Insulin Sensitivity Factor Calculator
Calculate your Insulin Sensitivity Factor (ISF) - also called the Correction Factor - to estimate how much one unit of insulin lowers your blood glucose.
Educational estimator only - not medical advice.
Insulin Type
TDD = all basal + all bolus insulin over 24 hours. Typical range: 20-100 units/day
Enter your Total Daily Dose above to calculate your Insulin Sensitivity Factor.
The ISF (also called Correction Factor or CF) is calculated using your Total Daily Dose of insulin:
Regular / Short-Acting: ISF = 1500 / TDD
Why two rules? Rapid-acting insulin analogues have a steeper, faster glucose-lowering effect than older regular insulin, so the "1800 rule" accounts for this greater potency. The "1500 rule" is used for Regular (human) insulin.
Total Daily Dose (TDD) = sum of all basal insulin (long-acting) + all bolus insulin (meal-time) taken over a typical 24-hour period. On a pump, it's the total daily delivery from the pump history.
Practical use: If your BG is 180 mg/dL and your target is 120 mg/dL, you need to correct by 60 mg/dL. With an ISF of 40, you would take 60 / 40 = 1.5 units to correct. Always verify with your endocrinologist before making dose changes.
Master insulin dosing calculations - from ISF and insulin-to-carb ratios to timing strategies for different meal types, exercise adjustments, and how to use pump data to refine your settings.
FOD Mature Ch 11: Insulin GuideThe 1800 and 1500 rules are starting-point estimates only. ISF varies by time of day, activity level, illness, stress, and other factors. Never adjust insulin doses based solely on calculator results. Always work with your endocrinologist or diabetes care team to determine safe and appropriate insulin doses for your situation. Incorrect dosing can cause hypoglycaemia, which can be life-threatening.