it is probably hard to define that actual end point … particularly if there was another C-IQ added bit of basal fairly late in this decay.

As a result, Tandem is doing the complex IOB calculation for us, but I suspect (I have no “insider information”) chose not to show the remaining time, because it is hard to define when you could have a significant series of “mini-boluses” over time.

At least that is my opinion.

Stay safe!


Well, yeah… that is what I referred to as cumulative IOB and hard to define the end. So why then do they even include the figure at all? There is nothing there to “hang my hat on.” And the cumulative number is so high that it scares me. It certainly does not help me determine what a correction dose should be. If I just let the pump run its course without any correction dose, then I spend many more hours in the much higher range than I would like.


Thanks, that was a great explanation and graphs too.

At the moment I am using SLEEP mode all day long.

I have two different IOB when Control-IQ SLEEP is on and off. When Control-IQ is off, the insulin on Board has no time next to it. I am looking at it now and with Control-IQ on, I have IOB of .17 and when Control-IQ is off I have IOB of 1.34 and time of 4:48. This makes sense to me as I have just given myself a correction bolus.

I agree that it makes no sense to see a time decay when Control-IQ is on and that it could have a tail of up to 8 hours.

What I really want to know is how should I utilize the IOB when Control-IQ is ON. Right now, Control-IQ is ON, my BG is 153 and IOB is .17. I assume that eventually Control-IQ SLEEP will bring me down to 110. I do not want to wait 8 hours (more than 4) to take me down. When I see a number for IOB (with Control-IQ ON I assume I could take a bolus and it would simply speed up the process of getting me to target.

I don’t like being high so I always give myself a bigger bolus to take it down faster. But if I did not give myself correction boluses could I simply rely on Control-IQ to do its thing BUT if I happen to look and see an IOB give myself a bolus (such as .17) to speed things up?


A couple of comments you have made in this thread lead me to believe that you think that the IOB in Control-IQ is fundamentally different than it is without Control-IQ turned on. While it is calculated differently because Control-IQ can add either increases in basal rate (that count as a bolus) or (if it is NOT in sleep mode and other conditions are met) can automatically give you a bolus once per hour based on projected glucose levels 30 minutes from now, IOB in either case is fundamentally the same.

In either case (Control-IQ on or Control-IQ off) a large value of IOB tells you that you have plenty of insulin available to help bring down high BG, whereas low or zero values tells you that there is little or no active insulin (other than basal …) remaining in your system.

Let me give me a couple of examples, all starting with a flat BG reading of 220 mg/dL. I’m assuming a correction factor of 50 mg/dL per unit of insulin and the Control-IQ target of 110 mg/ dL.

Case one: IOB = 3.5 U. In principle, that large IOB would reduce your current 220 mg/dL by 175 mg/dL down to 45 mg/dL. While that may not happen because Control-IQ will see that and begin to dial back basal insulin … you certainly don’t want or need a bolus.

Case 2: IOB = 2U. Ideally, those 2 U are enough to drop you from 220 mg/dL to 120 mg/dL … so you probably don’t want or need to do anything.

Case 3: IOB = 0.2 U. That amount of IOB is only likely to drop you from 220 to 210 mg/dL. Hopefully Control-IQ will either begin to ratchet up basal or give you 60% of the bolus that it thinks you need. However, that may be too slow for you so you might want to do a manual bolus. In that case, the bolus wizard would likely suggest a 1.8 U bolus: you need a total of 2.0 (ideally) to drop you from 220 to 110 mg/dL, but you already have 0.2 U of IOB, so the remaining amount you need is 1.8 U.

Do you agree with my numbers in all 3 cases?

For me, at least, I ALWAYS look at IOB to help me determine if I need to either prepare an additional bolus or reach for a snack.

Stay safe!



Yes, what I was showing in the plot was exactly the cumulative IOB that you described. To me, that is certainly a useful number to see (even without the remaining time).

Note: I have NOT noticed any appreciable change in IOB numbers since I began using Control-IQ. In my case, prior to Control-IQ my insulin duration was set to 4 hours 45 minutes … which is virtually identical to 5 hours. Also, I really didn’t change I/C or correction factors to make Control-IQ “more aggressive”.

Can any of those changes explain why you pre-C-IQ and post-C-IQ IOB numbers are so different?

Oh yes, I only run sleep mode when I am sleeping …

Stay safe!


I have been running CIQ since the beginning of this year. I did not run Sleep Mode 24/7 for the first 3 months. Three months later, my A1c was higher than before. At the average that the algorithm was keeping me, there was no way that I could get the consistently lower 6%s I had before. So I went to 24/7 Sleep Mode where the target range is lower. But Sleep Mode does not give any automatic corrections to temper highs. So that was up to me. I did reduce my ISF to 1:35, not a big difference from the previous 1:40. And I also changed my i:c ratio from 1:7 to 1:6. Not a huge difference either. I did have a duration of 3 hours previously. I find that the IOB numbers jump around a great deal. I just want to know what use I can safely make of the IOB now. Why provide the number if I can’t use it?

