Insulin on Board: What IOB Means and How to Avoid Stacking

The most common way to end up low is not a miscounted meal. It is a correction dose given on top of one that was still working. That leftover insulin has a name — insulin on board — and understanding how it decays is the difference between fixing a high and overshooting it into a low two hours later.

What Insulin on Board Actually Means

Insulin on board (IOB), sometimes called active insulin, is the portion of a previous bolus that has not finished working yet. Take 6 units at noon, and by 1:30pm perhaps 3.5 of those units have already done their job. The remaining 2.5 units are still in you, still lowering your blood sugar, and still unaccounted for if you look at your CGM at 1:30 and decide you need a correction.

That is the entire problem IOB exists to solve. Your glucose reading tells you where you are right now. It does not tell you where the insulin already in your body is going to take you.

How Long Insulin Actually Lasts

The window during which a bolus keeps working is called the duration of insulin action, or DIA. For the rapid-acting analogs most people use — lispro (Humalog), aspart (NovoLog), and glulisine (Apidra) — the general shape looks like this:

• Onset: roughly 10 to 20 minutes before it starts working
• Peak: around 1 to 3 hours, when it is pulling hardest
• Duration: 3 to 5 hours before it is effectively finished

The ultra-rapid formulations — Fiasp and Lyumjev — front-load that curve, starting sooner and peaking earlier, though the tail is not dramatically shorter. Regular human insulin sits at the other end entirely, with a duration closer to 6 to 8 hours.

Most pumps and bolus calculators ship with a default DIA of 4 hours. That default is a compromise, and it is worth knowing that it is a compromise rather than a measurement of your body.

Why the Decay Curve Is Not a Straight Line

Here is where a lot of mental math goes wrong. If you took 6 units three hours ago on a 4-hour DIA, it is tempting to assume a quarter of it is left — 1.5 units. Straight-line thinking: three quarters of the time has passed, so three quarters of the insulin is gone.

Insulin does not behave that way. It works slowly at first, hardest in the middle, and trails off at the end. Straight-line thinking under-states how much is still active in the first half and over-states it in the second — and those two errors fail in opposite directions. Under-stating it early means you think more has cleared than really has, which is the direction that stacks. Over-stating it late is the safer mistake: you hold back a correction you could have taken, and run high instead of low.

SweetLife uses the Walsh bilinear curve, a widely used approximation that assumes peak action at the midpoint of your DIA. On a 4-hour DIA and a 6-unit bolus, it works out roughly like this:

• 30 minutes in: about 5.8 units still active
• 1 hour in: about 5.2 units
• 2 hours in (the peak): about 3 units — half gone, half remaining
• 3 hours in: about 0.75 units
• 4 hours in: zero

Notice the shape. At one hour the straight-line guess says 4.5 units when about 5.2 are genuinely still working — you have three quarters of a unit more insulin in you than you think, right when a post-meal number is tempting you to correct. By three hours the error flips: the guess says 1.5 units when only 0.75 remain, and now you are holding back a correction you could safely take.

What Stacking Looks Like

Stacking is what happens when you give a correction while a previous dose is still active, without subtracting what is already on board. Two doses that would each be reasonable on their own become one dose that is too large.

A typical version goes like this. You bolus for lunch at noon. At 1:15pm you are still 220 mg/dL and it feels like the meal dose did not work, so you correct. At 2:30pm the lunch bolus is finally finishing, the correction is peaking, and both are pulling at once. You end up at 55 mg/dL, eat to treat it, and spend the afternoon on a rollercoaster you started yourself.

The insulin was never the problem. The timing was. A post-meal reading at 75 minutes is not a failure of the meal dose — it is a snapshot taken before that dose has finished.

Choosing a DIA That Matches You

The 4-hour default is a starting point, not a diagnosis. Some people clear insulin faster and do better at 3 to 3.5 hours; plenty of endocrinologists now favour 5 hours precisely because the shorter settings encourage stacking by declaring insulin finished while it is still working.

The direction of the error matters. A DIA set too short tells your calculator you have no insulin left when you still do, which invites an extra correction and a low. A DIA set too long is the conservative failure — it may leave you running a little high, but it will not stack you into a hypo.

This is a setting to change with your care team, not by intuition. What you can do on your own is gather the evidence: log your doses, watch what your glucose does in hours three and four after a bolus, and bring that pattern to your next appointment.

When IOB Matters Most

Three situations turn ordinary active insulin into a genuine risk.

Exercise. Physical activity increases insulin sensitivity, so the same units on board pull harder than they normally would. Starting a workout with 4 units active is a very different proposition from starting one with none.

Alcohol. The liver prioritises processing alcohol over releasing stored glucose, which removes the safety net that would normally catch a falling blood sugar. Active insulin plus a suppressed liver is the combination behind a lot of overnight lows.

Bedtime. A correction at 10pm on a 4-hour DIA is still working at 2am, when nobody is awake to notice. Checking IOB before bed is one of the highest-value habits in diabetes management, because it is the one window where a low goes unwitnessed.

What IOB Does Not Include

Two things commonly get confused with insulin on board.

Basal insulin is not IOB. Your long-acting dose, or your pump's basal rate, is background insulin covering the glucose your liver releases between meals. It is meant to be there. IOB tracks only the bolus doses layered on top — which is why SweetLife counts bolus entries toward IOB and ignores basal ones.

Food on board is the mirror image. Carbohydrates you ate 40 minutes ago are still raising your blood sugar the same way insulin from 40 minutes ago is still lowering it. A high reading with 3 units on board and a bagel half-digested is a very different situation from the same reading with 3 units on board and nothing eaten since breakfast.

How SweetLife Tracks It

Every bolus you log starts a decay curve. The home screen shows your current insulin on board and how long is left before it clears, so the number is there before you decide anything — not buried in a calculator you have to open deliberately.

The bolus calculator uses the same figure automatically: it works out your carb dose from your insulin-to-carb ratio, adds a correction from your sensitivity factor if you are above range, then subtracts what is already on board before suggesting a number. Your duration of insulin action lives in Profile under Insulin Settings, defaulting to 4 hours, and every calculation follows whatever you set there.

None of this replaces judgment, and the number the calculator produces is a suggestion for you and your care team to work from. But it does remove the arithmetic you were doing in your head at 1:15pm while feeling frustrated at a number — which is precisely the moment that arithmetic tends to go wrong.

The Short Version

Insulin on board is the dose still working from earlier. It decays on a curve, not a straight line, and it is at its most deceptive in hours two through four when it feels finished and is not. Before you correct, check what is already there. Before you exercise, drink, or go to sleep, check again.

The high you are looking at right now might already be handled.