Insulin Sensitivity Factor (ISF) Explained
Medically reviewed by Dr. Chhaya Makhija, MD, DipABLM — September 2026
By Ryan Mitchell · Medically reviewed by Dr. Chhaya Makhija, MD, DipABLM · Updated July 2026
The insulin sensitivity factor — also called the correction factor — answers one question: how far does one unit of rapid-acting insulin drop my blood glucose? An ISF of 50 means one unit lowers you roughly 50 mg/dL (2.8 mmol/L). It's the number behind every correction dose you take for a high reading, and every "correction bolus" your pump suggests.
Estimating ISF: the 1800 Rule and 1500 Rule
Example: TDD 45 u → 1800 ÷ 45 = 40 mg/dL (2.2 mmol/L) per unit. In mmol/L terms: 100 ÷ TDD (≈ 2.2 mmol/L per unit here).
- 1800 Rule — for rapid-acting analogs (Humalog, NovoLog, Apidra). The most widely taught version.
- 1500 Rule — the original, more conservative rule for Regular (short-acting) insulin; produces a stronger correction assumption.
- Some clinicians use divisors from 1500–2200 based on individual response — the divisor itself is titratable.
➜ Calculate yours: ISF Calculator (1800/1500 Rule)
Using ISF to correct a high reading
Example: Reading 260 mg/dL (14.4 mmol/L), target 120, ISF 40 → (260 − 120) ÷ 40 = 3.5 units — before subtracting insulin on board.
The subtraction step is what most new insulin users miss. If you corrected 90 minutes ago, much of that dose is still active. Taking a full second correction "stacks" insulin and is the classic cause of a severe low 2–3 hours later. Always net out insulin on board — pumps do this automatically; by injection you have to do it deliberately. Full tool: Correction Dose Calculator with IOB.
ISF vs ICR — the two ratios people confuse
| ISF (correction factor) | ICR (carb ratio) | |
|---|---|---|
| Answers | How far 1 unit drops glucose | How many carb grams 1 unit covers |
| Rule of thumb | 1800 ÷ TDD (rapid) / 1500 ÷ TDD (Regular) | 500 ÷ TDD (rapid) / 450 ÷ TDD (Regular) |
| Used for | Correcting highs | Dosing meals |
| Typical adult range | 20–100 mg/dL (1.1–5.5 mmol/L) per unit | 1:5 – 1:25 |
They combine at mealtimes: meal dose = carbs ÷ ICR, plus correction = (BG − target) ÷ ISF, minus IOB. See Carb Ratios Explained for the other half.
Why your ISF isn't constant
- Time of day: morning insulin resistance (the dawn phenomenon) can weaken your effective ISF at breakfast.
- Activity: exercise sensitizes muscle to insulin for up to 24–48 h, strengthening the effect of every unit.
- Illness & stress: counter-regulatory hormones raise resistance; corrections often run weaker during illness.
- CGM trend: a rising arrow means the same reading needs more correction than a falling one — see the trend-arrow correction calculator.
If corrections consistently overshoot (lows after correcting) your ISF number is too small — one unit drops you further than the math assumes. If highs barely budge, the number is too large. Either pattern, sustained over several days, is a conversation for your care team.
Frequently Asked Questions
What is a normal insulin sensitivity factor?
There is no single normal value, because ISF is inversely tied to your total daily dose. Someone on 30 units a day lands near 60 mg/dL (3.3 mmol/L) per unit under the 1800 Rule, while someone on 90 units lands near 20. A lower number means each unit does less work, which reflects greater insulin resistance.
Is the correction factor the same as the insulin sensitivity factor?
Yes — they are two names for the same number, which is a common source of confusion. Both describe how far one unit of rapid-acting insulin is expected to lower your blood glucose.
How is ISF different from the insulin-to-carb ratio?
They answer different questions. ISF tells you how much insulin to take to bring a high reading down; the insulin-to-carb ratio tells you how much to take to cover the carbohydrate you are about to eat. A mealtime bolus usually combines both.
Why does my ISF change during the day?
Insulin sensitivity is not constant. It typically falls overnight and in the early morning as cortisol and growth hormone rise, which is why the same correction can work well at lunch and under-correct at breakfast. Illness, steroids, poor sleep and menstrual-cycle phase all shift it too.
Sources & References
- Davidson PC, Hebblewhite HR, Steed RD, Bode BW. Analysis of guidelines for basal-bolus insulin dosing. Endocr Pract. 2008;14(9):1095-101.
- Walsh J, Roberts R. Using Insulin. Torrey Pines Press. (1500/1800 Rules.)
- American Diabetes Association. Standards of Care in Diabetes — 2026. Link
Written by Ryan Mitchell · Last updated: September 2026