HEALTH · INSULIN SENSITIVITY FACTOR CALCULATOR (1800 / 1700 RULE)
Insulin Sensitivity Factor Calculator (1800 / 1700 Rule)
Calculate your insulin sensitivity factor (ISF) — how much 1 unit of insulin lowers your blood glucose — using the 1800 Rule (rapid-acting) or 1700 Rule (regular insulin). Optionally estimate a correction dose for a given blood glucose reading.
FOR EDUCATIONAL USE ONLY. Never adjust insulin doses based solely on a calculator. All dosing decisions must be made with your diabetes care team.
About This Calculator
The Insulin Sensitivity Factor (ISF), also called the correction factor, tells you how much your blood glucose drops per unit of insulin. Knowing your ISF is essential for calculating correction doses when your blood glucose is above target. This calculator implements the 1800 Rule for rapid-acting insulin and the 1700 Rule for regular insulin, and optionally estimates a correction dose for your current blood glucose reading.
How It Works
Enter your total daily insulin dose (TDD) — the sum of all basal and bolus doses over 24 hours. Select your insulin type (rapid-acting for the 1800 Rule, regular for the 1700 Rule) and your preferred blood glucose unit. The calculator divides the rule constant by your TDD to produce your ISF. Optionally, enter your current blood glucose and target to see a suggested correction dose.
The Formula
ISF (mg/dL) = divisor ÷ TDD
- divisor
- 1800 for rapid-acting insulin; 1700 for regular insulin
- TDD
- Total Daily Dose — sum of all basal and bolus insulin units over 24 hours
Frequently Asked Questions
- What is total daily dose (TDD)?
- TDD is the total amount of insulin you use in a 24-hour period, including all basal insulin and all bolus doses (mealtime and correction). If you use an insulin pump, your TDD is shown in the pump history. If you use injections, add your daily basal dose to all bolus doses you took that day and average over several days for a stable number.
- Why is the divisor 1800 for rapid-acting and 1700 for regular insulin?
- Regular (short-acting) insulin acts more slowly and has a longer tail of activity than rapid-acting analogues. This slightly lower divisor accounts for the difference in onset and peak timing. Rapid-acting analogues (lispro, aspart, glulisine) use 1800; human regular insulin uses 1700. These divisors are empirical rules of thumb, not precise physiological constants — individual ISFs vary significantly.
- Is my ISF fixed?
- No. ISF can vary with time of day (commonly lower in the morning due to dawn phenomenon), illness, stress, menstrual cycle phase, exercise, and changes in total daily insulin dose. Many people use different ISFs for morning vs. evening corrections. Your diabetes care team can help you identify whether your ISF needs time-of-day adjustments.
- FOR EDUCATIONAL USE ONLY — why is this disclaimer here?
- Insulin is a high-risk medication. Incorrect dosing can lead to hypoglycemia, which can be life-threatening. This calculator is a starting-point reference to understand the 1800/1700 Rule — a well-established clinical concept. All actual dosing decisions must involve your diabetes care team (endocrinologist, diabetes educator, or nurse practitioner).