Carb-to-insulin ratio

Carb-to-insulin ratio (also ICR, insulin-to-carb ratio, I:C): The carb-to-insulin ratio (often abbreviated ICR or I:C) is the number of grams of carbohydrate that one unit of rapid-acting insulin covers for you. A ratio of 1:10 means one unit covers ten grams of carb; a ratio of 1:6 means one unit covers six grams.

How it's used

Mealtime dose = carbs in meal ÷ ICR + correction for current glucose. So a meal with 60g of carb at a 1:10 ratio is 6 units, plus any correction.

The 500 rule

A common starting heuristic: divide 500 by your total daily insulin dose to get a starting ICR. If you take 50 units a day, 500 ÷ 50 = 10, so 1:10 is a starting ratio. Most adults end up between 1:6 and 1:15.

It's not constant

ICR can vary by time of day (often more aggressive at breakfast due to dawn-phenomenon insulin resistance), by activity level, and across the menstrual cycle. Pumps and AID systems handle this with multiple ICR settings.