The IOB Guide for T1D — Part 3

Choosing Your Device-Specific IOB Settings: What Are You Optimising For?

Part 2 explained how different IOB models work mathematically and where the structural trade-offs lie. Part 3 looks at how commercial systems implement these models in practice — and what that means for real-world decision making.

The central question

What are you optimising for?

There is no universally correct AIT setting. The right choice depends on whether your priority is correction freedom, stacking protection, or exercise safety awareness. All three cannot be perfectly satisfied simultaneously by a single setting when meal and correction insulin are pooled. Understanding your system’s architecture is the first step toward managing its trade-offs deliberately.

Bolus calculator apps

Many people use standalone bolus calculator apps on their phone or in pen-connected devices. These apps vary considerably in their IOB architecture.

App	IOB architecture	What it optimises for	Structural limitation	Best used when
mySugr Accu-Chek Bolus Advisor	Expected glucose after-meal behaviour model (offset + meal rise + acting time). Correction IOB deducted for corrections.	Trajectory-aware correction timing and stacking governance	Does not display live insulin exposure (units on board hidden)	You want personalised after-meal glucose control and are comfortable without a visible IOB number.
Diabetes:M (1 & 3) mylife App (3) InPen (3) RapidCalc (3)	Linear/curvilinear decay model. (1) No IOB recognised for corrections (2) Correction IOB deducted (3) Meal & Correction IOB deducted	Visible active insulin and straightforward stacking control	Behaviour highly dependent on AIT setting and whether active insulin is enabled	You want to see total active insulin and are comfortable managing the correction-exposure trade-offs.

Common workarounds with apps

Shortening AIT to unlock corrections. Allows more aggressive corrections but increases stacking risk and gives a false sense of safety when exercising 2 to 4 hours after a bolus.
Repeated small corrections in the first 1 to 3 hours after eating. A common pathway to correction hypos — rage bolusing.
Using a behaviour-based app for exercise planning without a visible IOB number. If insulin exposure is hidden, you may underestimate risk and over-trust a screen that looks safe.

Standard pumps (without AID)

Standard pumps typically use a linear or curvilinear decay model. Meal and correction insulin are usually pooled, so the pump cannot know why insulin was given — only that it exists. Shorter AIT settings allow faster corrections. Longer AIT settings slow corrections. This means underestimated meals can feel like blocked corrections in protective configurations, while aggressive configurations increase the risk of stacking. Exercise safety is heavily dependent on how realistically AIT reflects insulin duration.

Manufacturer	Systems	Bolus calculator IOB model	User settings
Medtronic Bolus Wizard	530G → 780G (bolus calculator)	Curvilinear decay (meal + correction pooled and deducted from corrections)	AIT/DIA user adjustable (2 to 8 hours in 15-minute increments)
Insulet Omnipod	DASH, Omnipod 5 (Manual Mode)	Linear decay (meal + correction pooled and deducted from corrections)	AIT user adjustable (2 to 6 hours in 30-minute increments)
Tandem t:slim X2	Manual Mode	Linear decay (meal + correction pooled and deducted from corrections)	AIT user adjustable (2 to 8 hours in 15-minute increments)
Ypsomed mylife YpsoPump	Without AID	Linear decay (meal + correction pooled and deducted from corrections)	AIT/DIA user adjustable (2 to 8 hours in 15-minute increments)

Common workarounds for standard pumps

Shortening AIT to fix blocked corrections when meals are underestimated. This often converts a meal dosing problem into a stacking problem and increases late hypoglycaemia risk.
Chasing highs with multiple corrections in the 1 to 3 hours after a meal. With pooled IOB, this can compound insulin exposure even if the pump appears to permit it.
Using pump IOB as an exercise green light without considering meal boluses and recent corrections together. Total exposure matters, not just whether a correction was given.

AID and hybrid closed-loop systems

AID systems add a second layer: a user-facing bolus calculator and an internal control algorithm. These layers may not use the same insulin accounting. In practice, displayed IOB may not reflect total insulin exposure generated by automation. Automation can moderate, or add to, insulin exposure in ways the user does not directly see.

