Insulin-to-Carb Ratio Calculator
The insulin-to-carb ratio (ICR) tells you how many grams of carbohydrate one unit of rapid-acting insulin covers. A common starting estimate is the 500 rule: divide 500 by your total daily insulin dose (or 450 for regular insulin). This calculator returns the estimated ratio and, if you enter a meal's carbohydrate amount, an estimated carbohydrate bolus. These are starting points only: real ratios vary by person and time of day and are refined with monitoring. This tool is educational and not a substitute for guidance from your diabetes care team.
Insulin-to-carb ratio formula
ICR = constant / total daily dose
Constant = 500 (rapid-acting) or 450 (regular)
Meal bolus = meal carbohydrates / ICR
ICR is grams of carbohydrate per unit of insulin
For example, the 500 rule with a 50-unit total daily dose gives an ICR of 10 grams per unit, so a 60-gram meal needs about 6 units of carbohydrate-covering insulin (before any correction).
Using the ratio
- Total daily dose is all basal plus bolus insulin per day.
- A larger ICR means one unit covers more carbohydrate.
- Ratios often differ by meal and time of day.
- The meal bolus here excludes any correction dose.
- Confirm and adjust ratios with your care team.
Insulin-to-carb ratio: frequently asked questions
What is the insulin-to-carb ratio?
The insulin-to-carb ratio (ICR) is the number of grams of carbohydrate covered by one unit of rapid-acting insulin. An ICR of 10 means one unit covers about 10 grams of carbohydrate. It is a starting estimate, refined with monitoring.
What is the 500 rule?
The 500 rule divides 500 by your total daily insulin dose to estimate grams of carbohydrate covered by one unit. For example, 500 divided by 50 units gives an ICR of 10 grams per unit. A 450 rule is sometimes used for regular insulin.
How do I estimate a mealtime bolus?
Divide the carbohydrate grams in the meal by your insulin-to-carb ratio. If you eat 60 grams of carbohydrate and your ICR is 10, the carbohydrate bolus is about 6 units, before any correction dose.
Is the ratio fixed all day?
Not necessarily. Many people are more insulin resistant at breakfast and need a lower ICR (more insulin per gram) then. Ratios are individualized and adjusted with a clinician using real-world data.
Is this medical advice?
No. This is an educational estimate. Do not change insulin dosing based on a calculator alone. Insulin therapy must be guided by your healthcare team.
Official sources
- National Institute of Diabetes and Digestive and Kidney Diseases: Insulin and diabetes management.
- Centers for Disease Control and Prevention: Carbohydrate counting and diabetes.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.