An insulin correction factor (CF) — also called the insulin sensitivity factor — is the personalized number of mg/dL by which your blood glucose falls after 1 unit of rapid-acting insulin. It replaces the one-size-fits-all sliding scale with a calculation tailored to your total daily dose. The starting estimate uses the 1800 Rule for rapid analogs (1800 divided by TDD); the 1500 Rule applies to regular insulin. Correction dose equals (current BG minus target) divided by CF. Refine the number over 2 to 4 weeks using paired BG readings, and review it periodically as weight, activity, and sensitivity evolve. The CF is the foundation of safe correction dosing in any basal-bolus regimen.
What the Correction Factor Means
When you check your blood glucose before a meal or bedtime and the number is above target, you may need extra insulin in addition to the bolus that covers the carbs of the upcoming meal. The correction factor tells you exactly how much insulin to add — based on your personal sensitivity rather than a generic chart.
- Higher CF (e.g., 80) — you are more sensitive; 1 unit drops glucose a lot
- Lower CF (e.g., 20) — you are more resistant; 1 unit drops glucose less
- Typical adult range — 30 to 80 for rapid analogs
- Pediatrics often use much higher CFs (100 to 200) reflecting low total daily doses
Calculating Your Starting CF — the 1800 and 1500 Rules
| Insulin Type | Rule | Example TDD | Starting CF |
|---|---|---|---|
| Rapid analog (lispro, aspart, glulisine, Fiasp, Lyumjev) | 1800 ÷ TDD | 40 units | 45 mg/dL per unit |
| Rapid analog | 1800 ÷ TDD | 60 units | 30 mg/dL per unit |
| Rapid analog | 1800 ÷ TDD | 100 units | 18 mg/dL per unit |
| Regular insulin (Humulin R, Novolin R) | 1500 ÷ TDD | 50 units | 30 mg/dL per unit |
| Pediatric rapid analog (low TDD) | 1800 ÷ TDD | 15 units | 120 mg/dL per unit |
The 1800 Rule was empirically derived by analyzing pump data and adult insulin pharmacokinetics; some clinicians use 1700 or 2000 depending on patient profile. For mmol/L systems, the equivalent is the 100 Rule for rapid analogs (100 ÷ TDD = CF in mmol/L per unit).
The Correction Dose Formula
Correction dose (units) = (current BG − target BG) ÷ CF
| Current BG | Target BG | CF | Correction Units |
|---|---|---|---|
| 120 | 100 | 40 | 0.5 (round to 0 or 1) |
| 180 | 100 | 40 | 2 |
| 220 | 100 | 40 | 3 |
| 280 | 100 | 40 | 4.5 (round to 4 or 5) |
| 180 | 120 (bedtime) | 40 | 1.5 (round to 1 or 2) |
| 250 | 100 | 60 | 2.5 (round to 2 or 3) |
Adding Correction to a Meal Bolus
If you check BG pre-meal and find it above target, give both:
- Meal bolus — based on carbs and your insulin-to-carb ratio
- Correction — based on the formula above
Example: Premeal BG 180, target 100, CF 45, planned meal 50 g carbs, I:C 1:10:
- Meal bolus = 50 ÷ 10 = 5 units
- Correction = (180 − 100) ÷ 45 = 1.8 → 2 units
- Total premeal dose = 5 + 2 = 7 units of rapid analog
For complete regimen design, see basal-bolus insulin regimen and insulin dosing guidelines.
Target Glucose Values
| Situation | Common Target (mg/dL) | Notes |
|---|---|---|
| Premeal (most adults) | 80–130, often centered at 100 | ADA standard |
| Bedtime | 120–140 | Avoids overnight hypoglycemia |
| Postprandial (2 h) | <180 | ADA standard for adults |
| Pregnancy premeal | <95 | Gestational/pregestational diabetes |
| Older adult, high hypo risk | 110–150 premeal, <200 bedtime | Individualized |
| Severe hypo unawareness | 150 or higher premeal | Temporarily relaxed targets |
Active Insulin and Stacking
“Active insulin on board” (IOB) refers to the units from prior boluses still working. Rapid analogs have a duration of action of about 3 to 5 hours, so a correction given 1 hour after the last bolus still has 2 to 4 hours of unfinished effect.