System	User-facing bolus IOB model	User settings exposed	What the algorithm adds	Structural limitation and practical considerations
Medtronic 670G / 780G	Bolus Wizard and SmartGuard: Curvilinear decay (meal + correction IOB)	AIT/DIA adjustable for bolus calculator and algorithm correction behaviour (2 to 8 hours in 15-minute increments)	Automated basal adjustments and micro-corrections incorporating total IOB.	When AIT is set very short (2 hours), the bolus calculator permits aggressive correction dosing which can improve post-meal glucose control. Basal suspension provides some protection but cannot remove insulin already delivered. When AIT is at 2 hours, exercise-related hypoglycaemia risk can be high and displayed IOB will significantly underestimate physiological exposure. With longer AIT (4 hours or more), both meal and correction insulin remain counted as IOB for longer, which may require workarounds such as ghost carbohydrates. Many users choose the shortest AIT that does not produce excessive hypoglycaemia, often 2 to 3 hours, while recognising that displayed IOB still underestimates total exposure for exercise planning.
Omnipod 5 (Automated Mode)	Bolus calculator: linear decay (meal + correction IOB)	AIT adjustable for bolus calculator (2 to 6 hours in 30-minute increments)	Automation manages insulin delivery using a separate internal IOB pool. The exact method is not publicly defined. Negative IOB values can appear, suggesting basal reductions are being subtracted from a pooled estimate.	The Omnipod 5 uses a relatively conservative control algorithm that primarily looks around 60 minutes ahead when making automated adjustments. Because of this short prediction horizon, insulin suspension often occurs earlier than in some other systems. A shorter user-defined AIT (typically 2 to 2.5 hours) often allows more effective user-initiated corrections. Using a longer AIT often leads to prolonged hyperglycaemia because both meal and correction insulin remain counted as IOB for longer. Many users find that an AIT of around 2 to 2.5 hours, combined with a stronger correction factor and reverse correction disabled, allows more effective user corrections when required.
Tandem t:slim X2 with Control-IQ	Bolus calculator: linear decay (meal + correction IOB)	Fixed AIT 5 hours in Control-IQ	Control-IQ provides automated basal modulation and automated correction boluses based on the correction factor. User IOB from meal and correction boluses is incorporated using a fixed 5-hour insulin action curve.	Because IOB can become both positive and negative relative to programmed basal, it is helpful for basal insulin to represent at least 50 to 60% of total daily insulin. This gives the algorithm enough range to increase or decrease delivery effectively. Some users achieve this by using slightly weaker carbohydrate ratios so more insulin is delivered via automated basal adjustments rather than large bolus doses. A practical approach that often works well: basal 50 to 60% of total daily insulin, weaker carbohydrate ratios, strong correction factors. For exercise hypo risk prediction, the fixed 5-hour AIT is beneficial because displayed IOB is a good approximation of physiological insulin exposure.
CamAPS FX	Proprietary internal insulin action modelling (pharmacodynamic kinetics)	No meaningful user adjustment of insulin action model	The algorithm pairs carbohydrate entry and insulin delivery within its own internal model.	Highly specialised but opaque. The system uses a proprietary insulin action model and users cannot directly adjust insulin duration. Behaviour of the algorithm is therefore largely fixed and not transparent. The system includes a Boost mode, which strengthens the algorithm response without requiring additional carbohydrate entry. For many users this is preferable because it avoids manipulating carbohydrate entries or confusing data interpretation.

The question this leads to

Understanding these trade-offs reveals the central design limitation of current IOB systems. Every device is doing its best with one number that is being asked to do two jobs. Understanding the specific model your device uses is the first step toward managing its predictable risks deliberately rather than reactively.

This leads directly to the question at the centre of Part 4: “Could IOB be designed differently?”

Continue the guide

← Part 2
Part 4: The Future →

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