Carb-to-Insulin Ratio Calculator
Find out how many grams of carbohydrate one unit of insulin covers using the 500 Rule for rapid-acting insulin or the 450 Rule for regular insulin. Your insulin-to-carb ratio (ICR) is essential for accurate mealtime dosing.
ICR Reference Table
Estimated grams of carbs covered by 1 unit of insulin at common TDD values.
| TDD (units/day) | Rapid-acting (500 ÷ TDD) | Regular (450 ÷ TDD) |
|---|---|---|
| 20 units | 1:25.0 g | 1:22.5 g |
| 30 units | 1:16.7 g | 1:15.0 g |
| 40 units | 1:12.5 g | 1:11.3 g |
| 50 units | 1:10.0 g | 1:9.0 g |
| 60 units | 1:8.3 g | 1:7.5 g |
| 70 units | 1:7.1 g | 1:6.4 g |
Medical Disclaimer
This calculator is for educational purposes only. The 500 and 450 rules provide starting estimates — your actual carb-to-insulin ratio may differ. Always work with your doctor or diabetes educator to determine and adjust your ICR.
Glucoly – Blood Sugar Tracker
Log meals, insulin, and glucose together
Glucoly tracks your carbs, insulin doses, and blood sugar readings so you can see how your ICR holds up in real life.
What Is the Carb-to-Insulin Ratio?
The insulin-to-carbohydrate ratio (ICR), also called the carb ratio or carb factor, tells you how many grams of carbohydrate one unit of rapid-acting or regular insulin will cover. For example, a ratio of 1:15 means one unit of insulin covers 15 grams of carbs. Knowing your ICR allows you to calculate how much insulin to take before a meal based on the carbohydrates you plan to eat.
The 500 Rule vs the 450 Rule
The most common starting-point method is the “500 Rule” (for rapid-acting insulin): divide 500 by your Total Daily Dose (TDD) to estimate your ICR in grams of carbs per unit. The number 500 was derived empirically and works well for rapid-acting analogues such as lispro (Humalog), aspart (NovoLog), and glulisine (Apidra).
For regular (short-acting) insulin, a divisor of 450 is commonly used instead, reflecting the slower and longer action profile of regular insulin. Some clinicians use a 400 rule for patients with significant insulin resistance. These are starting estimates — your actual ICR will need to be verified and adjusted based on real-world glucose outcomes after meals.
When to Use Your ICR
Your ICR is used to calculate your mealtime (bolus) insulin dose: divide the grams of carbs in the meal by your ICR to get the number of units needed. Many people with type 1 diabetes and some with insulin-treated type 2 diabetes use their ICR daily. Your ICR may vary by time of day (e.g., higher insulin needs at breakfast due to the dawn phenomenon) and should be periodically reviewed with your diabetes care team.
Sources
- Walsh J, et al. Using Insulin: Everything You Need for Success with Insulin. Torrey Pines Press, 2003.
- American Diabetes Association. Insulin basics. Diabetes Care. 2024;47(Suppl 1).
- Davidson PC, et al. The 700 rule for regular insulin and the 1700/1800 rule for rapid-acting insulin. Endocr Pract. 2008;14(7):869-874.
Medical disclaimer: This tool is for informational and educational purposes only and does not constitute medical advice. Insulin dosing decisions should always be made in consultation with your doctor, diabetes nurse, or certified diabetes educator. Never adjust insulin doses without professional guidance.