Bolus insulin isn't only for meals. When blood glucose rises above your target range, you need a separate dose to bring it back down — this is called a correction bolus. The math is straightforward, but knowing when not to give one is just as important as knowing how.
This article is based on the educational video U-M Type 1 Diabetes 101 — Module 5: Carb Ratio & Correction Factor produced by Michigan Medicine. All medical concepts and examples are sourced from that video.
What Is the Correction Factor (ISF)?
The correction factor — also called the Insulin Sensitivity Factor (ISF) — tells you how many points your blood glucose will drop per one unit of rapid-acting insulin.
For example, a correction factor of 1:100 means that one unit of insulin will lower blood glucose by approximately 100 mg/dL.
Like the insulin-to-carb ratio, ISF:
- is unique to each person
- changes over time
- should be established and reviewed with your diabetes care team
How to Calculate a Correction Dose
First, you need a correction target — the blood glucose value you are aiming for. This is a number used for math purposes only. Any value between 70 and 180 mg/dL is considered acceptable.
Formula:
Correction dose = (Current BG − Correction target) ÷ ISF
Example
Correction target: 120 mg/dL Correction factor: 1:100 Current blood glucose: 220 mg/dL
(220 − 120) ÷ 100 = 1 unit
If blood glucose is 320 mg/dL:
(320 − 120) ÷ 100 = 2 units
Straightforward — as long as you know your ISF and your target.
When You Should NOT Give a Correction Bolus
This is the most important part. Giving a correction dose at the wrong time can cause hypoglycemia.
Do not give a correction bolus if any of the following is true:
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Current blood glucose is already below the correction target. There is nothing to correct — adding insulin will push glucose too low.
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Less than 3 hours have passed since the last correction dose. Rapid-acting insulin continues working for 3–4 hours after injection. Stacking doses before the previous one finishes increases the risk of a sharp drop.
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Less than 1 hour has passed since vigorous exercise. Exercise raises insulin sensitivity significantly. The same dose that worked before a workout will act much stronger afterward.
It is also good practice to account for insulin on board (IOB) — units from a previous bolus that are still active. Most insulin pumps and diabetes apps track this automatically.
Correction Factor vs. Carb Ratio — What's the Difference?
Both are called "ratios" and both affect your bolus dose, which makes it easy to mix them up.
| | Insulin-to-Carb Ratio (ICR) | Correction Factor (ISF) | | ----------------- | ------------------------------------- | -------------------------------------- | | Purpose | Calculate mealtime dose | Calculate correction dose | | What it tells you | How many grams of carbs 1 unit covers | How many mg/dL 1 unit will lower BG | | Example | ICR 10 → 1 unit per 10 g carbs | ISF 100 → 1 unit drops BG by 100 mg/dL |
Both values can vary by time of day. Morning insulin resistance is typically higher — so your ICR and ISF at breakfast may differ from your evening numbers.
How Often Should Corrections Be Needed?
Ideally, as rarely as possible. Frequent correction boluses are a signal worth investigating: the basal dose may be off, mealtime calculations may need refinement, or other factors like stress or illness are at play.
If you notice corrections are consistently needed at the same time of day, bring that pattern to your next clinic visit. It's likely a reason to adjust your settings.
Track Every Correction with Dia-Log
Keeping a record of your correction doses is one of the most useful things you can do for your diabetes management — and one of the easiest things to let slip.
Dia-Log logs every correction bolus automatically: the current glucose, the dose, and the time. Over days and weeks, those entries reveal patterns that are impossible to spot in the moment — which times of day your glucose tends to run high, whether your ISF is working, and what data to bring to your doctor.
The app works fully offline, so you can log a dose anywhere, even without a connection.
This article is for educational purposes only and is based on the Michigan Medicine educational series U-M Type 1 Diabetes 101. All insulin dosing decisions should be made in consultation with your diabetes care team.