The way you have CIQ set up and use is different from me. I do not see much relevance in your experience.


I wanted to thank you for all the effort and illustrations you provided to make your points. I am in agreement with most of your points. I think I understand your point that the IOB amounts are fundamentally the same. I also agree with your Cases except for what I think is a typo in case 2 where 110 should probably be 120.

Unlike you I do not want to have to “prepare” an additional bolus if my BG is high and there is little IOB. I also believe you do not have to. The Bolus Delivery Calculation screen shows you both the Correction required and IOB (both delivered and unabsorbed bolus and basal insulin) to get you back to target. Note that IOB on the home screen may be different than IOB on the Calculation screen… I believe Tandem is confusing people by showing both as being IOB. IOB on the home screen seems to simply be, a Net Bolus if you decide to step in and give yourself a bolus to get yourself to target without having Control-IQ do it over a much longer time frame.

I think I have figured out what I was asking but I will talk to Tandem on Monday to confirm. Of course I would always give myself a higher bolus than Calculated to get myself to target quicker and eat something later if I need to tweak it.

Do you agree?

Many users have suggested you can simply increase you weight in your profile for Control IQ to give you more insulin so you can end up closer to 100 rather than 110.


Thanks for catching my typo in my case #2 example. I have gone back in and fixed that.

I usually look at the IOB on the home screen, but will try to look more carefully at the IOB on the calculation screen to see why and by how much it differs for me. I agree that one would expect IOB to match in either location.

Stay safe!


I was incorrect they do match. But when you hit the bolus screen it gives you a blood glucose number which you can use rather than go up and calculate it yourself. It takes into account both delivered and unabsorbed bolus and basil insulin.

I am late to the conversation, and please forgive my uninformed questions/comments because I have never ever paid any attention to any IOB calcs. And I don’t have IOB calcs turned on with my OmniPod, so it is not even displayed.

Here is what I am wondering. If you are 300 and you take a bolus, will the IOB be the same as if you were 100 and took the same bolus? Because really, those 2 situations should result in a totally different amount of IOB shown. Insulin that has pushed glucose into your cells will no longer be “on board”. It will be gone.

Same if you eat. If you bolus X units, and eat a big meal, that insulin is not “on board” in the same way as if you did not eat.

Do pump IOB calculations consider stuff like that? I have those things in my head. I don’t use the pump IOB calcs, I just kind of track my own insulin/food/BG stuff. The Omnipod IOB calcs seemed primitive to me. Like once I have corrected a high BG, I don’t really need to worry too much about that insulin anymore.

Medtronic is different than T Slim with Control IQ ON. I don;t know anything about Omnipod.

Once you deliver a bolus, your IOB goes up by the amount of the bolus. Then it takes IOB down based on your selected duration (3 hrs, 5 hrs, etc.). I believe it uses a straight line method to take it down but it may be more sophisticated than that.

If you have more IOB than a required correction (how many units you need to get your current BG back down to your target).

Control-IQ is different in that it also figures how much additional insulin you have or don’t have because it has adjusted your basal insulin. I think I have this figured out now.

If you use Control IQ you can just let Control IQ do its thing and over time it will get you to target with multiple basal adjustments. For me I hate to be high and rather than waiting forever for Control IQ to get you to target, you can use their bolus screen and give yourself the bolus adjustment that they recommend or even a bigger bolus if you are like me.

If you have specific questions ask away.

No the IOB calculations are pretty strictly a cumulative record of time since injection, with a certain insulin duration that can be programmed or is fixed in some of the situations, but certainly won’t take into account the additional information you have suggested. Would be a good avenue to improve the algorithm’s though. I would expect that stuff like this will be included in pumps in a couple of generations.

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So none of the pumps IOB calcs are truly “insulin that has not yet been used”. They are all just what you have taken. That is what I was thinking, they all did it the same as omnipod.

Omnipod is also a linear depletion, which seems silly.

Chris, I don’t understand what you just said. Could you state differently.

Sure thing. The pumps IOB calculations that I have been exposed to are a First in, first out calculator with a non-programmable decay rate. So for example if you took 5 units of insulin. Immediately after injection the IOB would show 5 units, then after a unit of time expires you would have 4.75 units onboard, and it will decay until you hit the total insulin duration time (which can be programmed in most pumps, although not in Control IQ) then it would show 0 IOB.

If you take additional insulin while there is still IOB, it would add the second insulin bolus to the IOB and start calculating that in the same way.

So cumulative record of bolus insulin. None that I am aware take other factors into accounts such as what Eric hypothesized, which would be awesome.