Insulin Sensitivity Factor Calculator (1800 Rule)
Medically reviewed by Dr. Chhaya Makhija, MD, DipABLM — October 2026
Estimate your insulin sensitivity factor (ISF / correction factor) using the 1800 Rule for rapid-acting analogs or the 1500 Rule for Regular insulin. Shows both mg/dL and mmol/L results.
ISF is the abbreviation for insulin sensitivity factor — the drop in blood glucose (mg/dL or mmol/L) caused by one unit of rapid-acting insulin. Enter your total daily dose and the calculator estimates it with the 1800 rule.
📐 ISF Calculator
Sum of all insulin in a typical day. Calculate TDD →
Sensitivity Factor Estimate
per unit
units/day
mg/dL per unit
As total daily dose rises, each unit of insulin lowers glucose less — the curve shows why higher-TDD (more insulin-resistant) users have a smaller ISF.
How to Use the Insulin Sensitivity Factor Calculator (1800 Rule)
Enter Your TDD
Add all insulin you take in a typical day — basal plus all bolus doses. Include correction doses in your average.
Select Insulin Type
Rapid-acting analogs use the 1800 Rule. Regular (short-acting) insulin uses the 1500 Rule. Using the wrong rule gives an inaccurate ISF.
Read Your ISF
Results shown in both mg/dL per unit and mmol/L per unit. Use whichever matches your blood glucose meter.
Verify Experimentally
Test ISF when BG is elevated with no food or IOB. Correct and record the actual glucose drop. Compare to the formula estimate.
Use for Correction Doses
Plug your verified ISF into the correction dose calculator: (Current BG − Target BG) ÷ ISF.
Review With Your Team
ISF changes with weight, fitness, hormones, and TDD. Review at each diabetes appointment or when glucose patterns shift.
The 1800 Rule and 1500 Rule Explained
1800 Rule (Rapid-Acting Analogs)
Used for lispro (Humalog), aspart (NovoLog/NovoRapid), and glulisine (Apidra). Example: TDD = 40 → ISF = 1800 ÷ 40 = 45 mg/dL (2.5 mmol/L)/unit. Meaning: 1 unit of rapid analog is expected to lower BG by ~45 mg/dL (2.5 mmol/L) over 2–4 hours.
1500 Rule (Regular Insulin)
Used for Regular insulin (Humulin R, Novolin R). A unit of Regular lowers glucose as much as a unit of a rapid analog. The different divisor reflects clinical experience with the slower, longer action of Regular, not a difference in potency. TDD = 40 → ISF = 1500 ÷ 40 = 37.5 mg/dL/unit.
mmol/L Conversion
Or use the 100 Rule directly: ISF (mmol/L) = 100 ÷ TDD for rapid analogs; 83 ÷ TDD for Regular.
1500, 1700 or 1800 Rule: Which One Should You Use?
All three rules divide a constant by your total daily dose. A larger constant gives a larger ISF, which means each unit is expected to drop glucose further, so the correction dose comes out smaller. The choice changes the size of every correction you take.
| Rule | Typical use | ISF at TDD 40 | Correction for 250 → 120 mg/dL |
|---|---|---|---|
| 1500 Rule | Regular (short-acting) insulin; the original rule | 37.5 mg/dL (2.1 mmol/L) | 3.5 units |
| 1700 Rule | Rapid analogs; derived by Davidson et al. (2008) from 167 well-controlled type 1 patients | 42.5 mg/dL (2.4 mmol/L) | 3.1 units |
| 1800 Rule | Rapid analogs; the most widely taught version | 45 mg/dL (2.5 mmol/L) | 2.9 units |
Davidson's analysis behind the 1700 Rule also produced related starting estimates: basal insulin at about 47% of TDD and a carb ratio of 2.8 × body weight in pounds ÷ TDD. If 1800 Rule corrections keep leaving you above target, a smaller constant such as 1700 gives larger corrections; if they push you low, a larger constant gives smaller ones. Test any change with your care team.
ISF Chart by Total Daily Dose (1800 & 1500 Rules)
Each bar is the rapid-acting correction factor (1800 ÷ TDD) — longer bar means one unit drops your glucose further, i.e. you're more insulin-sensitive. The right column shows the Regular-insulin value (1500 ÷ TDD) for comparison. Carb ratio and basal tools built on the same TDD are in our insulin ratio calculators.
To convert any value to mmol/L, divide by 18 (e.g. ISF 45 → 2.5 mmol/L per unit). Apply your ISF to a high reading on the Correction Dose Calculator.
