Insulin Sensitivity Factor Calculator (1800 Rule)
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 Calculator
Sum of all insulin in a typical day. Calculate TDD →
Sensitivity Factor Estimate
per unit
units/day
mg/dL per unit
How to Use This Calculator
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/unit. Meaning: 1 unit of rapid analog is expected to lower BG by ~45 mg/dL over 2–4 hours.
1500 Rule (Regular Insulin)
Used for Regular insulin (Humulin R, Novolin R). Regular is less potent mg/dL per unit than analogs, hence the lower divisor. 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.
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.
- 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 Medical Care in Diabetes — 2024. Link
Last reviewed: June 2025
Frequently Asked Questions
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/unit, meaning each unit should lower BG by ~45 mg/dL. 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 calibrated for rapid-acting insulin analogs (lispro, aspart, glulisine), which are more potent than Regular insulin. The 1500 Rule is for Regular (short-acting) insulin, which is less potent and acts more slowly. 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.