What Is Insulin on Board?
By Ryan Mitchell · Referenced against insulin pharmacokinetics and pump-therapy practice · Updated July 2026
The idea
Insulin on board is the portion of a previous dose still working. Take 6 units of rapid-acting insulin and it does not act and finish — it works over several hours, tapering off. Two hours later a meaningful fraction is still lowering your glucose.
That matters at exactly one moment: when you are deciding whether to correct a high reading. If you check at 200 mg/dL (11.1 mmol/L) two hours after a meal bolus, some of that bolus is still working and will bring the number down without further help. Correcting the full gap as though nothing is active means taking insulin twice for the same glucose. That is stacking, and it is one of the more common causes of avoidable hypoglycemia.
Duration of action drives everything
IOB depends entirely on how long your insulin is assumed to work — the duration of insulin action, or DIA. Change the assumption and every IOB figure changes with it.
| Insulin | Typical DIA | Notes |
|---|---|---|
| Ultra-rapid (Fiasp, Lyumjev) | 3–4 hours | Faster onset and clearance |
| Rapid-acting (lispro, aspart, glulisine) | 4–5 hours | The common default |
| Regular human insulin | 6–8 hours | Longer tail than analogues |
Many pumps ship with a DIA of 3 hours, which is shorter than most rapid-acting insulins actually last. A short DIA setting makes the pump believe insulin has finished when some is still working, so it under-reports IOB and permits corrections it should be discouraging. If you stack without meaning to, the DIA setting is the first place to look.
Linear versus exponential decay
There are two ways to model the decline, and they disagree most in the middle — which is precisely when people check.
A linear model assumes insulin disappears at a constant rate: with a 4-hour DIA, 25% is gone every hour. Simple, and what many older calculators use. An exponential model assumes it clears more slowly at first, then faster, which better matches how rapid-acting analogues actually behave.
| Time since dose | Linear (4h DIA) | Exponential (4h DIA) |
|---|---|---|
| 1 hour | 75% remaining | ~80% remaining |
| 2 hours | 50% remaining | ~50% remaining |
| 3 hours | 25% remaining | ~20% remaining |
| 4 hours | 0% | ~0% |
Both agree at the halfway point and at the ends, and differ by roughly five percentage points either side. On a 6-unit bolus that is about a third of a unit — small for an adult, but not negligible for a child on half-unit dosing. Our IOB calculator shows both curves.
How stacking actually happens
The classic sequence: bolus for a meal, check two hours later, see a number that has not come down as far as expected, correct it. The bolus was still working. The correction lands on top, and roughly an hour after that the two together overshoot.
What makes it hard to spot is the delay. The low arrives hours after the decision that caused it, by which point the obvious explanation is the most recent thing you did rather than the correction you took earlier. People often conclude their correction factor is too strong and weaken it — which then leaves genuine highs undertreated.
The practical rule is to subtract IOB from any correction before taking it. If the calculation says 3 units and you have 1.5 units still active, the correction is 1.5. Most pumps and bolus calculators do this automatically, which is much of their value; on injections it has to be done deliberately.
One important limit: IOB accounting applies to corrections, not to carbohydrate. Insulin still working from an earlier dose is committed to the food you already ate, so subtracting IOB from a meal bolus systematically underdoses meals.
Limitations & Safety Notes
- IOB is a modelled estimate, not a measurement. No device measures active insulin in the body.
- The figure depends entirely on the assumed duration of action. A DIA set shorter than your insulin actually lasts under-reports IOB and permits stacking.
- Linear and exponential models disagree by roughly five percentage points in the middle hours — small in absolute units for adults, more significant on paediatric doses.
- Absorption varies with site, temperature, exercise and scar tissue, none of which any IOB model accounts for.
- Subtract IOB from corrections, not from carbohydrate coverage — insulin already given is committed to food already eaten.
Frequently Asked Questions
What does insulin on board mean?
It is the amount of a previous insulin dose still expected to be working. It matters most when deciding whether to correct a high reading, because insulin already active will keep lowering glucose without further help.
How long does insulin stay on board?
As long as its duration of action — typically 4 to 5 hours for rapid-acting analogues, 3 to 4 for ultra-rapid formulations, and 6 to 8 for regular human insulin. The DIA setting on a pump defines this for its calculations.
What is insulin stacking?
Taking a correction while a previous dose is still working, so the two overlap and drive glucose lower than either would alone. The resulting low typically arrives hours later, which makes the cause easy to misattribute.
Should I subtract IOB from my meal bolus?
No — only from corrections. Insulin still active from an earlier dose is already committed to carbohydrate you have eaten. Subtracting it from a new meal bolus systematically underdoses the meal.
Why does my pump show a different IOB than I expect?
Almost always the DIA setting. Many pumps default to 3 hours, shorter than most rapid-acting insulins actually last, so IOB reads lower than reality and the pump permits corrections it should be discouraging.
Is linear or exponential IOB more accurate?
Exponential better reflects how rapid-acting analogues clear. The practical difference is around five percentage points in the middle hours — roughly a third of a unit on a 6-unit bolus, which matters more for children on half-unit dosing than for most adults.
Sources
- American Diabetes Association. Standards of Care in Diabetes — 2026. Diabetes Care. 2026;49(Suppl 1). Link — insulin therapy and pump settings
- Walsh J, Roberts R. Pumping Insulin. Torrey Pines Press. — duration of action and bolus-calculator practice
- Exploring the Burden of Mealtime Insulin Dosing in Adults and Children With Type 1 Diabetes. PMC. Link — bolus decision-making in practice
Last reviewed: July 2026