Insulin Sensitivity Factor Calculator
Calculate your insulin sensitivity factor (ISF) using the 1800 or 1500 rule. Enter values for instant results with step-by-step formulas.
Formula
ISF = 1800 / TDD (rapid-acting) or ISF = 1500 / TDD (regular insulin)
Where ISF = Insulin Sensitivity Factor (mg/dL drop per unit), TDD = Total Daily Dose of insulin (units). The 1800 rule is used for rapid-acting insulin analogs, while the 1500 rule is used for regular human insulin. Correction Dose = (Current BG - Target BG) / ISF.
Worked Examples
Example 1: Rapid-Acting Insulin Correction
Problem: A patient takes 45 units total daily dose of rapid-acting insulin. Current blood glucose is 280 mg/dL with a target of 120 mg/dL. Calculate ISF and correction dose.
Solution: ISF (1800 Rule) = 1800 / 45 = 40 mg/dL per unit\nBlood glucose difference = 280 - 120 = 160 mg/dL\nCorrection dose = 160 / 40 = 4.0 units\nEach unit of rapid-acting insulin will lower BG by approximately 40 mg/dL.
Result: ISF: 40 mg/dL per unit | Correction Dose: 4.0 units of rapid-acting insulin
Example 2: Regular Insulin Correction
Problem: A patient uses regular insulin with a TDD of 60 units. Current BG is 320 mg/dL, target is 140 mg/dL. Calculate ISF and correction dose.
Solution: ISF (1500 Rule) = 1500 / 60 = 25 mg/dL per unit\nBlood glucose difference = 320 - 140 = 180 mg/dL\nCorrection dose = 180 / 25 = 7.2 units\nEach unit of regular insulin will lower BG by approximately 25 mg/dL.
Result: ISF: 25 mg/dL per unit | Correction Dose: 7.2 units of regular insulin
Frequently Asked Questions
What is an insulin sensitivity factor and why does it matter?
The insulin sensitivity factor (ISF), also called a correction factor, tells you how much one unit of insulin will lower your blood glucose level in mg/dL. For example, an ISF of 50 means one unit of insulin will drop your blood sugar by approximately 50 mg/dL. This value is essential for calculating correction doses when your blood glucose is above your target range. Without knowing your ISF, you risk either giving too little insulin (leaving blood sugar elevated) or too much insulin (causing dangerous hypoglycemia). Your endocrinologist typically helps determine your initial ISF, which is then fine-tuned based on your individual response patterns over time.
What factors can change my insulin sensitivity throughout the day?
Insulin sensitivity fluctuates significantly throughout the day due to multiple physiological factors. The dawn phenomenon causes increased insulin resistance in the early morning hours due to growth hormone and cortisol surges, meaning you may need more insulin at breakfast. Exercise dramatically increases insulin sensitivity for 24 to 48 hours afterward, potentially requiring lower doses. Stress, illness, and infections trigger counter-regulatory hormones that reduce sensitivity. Menstrual cycle phases can affect insulin needs in women, with resistance typically increasing in the luteal phase. Sleep deprivation, high-fat meals, and certain medications like corticosteroids also alter sensitivity substantially.
How often should I reassess my insulin sensitivity factor?
Most endocrinologists recommend reassessing your ISF every two to four weeks when first establishing it, and at least quarterly once stable. You should also reassess whenever there are significant changes in your total daily dose, body weight, activity level, or overall health status. Pregnancy, new medications (especially steroids), and major lifestyle changes all warrant ISF recalculation. Signs that your ISF needs adjustment include consistently overshooting your target (ISF too high, meaning you are giving too much correction insulin) or consistently undershooting (ISF too low, meaning corrections are insufficient). Keeping a detailed log of correction doses and subsequent blood glucose readings helps identify patterns requiring adjustment.
What is the relationship between insulin sensitivity and insulin resistance?
Insulin sensitivity and insulin resistance are opposite ends of the same spectrum. High insulin sensitivity means your body responds well to insulin and you need smaller doses to lower blood glucose, reflected by a higher ISF number. Insulin resistance means your cells do not respond as effectively to insulin, requiring larger doses for the same effect, shown by a lower ISF number. Type 2 diabetes is primarily characterized by insulin resistance, often requiring much higher total daily doses than Type 1 diabetes. Factors that improve insulin sensitivity include regular exercise, weight loss, adequate sleep, and stress management. Metformin and thiazolidinediones are medications specifically designed to reduce insulin resistance.
What role does insulin on board play when using correction doses?
Insulin on board (IOB), also called active insulin, refers to insulin from previous bolus doses that is still working in your body. Rapid-acting insulin typically remains active for three to five hours after injection, with peak activity at one to two hours. If you calculate a correction dose without accounting for IOB, you risk insulin stacking, where multiple overlapping doses cause a dangerous hypoglycemic episode. For example, if you dosed two units two hours ago and your pump estimates one unit is still active, your correction dose should be reduced by that one unit of IOB. Modern insulin pumps and continuous glucose monitors with dosing calculators automatically subtract IOB from recommended correction doses, significantly improving safety.
How does the basal-bolus split affect insulin sensitivity calculations?
The standard recommendation is approximately a 50/50 split between basal and bolus insulin, though individual needs vary from 40/60 to 60/40. If your basal dose is too high, it can mask a true ISF by causing frequent low blood sugars that get corrected with food, artificially inflating your apparent TDD. Conversely, insufficient basal insulin leads to persistent hyperglycemia and overcorrection with bolus doses. An accurate basal rate is verified through basal rate testing, where you skip meals and monitor whether blood glucose remains stable. Only after confirming your basal rate is correct should you fine-tune your ISF and carbohydrate-to-insulin ratio for accurate bolus dosing.