Correction Factor / Insulin Sensitivity Factor (ISF) Calculator

The Correction Factor (or Insulin Sensitivity Factor, ISF) defines how much one unit of rapid-acting insulin will drop your blood glucose level. It is used to correct high blood sugar readings. This tool uses the standard "1800 Rule" (or 1500 Rule for Regular insulin).

The Math (Manual Calculation)

The 1800 Rule (for mg/dL): 1800 / Total Daily Dose = ISF

Example: If you take 45 units total per day, 1800 / 45 = 40. One unit of insulin drops your blood sugar by 40 mg/dL.

Once known, use this in a correction formula: (Current BG - Target BG) / ISF = Correction Dose. Or, use our Mealtime Bolus Calculator which does this math automatically.

Why It Matters

Guessing correction doses leads to dangerous outcomes. "Rage bolusing" (taking excessive insulin out of frustration during a stubborn high) often results in severe subsequent hypoglycemia. A mathematically sound ISF ensures safe corrections.

Common Mistakes

• Ignoring Insulin on Board (IOB): If you correct a high blood sugar 1 hour after eating, you still have active insulin from the meal working in your body. Taking a full correction dose will lead to a crash. Always subtract IOB when calculating manual corrections. Most insulin pumps calculate this automatically.
• Not adjusting for exercise: Exercise vastly increases insulin sensitivity. A correction factor that works while sedentary may cause hypoglycemia if used right before a run.

Frequently Asked Questions (FAQ)

Is my Correction Factor the same all day?

Not necessarily. Like the Carb Ratio, insulin sensitivity can be lower in the morning (requiring a smaller ISF number, like 30) and higher in the afternoon (requiring a larger number, like 50).