Key Takeaways
- The I:C ratio tells you how many grams of carbs are covered by 1 unit of insulin
- Common ratios range from 1:5 to 1:20 depending on insulin sensitivity
- Your ratio may vary at different times of day (often lower in the morning)
- The formula is simple: Carbs / Ratio = Insulin Units
- Always verify your calculated dose with your healthcare provider
What Is the Carb to Insulin Ratio?
The carbohydrate-to-insulin ratio (I:C ratio) indicates how many grams of carbohydrate are covered by one unit of rapid-acting insulin. For example, a 1:10 ratio means that 1 unit of insulin covers 10 grams of carbohydrates.
This ratio is essential for people with diabetes who use mealtime insulin (such as insulin aspart, lispro, or glulisine) to manage blood sugar levels after eating. By accurately calculating your insulin dose based on carb intake, you can better maintain stable glucose levels.
The Formula
Insulin Dose = Carbohydrates (g) / I:C Ratio
Medical Disclaimer
This calculator is for educational purposes only. Insulin dosing is highly individual and depends on many factors including current blood sugar, activity level, illness, and more. Always consult your healthcare provider or certified diabetes educator before adjusting your insulin doses. Incorrect dosing can lead to dangerous hypoglycemia or hyperglycemia.
How to Determine Your I:C Ratio
Your healthcare team typically determines your I:C ratio through a combination of:
- The 450/500 Rule: Divide 450 (or 500 for more conservative estimate) by your Total Daily Dose (TDD) of insulin
- Continuous glucose monitoring data: Analyzing post-meal glucose patterns
- Food diary analysis: Tracking carbs eaten and insulin taken with resulting blood sugars
- Trial and adjustment: Fine-tuning based on real-world results
Pro Tip: Ratio May Vary by Time of Day
Many people need different I:C ratios at different times. Morning ratios are often lower (e.g., 1:8) due to the "dawn phenomenon," while evening ratios may be higher (e.g., 1:12). Work with your diabetes care team to determine if you need multiple ratios.
Factors That Affect Your Insulin Needs
- Current blood glucose level: May need correction dose in addition to meal coverage
- Physical activity: Exercise increases insulin sensitivity
- Illness or stress: Often requires more insulin
- Type of carbohydrates: High glycemic foods may require different timing
- Fat and protein content: Can delay glucose absorption
- Hormonal changes: Menstrual cycle, pregnancy can affect needs
About This Calculator
This free online calculator helps you quickly determine your mealtime insulin dose based on carbohydrate intake. Simply enter the grams of carbohydrates you plan to eat and your prescribed I:C ratio to get an instant calculation. Remember, this is a tool to assist with calculations - it does not replace medical advice or account for correction doses.