Insulin Sensitivity Factor Calculator

What Is the Insulin Sensitivity Factor?

The Insulin Sensitivity Factor (ISF), also called the Correction Factor (CF), estimates how many points your blood glucose drops for each unit of rapid- or short-acting insulin. For example, an ISF of 50 mg/dL means one unit of insulin lowers your blood sugar by about 50 mg/dL. A higher ISF means you are more sensitive to insulin (a little goes a long way), while a lower ISF means you need more insulin to move your glucose.

How the ISF Is Calculated

The ISF is derived from your Total Daily Dose (TDD) of insulin — the sum of all basal and bolus insulin you take in a typical day — using a simple division rule:

mg/dL: ISF = 1800 ÷ TDD (1800 rule) | 1500 ÷ TDD (1500 rule)

mmol/L: ISF = 100 ÷ TDD (1800 rule) | 83 ÷ TDD (1500 rule)

1500 Rule vs 1800 Rule

• 1800 Rule — the most common choice for rapid-acting analog insulin (lispro/Humalog, aspart/Novolog, glulisine/Apidra). Some clinicians also use 1700 or 2000.
• 1500 Rule — traditionally used for older regular (short-acting) human insulin, which acts more slowly and is less potent per unit.
• The larger the numerator, the larger (more conservative) the ISF, meaning each unit is assumed to do less work. When unsure, your care team decides which constant fits you best.

How to Use This Calculator

1. Enter your Total Daily Dose (TDD) in insulin units.
2. Pick the rule that matches your insulin type (1800 for rapid-acting is the default).
3. Choose your blood glucose unit — mg/dL (US) or mmol/L (most other countries).
4. Optional: enter your current and target glucose to see an estimated correction dose.
5. The ISF value updates instantly along with a comparison of all four rules.

Worked Example

Suppose your TDD is 40 units of rapid-acting insulin and you use the 1800 rule:

ISF = 1800 ÷ 40 = 45 mg/dL per unit

If your blood glucose is 250 mg/dL and your target is 120 mg/dL, the correction dose estimate is (250 − 120) ÷ 45 = 2.9 units (about 3 units). This is only an illustration — real dosing must account for insulin already active in your body.

Medical Disclaimer

This Insulin Sensitivity Factor Calculator is for educational purposes only and is not medical advice. It provides a rough starting estimate based on simple "rules of thumb." Your real ISF can vary by time of day, activity, hormones and illness. Never start, stop or change an insulin dose without guidance from your doctor or diabetes care team.

Frequently Asked Questions