Insulin Sensitivity Factor Calculator

The Insulin Sensitivity Factor (ISF) Calculator estimates how far one unit of rapid- or short-acting insulin lowers your blood glucose, using the 1500, 1700, 1800 or 2000 rule applied to your total daily insulin dose. It compares all four rules side by side and can also estimate an optional correction dose in mg/dL or mmol/L.

What Is the Insulin Sensitivity Factor (ISF)?

The Insulin Sensitivity Factor — also called the Correction Factor (CF) — estimates how many points your blood glucose drops for each 1 unit of rapid- or short-acting insulin. For example, an ISF of 50 mg/dL means one unit of insulin is expected to lower your blood sugar by about 50 mg/dL. A higher ISF means you are more sensitive to insulin (a little goes a long way); 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 use in a typical day — using a simple division rule:

1500 Rule vs 1800 Rule — Which Should I Use?

• 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. Click Calculate ISF to see your factor, a comparison of all four rules, and the math.

Worked Example

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

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.

Frequently Asked Questions

What is a normal insulin sensitivity factor?

There is no single "normal." ISF commonly ranges from roughly 20 to 100 mg/dL per unit (about 1–6 mmol/L per unit) depending on your total daily dose and insulin resistance. Lower TDD usually means a higher ISF.

How do I find my total daily dose (TDD)?

Add up every unit of insulin you take in an average day — long-acting/basal plus all meal-time and correction boluses. Averaging several recent days gives a more reliable number.

Should I use the 1500 or 1800 rule?

Use 1800 (or 1700) for modern rapid-acting analog insulin, and 1500 for older regular human insulin. Your healthcare provider can confirm the right constant for you.

What is the difference between ISF and the carb ratio?

The ISF tells you how far 1 unit lowers high blood sugar (correction). The insulin-to-carb ratio tells you how many grams of carbohydrate 1 unit covers at a meal. They are two separate settings.

Is this calculator safe to dose from?

No. It is an educational estimate only. Always follow the dosing plan from your doctor or diabetes care team, and account for insulin on board, food and activity.