🥗Insulin-to-Carb Ratio (ICR) & the 500 Rule
Your Insulin-to-Carb Ratio tells you how many grams of carbohydrate one unit of rapid-acting insulin will cover. Here's how to calculate it, use it correctly, and know when to adjust it.
What Is an Insulin-to-Carb Ratio?
An Insulin-to-Carb Ratio (ICR) — also written as I:C ratio — expresses how many grams of carbohydrate are covered by 1 unit of rapid-acting insulin. For example:
- An ICR of 1:15 means 1 unit covers 15 grams of carbohydrate.
- An ICR of 1:10 means 1 unit covers only 10 grams (a more insulin-sensitive person needs more units per gram).
- An ICR of 1:20 means 1 unit covers 20 grams (a less insulin-resistant person).
The ICR is specific to rapid-acting and short-acting mealtime insulins. It is not used for basal insulins, which are dosed separately.
The 500 Rule: Calculating Your ICR from TDD
The most widely used method to estimate an initial ICR is the 500 Rule, also called the Insulin-to-Carb Factor formula:
The constant "500" was empirically derived from observations of thousands of patients on intensive insulin regimens. It reflects the approximate relationship between a person's total daily insulin requirement and their carbohydrate metabolism.
Worked Example: Calculating ICR
Let's say a patient takes:
- Basal insulin glargine: 20 units/day
- Rapid-acting insulin lispro: 8 units at breakfast, 10 units at lunch, 12 units at dinner
TDD = 20 (basal) + 8 + 10 + 12 (bolus) = 50 units/day
ICR = 500 ÷ 50 = 10 grams per unit
This patient needs 1 unit of rapid-acting insulin for every 10 grams of carbohydrate consumed.
Using the ICR: Calculating a Meal Bolus
Once you know your ICR, calculating a meal bolus is straightforward:
Example: If your ICR is 1:10 and you eat a meal with 60 grams of carbs:
Meal Bolus = 60 ÷ 10 = 6 units of rapid-acting insulin
Combining Meal Bolus with Correction Bolus
In clinical practice, the meal bolus is often combined with a correction dose (if blood glucose is above target) and adjusted for active Insulin on Board (IOB). InsulinPoint's bolus calculator handles all of this automatically:
- Enter your TDD → the calculator derives your ICR using the 500 Rule
- Enter carbs to be eaten → meal bolus is computed
- Enter current blood glucose → correction bolus is computed using your ISF (1800 Rule)
- Enter time since last dose → IOB is deducted from the total
- Final recommended dose = Meal Bolus + Correction Bolus − Active IOB
When Is the ICR Different by Meal?
Many people find their ICR is not constant throughout the day. Common patterns include:
- Dawn phenomenon: Higher insulin resistance in the morning (due to cortisol and growth hormone surges) means a lower ICR at breakfast (e.g., 1:8) compared to lunch or dinner (e.g., 1:12).
- Exercise effects: Exercise improves insulin sensitivity for hours after activity, which may require a higher ICR (fewer units per gram of carb).
- Illness/stress: Physiological stress raises cortisol and glucagon, increasing insulin resistance. ICR may temporarily decrease (requiring more insulin per gram of carb).
Advanced insulin users and continuous glucose monitor (CGM) users often individualize ICR for different times of day in collaboration with their endocrinologist or Certified Diabetes Care and Education Specialist (CDCES).
Signs Your ICR Needs Adjustment
| Pattern After Meals | What It Suggests | ICR Adjustment |
|---|---|---|
| Consistently too high (hyperglycemia) | ICR is too high — not enough insulin per gram of carb | Decrease ICR (e.g., 1:15 → 1:12) |
| Consistently too low (hypoglycemia) | ICR is too low — too much insulin per gram of carb | Increase ICR (e.g., 1:10 → 1:13) |
| Variable — sometimes high, sometimes low | Carb counting may be inaccurate, or ICR varies by meal | Improve carb counting; consider meal-specific ICRs |