Insulin Sensitivity Factor Calculator (1800 Rule): Limitations & Safety Notes
- ISF is derived from total daily dose, so an inaccurate TDD produces an inaccurate ISF. Confirm your TDD first.
- Use 1800 with rapid-acting analogues and 1500 with regular human insulin. Applying the wrong constant systematically mis-sizes every correction.
- Sensitivity is not constant through the day. It typically falls overnight and pre-dawn, so the same correction can behave differently at breakfast and at dinner.
- The formula takes no account of insulin still active from a previous dose. Correcting without checking insulin on board is how stacking and hypoglycemia happen.
Sources & References
- Davidson PC, Hebblewhite HR, Steed RD, Bode BW. "Analysis of guidelines for basal-bolus insulin dosing: basal insulin, correction factor, and carbohydrate-to-insulin ratio." Endocr Pract. 2008;14(9):1095–1101. Link — source of the 1700 Rule (CF = 1700 ÷ TDD).
- Walsh J, Roberts R, Bailey T. "Guidelines for Optimal Bolus Calculator Settings." J Diabetes Sci Technol. 2011;5(1):129–135.
- American Diabetes Association. Standards of Care in Diabetes — 2026. Link
Written by Ryan Mitchell · Last updated: October 2026
Frequently Asked Questions: Insulin Sensitivity Factor Calculator (1800 Rule)
What is insulin sensitivity factor (ISF)?
ISF (also called correction factor or CF) tells you how much 1 unit of rapid-acting insulin lowers your blood glucose over 2–4 hours. The 1800 Rule estimates it as 1800 ÷ TDD for rapid-acting analogs. A TDD of 40 gives ISF = 45 mg/dL (2.5 mmol/L)/unit, meaning each unit should lower BG by ~45 mg/dL (2.5 mmol/L). ISF is used to calculate correction doses when BG is above target.
How do I verify my ISF is correct?
To verify ISF: when BG is elevated (180–250 mg/dL) with no food eaten in the last 4 hours and no active insulin on board (IOB), give a correction dose based on your estimated ISF. Record BG at 2 and 4 hours. If the drop matches your ISF estimate, it is accurate. Repeat 2–3 times to confirm. Work with your diabetes care team to adjust if the drop is consistently larger or smaller than expected.
What is the difference between the 1800 and 1500 Rule?
The 1800 Rule is taught for rapid-acting insulin analogs (lispro, aspart, glulisine). The 1500 Rule is the older rule for Regular (short-acting) insulin, which starts more slowly and lasts longer. Unit for unit, both insulins lower glucose by the same amount; the rules differ because they were built for different regimens. Using the 1800 Rule for Regular insulin would over-estimate the correction effect, potentially leading to under-correction. Always match the rule to your insulin type.
What is a normal ISF range?
ISF for adults with Type 1 diabetes typically ranges from 20–100 mg/dL per unit, with many falling in the 30–60 range. An ISF below 20 indicates high insulin resistance (common in Type 2, puberty, pregnancy, Cushing's, or high BMI). An ISF above 100 indicates high sensitivity (common in lean, very active Type 1 adults or during intense activity). ISF is highly individual — the formula is a starting estimate only.
Can I use ISF in mmol/L?
Yes. This calculator automatically shows ISF in both mg/dL and mmol/L. The mmol/L ISF is derived by dividing the mg/dL value by 18.016 (the conversion factor). Equivalently, the 100 Rule gives ISF directly in mmol/L for rapid analogs: ISF (mmol/L) = 100 ÷ TDD. Use whichever unit matches your glucose meter and how your care team discusses your targets.
Is the correction factor the same as the insulin sensitivity factor?
Yes. Correction factor (CF), insulin sensitivity factor (ISF) and sensitivity factor all name the same number: how far one unit of rapid-acting insulin lowers your glucose, in mg/dL or mmol/L. Pump menus often call it "insulin sensitivity", while many clinic handouts say "correction factor".
What is the 1700 rule for insulin?
The 1700 Rule estimates the correction factor as 1700 ÷ total daily dose. Davidson and colleagues derived it in 2008 from 167 well-controlled adults with type 1 diabetes. At a TDD of 40 units it gives 42.5 mg/dL per unit, slightly smaller than the 1800 Rule's 45, so corrections come out a little larger.
What does ISF mean?
ISF stands for insulin sensitivity factor: the expected blood-glucose drop from one unit of rapid-acting insulin. It is typically estimated as 1800 ÷ total daily dose in mg/dL (100 ÷ TDD in mmol/L) and then refined against real corrections, because two people with the same weight can have very different ISFs.