Carb/Insulin Ratio (ICR) and Insulin Sensitivity Factor (ISF)

Insulin-to-carb ratio (ICR) and sensitivity factor (ISF)

If you're on intensive insulin therapy, two personal numbers do most of the work at mealtimes and when you correct a high reading. The insulin-to-carb ratio (ICR) tells you how many grams of carbohydrate one unit of insulin will cover. The insulin sensitivity factor (ISF), sometimes called the correction factor, tells you how many mg/dL one unit drops your blood glucose. People usually estimate both from their total daily dose (TDD), the sum of all the basal plus bolus insulin used on a typical day, with a few simple rules taught in diabetes education.

How it works

With rapid-acting analog insulin (lispro, aspart, glulisine), the 500 rule gives ICR = 500 / TDD and the 1800 rule gives ISF = 1800 / TDD. Regular human insulin acts more slowly, so it uses the 450 rule (ICR = 450 / TDD) and the 1500 rule (ISF = 1500 / TDD). With those in hand, a meal bolus comes out to carbs / ICR and a correction bolus to (current glucose − target glucose) / ISF. Treat the formulas as rough starting points. They fit people on a basal/bolus regimen best, and you tighten them over time using your own glucose data.

Worked example

Take a TDD of 50 units of rapid-acting analog. That gives ICR = 500 / 50 = 10 g per unit and ISF = 1800 / 50 = 36 mg/dL per unit. A 60 g meal needs 60 / 10 = 6 units. Say the current glucose reads 200 mg/dL against a target of 110; the correction works out to (200 − 110) / 36 ≈ 2.5 units. Add them up and you land near 6 + 2.5 = 8.5 units, before whatever rounding your clinician advises.

FAQ

Why two different rules? Each unit of a rapid analog acts faster and harder, which is why they use the larger numerators (500/1800). Regular insulin is slower, and the smaller numerators (450/1500) match its weaker per-unit effect.

Can my ICR and ISF change during the day? They can. Lots of people are more insulin-resistant at breakfast and need a tighter morning ICR. Exercise, illness, stress, hormones, all of it shifts these numbers.

Medical disclaimer: This tool is meant for education and is not a substitute for advice from an endocrinologist or your diabetes care team. The 500/450 and 1800/1500 rules only give rough estimates. Real insulin doses have to be individualized, prescribed and supervised by a qualified healthcare professional working from your own glucose data. Never start or change insulin dosing on the strength of this calculator alone.