- Do not stack corrections within 3 hours unless BG is rapidly climbing despite the prior dose
- Pump and smart pen bolus calculators track IOB automatically — they subtract IOB from the calculated correction
- For manual injections, write down your last bolus time or use an app to avoid hypoglycemia from overlap
- If you must correct at hour 2 and feel uncertain, halve the calculated correction and check again at hour 3
How CF Changes Across the Day
Insulin sensitivity is not constant. Common patterns:
- Dawn phenomenon — cortisol and growth hormone rise overnight, peaking around 4 to 8 a.m., reducing morning sensitivity; morning CF may be 30 to 50 percent stronger (lower number) than afternoon CF
- Afternoon and evening — usually most sensitive; CF can be relatively high (less insulin needed per glucose drop)
- Late evening — sensitivity may rise again with bedtime; reduce correction or apply higher target
- Exercise window — muscle uptake increases sensitivity for up to 24 hours; CF should be higher temporarily
- Illness or steroid use — sensitivity drops; CF temporarily lower (more insulin needed); see ADA sick-day rules
- Menstrual cycle — sensitivity may shift across phases for some women
How to Test Your CF at Home
- Pick an afternoon when you are 4+ hours from any food or insulin and your BG is >180 mg/dL.
- Give your calculated correction dose only — no meal bolus, no food.
- Check BG at 60, 120, and 180 minutes.
- Note the lowest BG (typically at 90 to 120 minutes for rapid analogs).
- Compute actual drop ÷ units given = your real CF.
- Repeat 2 to 3 times across different days, then average.
- Update your CF for that time of day if the real value differs by more than 20 percent from your starting estimate.
This test is safest with CGM. Have fast-acting carbs available, and stop the test if BG drops <80 mg/dL.
Common Mistakes
- Using the same CF in the morning as the evening — most people need stronger morning correction
- Correcting when active insulin from a recent bolus is still working (stacking)
- Forgetting to subtract IOB when using a manual pen rather than a smart pen or pump
- Using the same CF after losing or gaining 5+ kg without recalculating
- Targeting too low (e.g., 80 mg/dL) and causing overnight hypos
- Mixing up the 1800 and 1500 rules across rapid and regular insulin
Special Situations
| Situation | Adjustment |
|---|---|
| Exercise within 90 minutes | Reduce correction by 25–50% |
| Illness with high BG | Lower CF (more units) by 10–20%; follow sick-day rules |
| Steroid burst (prednisone, etc.) | Lower CF 25–50% for course duration; restore after taper |
| Pregnancy (3rd trimester) | CF typically halves as TDD doubles; review every 1–2 weeks |
| Renal insufficiency | Higher CF (less insulin needed); slower correction response |
| Recent severe hypo | Higher target temporarily; reduce correction by 25–50% |
CF vs. Sliding Scale
A traditional sliding scale applies the same units of insulin to the same BG range for every patient regardless of personal sensitivity. The correction factor is the modern individualized replacement — same logical purpose, much more accurate. Most clinicians today provide patients with their CF, I:C ratio, and target BG rather than a fixed sliding-scale chart.
External Sources
For the dosing-rule derivation, see Davidson et al. in Endocrine Practice 2008. For the latest CF and target guidance, see the ADA Standards of Care in Diabetes 2024.
The Bottom Line
Your insulin correction factor is the personalized number of mg/dL that 1 unit of rapid-acting insulin lowers your blood glucose. Calculate the starting value with the 1800 Rule (1800 divided by total daily dose) and refine over 2 to 4 weeks using BG-response testing. Correction units equal (current BG minus target) divided by CF, layered on top of meal bolus for premeal high readings. Do not stack corrections within 3 to 4 hours. Review your CF every 3 to 6 months and after weight, activity, illness, or medication changes — it is not a fixed number for life. The CF is the math foundation of any safe and effective insulin regimen.
