Correction insulin: why patience matters more than maths

Overview

Correction insulin is fast-acting insulin used to bring a high glucose level back into target between meals. The maths is simple, but the physiology isn’t: insulin takes hours to work, so repeating corrections too soon leads to stacking, hypos, and the glucose rollercoaster.

This page explains how to calculate corrections, why “fast-acting” insulin still feels slow, and how impatience — not bad maths — causes most correction-related problems.

The detail

How do you calculate a correction dose?

To calculate a fast-acting insulin correction dose, you need three things:

1. Target glucose. A sensible default is ~5.5 mmol/L (100 mg/dL), unless your team has set something different.
2. Correction factor (insulin sensitivity factor). How much 1 unit of insulin lowers glucose.
3. Current glucose level.

Correction factors vary by time of day for the same reasons carb ratios do: insulin sensitivity changes with activity, fitness, and circadian rhythm.

Example correction factors:

• Breakfast: 1u : 3.0 mmol/L (≈54 mg/dL)
• Lunch: 1u : 4.0 mmol/L (≈72 mg/dL)
• Evening meal: 1u : 3.5 mmol/L (≈63 mg/dL)

Example calculation:

(Current glucose − target glucose) ÷ correction factor = correction units

• mmol/L: (11.0 − 5.5) ÷ 3.0 = 1.8 units
• mg/dL: (200 − 100) ÷ 54 = 1.8 units

Why “fast-acting” still feels slow

This graphic shows what happens after a 1.8-unit correction:

• ~3 hours to return below 10.0 mmol/L (180 mg/dL)
• ~4 hours to reach ~5.5 mmol/L (100 mg/dL)

The trap: insulin stacking

The classic mistake is repeating a correction too soon because glucose hasn’t moved yet.

The result: insulin piles up. Two hours later, glucose drops hard. That’s insulin stacking.

Real-world CGM traces show this pattern clearly: an early second correction creates a delayed hypo, followed by defensive eating and a rebound high — the glucose rollercoaster.

Bottom line: don’t judge a correction until the insulin has had time to work. Use CGM trends, not impatience.

Practical foundations

If you want glucose to fall faster between meals, insulin is not the best tool. There is a faster lever — and it works in ~30 minutes rather than hours.

What’s next

Next up: Activity and exercise.

Foundations / related content

The order shown below is recommended, but navigate as you see fit.

• What is Diabetes?
• Continuous glucose monitoring (CGM)
• Basal insulin
• Bolus insulin
• Correction insulin
• Carbohydrate counting
• Three balanced meals
• Hypoglycaemia
• Measuring success

References

• Bolus insulin
• Dynamic Glucose Management
• Activity & exercise
• Activity snacking podcast