Insulin Correction Factor Calculator
Estimates your Insulin Sensitivity Factor (ISF) — how many mg/dL one unit of insulin lowers your blood glucose — using the 1800 Rule or 1500 Rule.
All insulin in 24 hours (basal + all bolus doses).
1800 Rule vs 1500 Rule
The 1800 Rule was derived empirically — for most patients using rapid-acting analogues, their total daily dose × ISF ≈ 1800. Regular insulin has a slower, broader action profile, so the divisor is smaller (1500), yielding a lower (more conservative) ISF.
| TDD | 1800 Rule (rapid) | 1500 Rule (Regular) |
|---|---|---|
| 20 u/day | 90 mg/dL/unit | 75 mg/dL/unit |
| 30 u/day | 60 mg/dL/unit | 50 mg/dL/unit |
| 40 u/day | 45 mg/dL/unit | 37.5 mg/dL/unit |
| 50 u/day | 36 mg/dL/unit | 30 mg/dL/unit |
| 70 u/day | 26 mg/dL/unit | 21 mg/dL/unit |
Correction Factor vs Insulin Sensitivity Factor
These are two names for the same number. "Insulin sensitivity factor (ISF)" is the clinical term; "correction factor (CF)" is the everyday one. Both tell you how many mg/dL — or mmol/L — one unit of rapid-acting insulin lowers your blood glucose. If your clinician gives you an ISF of 50, your correction factor is also 50.
This page focuses on deriving that number from your total daily dose and choosing the right rule for your insulin. For the broader walkthrough of sensitivity and how it pairs with your carb ratio, see the Insulin Sensitivity Factor Calculator.
How to Use Your Correction Factor
Once you have your correction factor, a correction dose is a single step: (Current BG − Target BG) ÷ correction factor. With a CF of 50, a reading of 220 against a target of 100 gives (220 − 100) ÷ 50 = 2.4 units.
Two cautions before acting on that number:
- Subtract insulin on board first if you have dosed in the last few hours — the Correction Dose Calculator builds IOB into the result.
- Don't correct repeatedly. Wait 3–4 hours so the previous correction can finish working before stacking another.
Signs Your Correction Factor Needs Adjusting
| What you notice after a correction | What it suggests |
|---|---|
| Corrections drop you too low | CF is too small — one unit lowers glucose more than expected; your team may raise it (e.g., 50 → 60) |
| Corrections don't reach target | CF is too large; your team may lower it (e.g., 50 → 40) |
| Works at night but not in the morning | Time-of-day variation; a separate, stronger morning CF may help |
Frequently Asked Questions
What is an insulin correction factor?
It is how many mg/dL (or mmol/L) one unit of rapid-acting insulin lowers your blood glucose. It is the same value as the insulin sensitivity factor (ISF). A common estimate is 1800 ÷ your total daily dose for rapid-acting insulin.
How do I calculate my correction factor?
Use the 1800 Rule for rapid-acting analogs: CF = 1800 ÷ TDD. For Regular insulin use the 1500 Rule, and for mmol/L use the 100 Rule (100 ÷ TDD). Example: TDD 36 → 1800 ÷ 36 = 50 mg/dL per unit.
Is correction factor the same as sensitivity factor?
Yes. "Correction factor" and "insulin sensitivity factor (ISF)" are interchangeable terms for the same number — the expected glucose drop per unit of insulin.
What is a typical correction factor?
It varies widely with total daily dose — a lower TDD means a larger correction factor (more glucose drop per unit). Values commonly fall between about 25 and 90 mg/dL per unit, but yours must be confirmed through titration with your care team.
Source
- Walsh J, Roberts R. Pumping Insulin. 5th ed. 2012.
Last reviewed: June 2025