The insulin sensitivity factor, commonly abbreviated ISF, estimates how much one unit of rapid acting insulin is expected to lower blood glucose. Knowing your ISF helps tailor correction doses when your readings are higher than desired. A typical approach, called the 1800 rule, divides 1800 by your total daily insulin dose. This rule of thumb assumes most adults need roughly the same amount of insulin to correct a high reading, though actual sensitivity varies by person and even by time of day.
Simply input your total daily dose of insulin—the sum of all basal and bolus units over twenty-four hours. The calculator then applies the 1800 rule, displaying your estimated ISF in mg/dL. If your daily dose varies from day to day, you can average several recent days to improve accuracy. The resulting ISF guides how many units you might take to correct for high glucose. For instance, if your ISF is 40 mg/dL, one unit should lower your reading by about 40 points.
The 1800 rule provides a convenient starting point but cannot account for every variable that affects blood sugar. Factors like insulin sensitivity changes during illness, physical activity, or hormonal fluctuations may alter how much correction you require. Additionally, very low total daily doses may yield an unrealistically high ISF, while very high doses may underestimate sensitivity. Work with your healthcare provider to refine the number based on real-world glucose data.
Suppose your total daily dose is 50 units. Dividing 1800 by 50 yields an ISF of 36 mg/dL. That means each unit of rapid acting insulin should drop your glucose by approximately 36 milligrams per deciliter. If your target is 110 mg/dL but your current reading is 200 mg/dL, the difference is 90. Dividing that by 36 suggests you would need about 2.5 units to correct down to your goal. Always round cautiously and monitor your results.
Adjusting insulin precisely helps avoid both hyperglycemia and hypoglycemia. When you correct high readings too aggressively, you risk crashing later. Too conservatively, and your levels may remain elevated. Knowing your estimated ISF makes corrections more predictable, especially when paired with a carbohydrate-to-insulin ratio for meal bolusing. It is a cornerstone of effective intensive insulin therapy.
Your insulin needs are dynamic. As your lifestyle or health status changes, so does your sensitivity. Keep a log of corrections and results, then revisit your ISF with your diabetes care team. Some people use different factors for different times of day—perhaps more sensitive in the afternoon and less in the early morning. Continuous glucose monitoring systems make it easier to see trends and adjust accordingly.
The 1800 rule can be expressed as , where represents your total daily dose. While some clinicians use the 1500 rule with regular insulin, rapid acting analogs more commonly employ 1800. Use the formula that best matches your medication.
This calculator is intended for educational purposes and should not replace personal medical advice. Discuss any dosing changes with your doctor or diabetes educator. They may suggest a trial period to fine tune the number and confirm it aligns with your daily glucose patterns.
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