Insulin Sensitivity Factor (ISF) Explained
By Ryan Mitchell · Reviewed against ADA Standards of Care · 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. 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 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, 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 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.
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 Medical Care in Diabetes — 2024. Link
Last reviewed: July 2026 · See our methodology