Insulin-to-Carb Ratio (ICR) Calculator
The Insulin-to-Carbohydrate Ratio (ICR) specifies how many grams of carbohydrates are covered by one unit of rapid-acting insulin. This calculation is vital for anyone on intensive insulin therapy (MDI or pump) to accurately dose for meals. This tool uses the "500 Rule" (or 450 Rule for Regular insulin) to estimate your starting ICR based on your Total Daily Dose (TDD) of insulin.
The Math (Manual Calculation)
The 500 Rule (Rapid-Acting Insulin): 500 / Total Daily Dose = ICR
The 450 Rule (Short-Acting Insulin): 450 / Total Daily Dose = ICR
Example: If you take 50 units of insulin per day total (basal + bolus), 500 / 50 = 10. Your ratio is 1:10 (1 unit for every 10g of carbs).
Why It Matters
Using a flat scale for meals (e.g., "take 5 units for dinner") ignores the actual carbohydrate load of the meal, resulting in hyperglycemia if the meal is carb-heavy, or severe hypoglycemia if the meal is low-carb. Once you establish your ICR, use our Bolus Calculator to calculate specific mealtime doses.
Common Mistakes
- Using one ratio all day: Insulin sensitivity fluctuates due to circadian rhythms. Many people are more insulin-resistant in the morning due to cortisol and growth hormone, requiring a stronger ratio (e.g., 1:8) for breakfast, and a weaker one (e.g., 1:12) for dinner.
- Failing to test the ratio: The 500 Rule provides a starting point. To test it, eat a meal with a known carbohydrate amount, take the calculated insulin, and check blood sugar 3 hours later. If it's significantly higher or lower than the starting value, the ratio needs adjustment by a clinician.
Frequently Asked Questions (FAQ)
- How do I find my Total Daily Dose (TDD)?
- Add up all your basal (long-acting) insulin and all bolus (meal/correction) insulin taken in a 24-hour period. Average this over 3-5 days. See our TDD Estimator.