What Is the Insulin Sensitivity Factor?
The Insulin Sensitivity Factor (ISF), also called the correction factor or sensitivity factor, answers the question: How much will 1 unit of insulin lower my blood glucose?
For someone managing insulin-requiring diabetes — whether type 1 or insulin-dependent type 2 — the ISF is one of the most practically important numbers in daily management. It is used to calculate correction doses: the amount of additional insulin needed when blood glucose is above target.
Without knowing your ISF, you cannot make accurate correction-dose calculations. Too little insulin leaves blood glucose elevated; too much causes hypoglycemia, which can be dangerous.
Critical disclaimer: ISF is a starting-point reference calculated from population-level formulas. Individual insulin sensitivity varies substantially and changes with time of day, illness, stress, exercise, and hormonal changes. All dosing decisions must be made in consultation with your diabetes care team.
The 1800 Rule and 1700 Rule
Two formulas are used to estimate ISF from total daily dose (TDD):
1800 Rule (for rapid-acting insulin analogues):
ISF (mg/dL per unit) = 1800 ÷ TDD1700 Rule (for regular human insulin):
ISF (mg/dL per unit) = 1700 ÷ TDDThe divisor (1800 or 1700) reflects empirical observations about how much blood glucose drop one unit of insulin produces across a large sample of people. The 1800 Rule was popularised by John Walsh and Ruth Roberts in Pumping Insulin for use with rapid-acting analogues (lispro, aspart, glulisine). The 1700 Rule applies to regular insulin, which has a slower onset and longer tail of action.
Why rapid-acting insulin uses a higher divisor: Rapid-acting analogues have a more predictable and narrower action profile than regular insulin. The 1800 vs 1700 difference accounts for the different pharmacokinetics. Some providers use 1650–2000 depending on the individual and their insulin type.
How Total Daily Dose Is Calculated
Total Daily Dose (TDD) is the sum of all insulin taken over 24 hours:
TDD = basal insulin + all bolus doses (mealtime + corrections)For pump users: TDD is shown directly in the pump’s history.
For multiple daily injection (MDI) users: Add up the basal dose (e.g. 20 units of glargine) and all bolus units taken that day. Average over several representative days for a stable TDD.
The TDD used in the ISF formula should reflect a typical day, not an unusual high- or low-dose day. Many clinicians recommend averaging TDD over 7–14 days.
Worked Example: TDD 45 Units, Rapid-Acting, Blood Glucose 180 mg/dL
Step 1: Calculate ISF
ISF = 1800 ÷ 45 = 40 mg/dL per unit
This means: 1 unit of rapid-acting insulin is expected to lower blood glucose by approximately 40 mg/dL.
Step 2: Calculate correction dose
Correction dose = (current BG − target BG) ÷ ISF
With a current BG of 180 mg/dL and a target of 100 mg/dL:
Correction = (180 − 100) ÷ 40 = 80 ÷ 40 = 2 units
In mmol/L: ISF = 40 ÷ 18.0182 ≈ 2.2 mmol/L per unit
If BG is 10.0 mmol/L and target is 5.6 mmol/L: Correction = (10.0 − 5.6) ÷ 2.2 ≈ 2 units.
When Not to Take a Correction Dose
The correction dose is clamped at zero — never negative. If your blood glucose is at or below target, no correction is needed. Taking insulin when BG is already at target risks hypoglycemia.
Additionally, correction doses are rarely given if active insulin from a recent meal bolus is still working. Many insulin management systems track “insulin on board” (IOB) to prevent stacking corrections on top of still-active boluses.
ISF Varies Throughout the Day
A single ISF value calculated from TDD is an average for the entire day. In practice, many people find that their ISF is different at different times:
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Dawn phenomenon: Cortisol and growth hormone release in early morning (roughly 4–8 AM) increase blood glucose and can cause insulin resistance. An ISF of 30 mg/dL/unit might be appropriate in the morning when 40 mg/dL/unit is right for afternoon.
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Post-exercise: Aerobic exercise typically increases insulin sensitivity for hours afterward, raising ISF (less insulin is needed to lower BG by the same amount).
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Illness and stress: Both tend to reduce insulin sensitivity (lower ISF), meaning more insulin is needed to achieve the same BG reduction.
Diabetes care providers sometimes prescribe different ISF values for different time periods — for example, 30 mg/dL/unit before breakfast, 40 mg/dL/unit at other times. Insulin pumps can store multiple ISF settings programmed by time of day.
Converting Between mg/dL and mmol/L
Blood glucose measurements use mg/dL in the United States and mmol/L in most other countries. The conversion factor is approximately 18.0182:
ISF (mmol/L) = ISF (mg/dL) ÷ 18.0182Example: ISF of 40 mg/dL/unit = 40 ÷ 18.0182 = 2.2 mmol/L per unit (rounded to 1 decimal place per clinical convention).
The blood sugar converter calculator on this site performs these conversions for individual readings.
How ISF Fits Into a Broader Insulin Management Plan
ISF is one of three key insulin management parameters:
- Insulin-to-carb ratio (ICR): How many carbohydrates one unit of insulin covers (e.g., 1:10 means 1 unit covers 10 grams of carbs)
- Insulin sensitivity factor (ISF): How much 1 unit lowers blood glucose (used for corrections)
- Basal rate / basal dose: The background insulin that keeps blood glucose stable between meals
These three parameters are set by the diabetes care team based on history, TDD, current control, and individual response. Adjusting any one of them requires careful observation and follow-up. The 1800/1700 Rule provides a starting-point estimate for ISF, but it is not a substitute for clinical dose adjustment.
Frequently Asked Questions
How do I know if my ISF is correct? Test it by taking a correction dose when your blood glucose is elevated and no food or active insulin is involved. Measure BG at the start, then 2 and 3 hours later. If BG dropped by approximately the ISF value per unit taken, the estimate is reasonable. If it dropped significantly more or less, your actual ISF differs from the formula estimate. Discuss adjustments with your care team.
Can I use this calculator to set my pump’s ISF? No — not without clinical review. Pump settings are prescribed by your care team. This calculator provides an educational estimate based on the 1800/1700 Rule starting point. Whether this estimate is appropriate for you depends on your actual insulin response, which can differ substantially from population averages.
What if my TDD varies a lot from day to day? High day-to-day TDD variation often reflects inconsistent eating patterns, physical activity changes, or glucose-management challenges. For a stable ISF estimate, use a 7–14 day average TDD from a representative period. Your diabetes educator can help identify an appropriate averaging period.
Is the 1800 Rule always 1800? No. Different providers and references use divisors from 1650 to 2200 depending on the insulin type and the patient population. The 1800 Rule is the most widely cited for rapid-acting analogues. Some clinicians in the UK use 1800 × (1/18) ≈ 100 as a starting estimate in mmol/L units. The formula is an empirical guide, not a physiological constant.