Insulin-to-Carb Ratio Explained
The insulin-to-carb ratio (ICR) is the number of grams of carbohydrate covered by one unit of rapid-acting insulin. Understanding ICR is the foundation of flexible mealtime insulin dosing — here's what it means and how to use it.
What Is the Insulin-to-Carb Ratio?
The ICR tells you: "For every X grams of carbohydrate I eat, I need 1 unit of rapid-acting insulin."
If your ICR is 15 g/unit, that means one unit of rapid-acting insulin (like lispro, aspart, or glulisine) covers 15 grams of carbohydrate. To eat 60 grams of carbs, you would need 60 ÷ 15 = 4 units.
Meal Bolus Formula
How Is ICR Calculated? — The 500 Rule
500 Rule
The constant 500 was derived empirically from clinical experience with rapid-acting insulin analogues. It assumes roughly 50% of TDD is used as bolus insulin. If your TDD is 40 units: ICR = 500 ÷ 40 = 12.5 g/unit — round to 12 or 13 g/unit depending on your pen's increments.
The 450 Rule is sometimes used instead — for patients on Regular (short-acting) insulin, or those who find the 500 Rule produces doses that are slightly too low. Compare: ICR (500) = 500 ÷ 40 = 12.5 vs ICR (450) = 450 ÷ 40 = 11.25.
Does ICR Stay the Same All Day?
Not always. Many people with Type 1 diabetes — and some Type 2 — find their ICR varies by meal time:
| Meal | Typical ICR Pattern | Reason |
|---|---|---|
| Breakfast | Lower ICR (more insulin per gram) | Dawn phenomenon — cortisol increases insulin resistance in the morning |
| Lunch | Moderate ICR | Relatively stable insulin sensitivity mid-day |
| Dinner | Higher ICR (less insulin per gram) | Increased activity in afternoon may improve sensitivity |
These are general patterns — individuals vary. Your diabetes educator can help identify if you need time-varied ICRs by reviewing your post-meal glucose logs.
Signs Your ICR Needs Adjustment
- Consistently high 2-hour post-meal glucose: ICR may be too high (not enough insulin per gram) — try a lower ICR (e.g., 12 instead of 15)
- Consistently low 2-hour post-meal glucose: ICR may be too low — try a higher ICR
- Pre-meal glucose fine, post-meal consistently off: ICR is the variable to adjust, not basal
ICR adjustments should be made with your diabetes care team, typically by reviewing 3–5 days of data for the same meal type.
Sources
- Walsh J, Roberts R. Pumping Insulin. 5th ed. 2012.
- ADA. Standards of Medical Care — 2024. Section 7.
Last reviewed: June 2025