Insulin on Board (IOB) Calculator
Estimate how much active insulin remains from a previous bolus dose. Accounting for IOB prevents dangerous insulin stacking — a leading cause of hypoglycemia. Supports linear and exponential decay models.
⏳ IOB Estimator
Insulin on Board Estimate
still active
consumed
fully cleared
used
| Time (min) | % Active | Units Active |
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Why IOB Matters
Insulin stacking — giving a correction dose without accounting for insulin still active from a previous bolus — is one of the most common causes of hypoglycemia. If 3 units from a meal bolus are still active, a correction dose should be reduced by 3 units to avoid over-correcting.
Adjusted Correction Formula
If the result is negative, no correction is needed — existing active insulin is already sufficient (or will cause low BG). Never give a negative correction; it simply means you should wait and recheck your glucose.
IOB models are approximations. Exercise accelerates insulin absorption. Injection into muscle vs. subcutaneous fat changes onset and duration. Heat increases absorption rate. Your personal DIA may differ from the selected value. If your CGM or pump calculates IOB, use that instead — it has more data about your actual doses and timing.
Why active insulin is the key to avoiding lows
Insulin on board (IOB) is the amount of a previous dose still working in your body. It matters because rapid-acting insulin doesn't finish the moment your glucose looks better — it keeps lowering blood sugar for hours afterward. If you forget about that lingering insulin and dose again, the two overlap and can drive you into a hypoglycemic low. Knowing your IOB is the simplest way to answer the question every person on insulin eventually asks: "I'm still high, but did my last dose already handle it?"
Most pumps and many apps track IOB automatically for exactly this reason. Understanding the concept yourself, though, helps you make safer decisions even with a pen and a meter.
How long your rapid-acting insulin keeps working
The window over which insulin stays active is called the duration of insulin action, and for modern rapid-acting insulins it's typically three to five hours. This calculator uses that duration to estimate how much of a dose remains, either by assuming a steady linear decline or a more realistic curved (exponential) decay that front-loads the action.
Why the decay isn't a straight line
In reality, insulin works hardest in the first hour or two and then tails off. A curved model captures that better than a simple straight-line estimate, which is why pump settings let you choose a duration and often assume a curved profile. Your personal duration can vary, so treat the result as a well-reasoned estimate rather than an exact figure.
Using insulin on board before you correct a high
The practical payoff is in correction doses. Before adding insulin to bring down a high reading, subtract your IOB from the correction your numbers suggest — because that active insulin is already on its way to lowering you. Skipping this step is the classic cause of "stacking," where back-to-back corrections pile up and tip you into a low a couple of hours later. Pair this tool with our bolus calculator when you're deciding whether a correction is truly needed, and confirm your insulin duration with your care team.
Frequently Asked Questions
What is insulin on board (IOB)?
Insulin on board is the amount of rapid-acting insulin from a previous dose that is still active and lowering your blood glucose. Tracking it prevents "stacking" doses, a leading cause of hypoglycemia.
How is IOB calculated?
IOB is estimated from how long ago you dosed and your duration of insulin action (DIA). A linear model assumes insulin clears evenly over the DIA; curvilinear models clear faster early and slower late. This tool shows both — for example, 2 hours into a 4-hour DIA leaves roughly half the dose active.
What is duration of insulin action (DIA)?
DIA is how long a dose of rapid-acting insulin keeps working — commonly 3 to 5 hours. Your true DIA can vary, so use the value your care team or pump uses for you.
Why subtract IOB before a correction dose?
Because insulin already working will keep lowering your glucose. Adding a full correction on top of it can push you low. Subtract IOB so you only take the insulin still needed: Net correction = ((BG − Target) ÷ ISF) − IOB.
Sources
- Walsh J, Roberts R. Pumping Insulin. 5th ed. Torrey Pines Press; 2012.
- Schiavon M et al. "Modeling of insulin action in closed-loop insulin delivery." Diabetes Care. 2020.
Last reviewed: June 2025