The correction factor (CF) tells you how many mg/dL one unit of insulin will lower your blood glucose. For example, if your CF is 45, then 1 unit of insulin will lower your blood glucose by approximately 45 mg/dL. It is used to calculate correction doses for hyperglycemia.

The 1800 rule (CF = 1800 ÷ TDD) is used for rapid-acting insulin analogs (lispro, aspart, glulisine). The 1500 rule (CF = 1500 ÷ TDD) is used for regular insulin. The different numbers account for the different pharmacokinetic profiles.

Example: Target glucose 120 mg/dL, current glucose 240 mg/dL, CF = 45. Correction dose = (240 − 120) ÷ 45 = 2.7 units, rounded to 3 units. Add this to any meal-time insulin dose. Always account for insulin-on-board from recent doses.

If correction doses consistently fail to bring glucose to target, the CF may be too high (not enough insulin per unit). If glucose drops too low after correction, the CF may be too low. Adjust in 10–20% increments and monitor for 3–5 days before adjusting again.