TL;DR
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.
Simple
Watch the video or read on:
Deep
How do you calculate a correction dose?
To calculate a fast-acting insulin correction dose, you need three things:
- Target glucose. A sensible default is ~5.5 mmol/L (100 mg/dL), unless your team has set something different.
- Correction factor (insulin sensitivity factor). This tells you how much 1 unit of fast-acting insulin drops your glucose. Example: 1u lowers glucose by 3.0 mmol/L (≈54 mg/dL).
- Current glucose level.
Your diabetes team will usually give you correction factors for different times of day. The same logic we covered for carb ratios applies: factors vary with fitness and with time-of-day insulin sensitivity.
Mine are currently:
- Breakfast: 1u : 3.0 mmol/L (1u : 54 mg/dL)
- Lunch: 1u : 4.0 mmol/L (1u : 72 mg/dL)
- Evening meal: 1u : 3.5 mmol/L (1u : 63 mg/dL)
Example: if my glucose is 11.0 mmol/L (200 mg/dL) and I want to correct down to target, here’s the formula:
(Current glucose − target glucose) ÷ correction factor = correction insulin units
Breakfast example:
- 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 that 1.8-unit correction:
- ~3 hours to get back into range (<10.0 mmol/L / 180 mg/dL).
- ~4 hours to reach 5.5 mmol/L (100 mg/dL).
The trap: insulin stacking
Here’s the classic mistake: an hour or so after correcting, glucose still hasn’t moved much, so you feel tempted to take the same correction again.
What happens? The insulin piles up. Two hours later, glucose drops hard. That’s insulin stacking.
The only way to avoid a hypo disaster is to eat extra carbs and overshoot, which creates the glucose rollercoaster. It’s not a fun ride.
This is exactly what happened to me on Christmas Day 2018 — and we’ll unpack it properly in Dynamic Glucose Management.
Bottom line: don’t judge a correction until the insulin has had time to work. Use CGM trends, not impatience.
Practical
If you want something that drops glucose faster than insulin between meals, there is a better tool — and it works in ~30 minutes rather than four hours.
That brings us to the next section: Activity and exercise.
For a deeper dive, jump to the podcast on activity snacking.
References
What’s next
Next step: Activity and exercise.
