Insulin to Carb Ratio Calculator
The insulin-to-carbohydrate ratio (ICR) is a key tool in flexible insulin therapy for Type 1 diabetes and some Type 2 patients. It determines how many grams of carbohydrate one unit of rapid-acting insulin will cover. The mealtime insulin dose is calculated as: Units = Carbohydrates (g) / ICR (g per unit). ICR is typically established by the diabetes care team using the 500 Rule (500 / Total Daily Dose) as a starting estimate, then fine-tuned through glucose monitoring. This calculator works in both directions: calculate your dose from carb intake and ICR, or estimate your ICR from known dose and carb intake.
Insulin to carb ratio formula
Mealtime units = Carbohydrates (g) / ICR (g per unit)
ICR estimate (500 rule) = 500 / Total Daily Dose (units)
ICR varies by individual, time of day, and other factors. An ICR of 1:10 means 1 unit per 10 g carbohydrate; 1:15 means 1 unit per 15 g. Morning ICR is often lower (more insulin sensitive later in the day for many patients).
Carbohydrate counting and ICR
- ICR is usually estimated starting with the 500 rule: 500 / TDD = grams covered per unit.
- ICR often varies through the day; many patients have different ratios for breakfast, lunch, and dinner.
- Glycemic index and fat/protein content of meals can also affect post-meal glucose response beyond simple carbohydrate counting.
- Pre-meal blood glucose correction (using the insulin sensitivity factor) is separate from and additive to the carbohydrate-based dose.
- Only rapid-acting insulins (lispro, aspart, glulisine) or short-acting regular insulin are used for mealtime dosing.
Insulin to carb ratio: frequently asked questions
What is insulin-to-carbohydrate ratio?
The insulin-to-carbohydrate ratio (ICR or I:C ratio) is the amount of carbohydrate in grams that 1 unit of rapid-acting insulin will cover. For example, an ICR of 1:15 means 1 unit covers 15 g of carbohydrates. ICR is used to calculate mealtime insulin doses in carbohydrate counting therapy.
How is ICR determined for a patient?
ICR is determined through clinical assessment by a diabetes care team, typically using the 500 rule for rapid-acting analogues: 500 / Total Daily Dose (TDD) of insulin = grams of carbohydrate covered by 1 unit. This is a starting estimate requiring individual adjustment based on blood glucose monitoring.
How do I use this calculator for a meal?
Enter the grams of carbohydrates in the meal and your ICR (grams per unit). The calculator divides carb grams by the ICR to give the units of rapid-acting insulin to inject for that meal. Always verify with your diabetes care team and adjust for pre-meal blood glucose using your correction factor.
What is the correction factor (insulin sensitivity factor)?
The correction factor (CF or ISF) is the drop in blood glucose (mg/dL) produced by 1 unit of rapid-acting insulin. It is estimated using the 1800 rule: 1800 / TDD. ICR and CF together form the basis of advanced carbohydrate counting for insulin-dependent diabetes.
Can this calculator be used to self-manage insulin doses?
No. Insulin dosing must be managed under the supervision of a licensed diabetes care professional. This tool is for educational and informational purposes only. Incorrect insulin dosing can cause hypoglycemia or hyperglycemia, which can be life-threatening.
Official sources
- American Diabetes Association: ADA Diabetes Technology Standards.
- NIH National Institute of Diabetes and Digestive and Kidney Diseases: NIDDK Insulin and Diabetes Treatments.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.