Insulin Sensitivity Factor Calculator

About this calculator

The insulin sensitivity factor (ISF), also called the correction factor, estimates how many mg/dL your blood glucose will drop per unit of rapid-acting insulin. It's most commonly estimated with the “Rule of 1800” for rapid-acting insulin (ISF = 1800 ÷ total daily insulin dose) or the “Rule of 1500” for regular insulin (ISF = 1500 ÷ TDD) — so someone taking 45 units per day would have an estimated ISF of 1800 ÷ 45 = 40 mg/dL per unit.

ISF is the key number behind a correction dose: if your blood glucose is above target, dividing the difference by your ISF tells you how many extra units to give. People with type 1 diabetes, caregivers, and endocrinologists use this starting estimate to build or fine-tune insulin pump settings and correction-dose calculators, always verifying the result against real glucose response and adjusting under medical supervision.