FitDose

Insulin Sensitivity Factor (Correction Factor) Calculator

Estimate how far one unit of insulin lowers your blood sugar from your total daily dose, using the 1800 or 1500 rule. For education only: your real correction factor comes from your diabetes care team. Nothing you enter is stored.

Your data is not stored or sent to the server. All calculations are performed locally in your browser.

Before you use this calculator
  • It uses general rules of thumb. Your own ratios and correction factor should come from your diabetes care team and can differ a lot from these estimates.
  • It is for education and for discussing settings with your team. Do not use it to change your insulin doses on your own.
  • A wrong insulin dose can cause dangerous low or high blood sugar. If you feel unwell, follow your care plan or get medical help.

Correction factor by total daily dose

Rule-of-thumb estimates for common daily doses.

Total daily dose1800 rule (rapid-acting)1500 rule (regular)
20 units90 mg/dL (5.0 mmol/L)75 mg/dL (4.2 mmol/L)
30 units60 mg/dL (3.3 mmol/L)50 mg/dL (2.8 mmol/L)
40 units45 mg/dL (2.5 mmol/L)38 mg/dL (2.1 mmol/L)
50 units36 mg/dL (2.0 mmol/L)30 mg/dL (1.7 mmol/L)
60 units30 mg/dL (1.7 mmol/L)25 mg/dL (1.4 mmol/L)
80 units23 mg/dL (1.2 mmol/L)19 mg/dL (1.0 mmol/L)

How it's calculated

Rapid-acting: correction factor (mg/dL per unit) = 1800 ÷ total daily doseRegular insulin: correction factor (mg/dL per unit) = 1500 ÷ total daily dosemmol/L per unit = mg/dL per unit ÷ 18.016 (about 100 ÷ total daily dose)

Sources: Texas DSHS Diabetes Toolkit: Insulin Pump Therapy; Whittington Health NHS Trust: Adjusting insulin

Last reviewed

Frequently asked questions

What is an insulin sensitivity factor?

The insulin sensitivity factor (ISF), also called the correction factor, is how much one unit of mealtime insulin is expected to lower your blood glucose. It is used to work out a correction dose when you are above target.

What is the 1800 rule?

Divide 1800 by your total daily insulin dose to estimate how many mg/dL one unit of rapid-acting insulin will lower your glucose. For example, with 40 units a day, 1800 ÷ 40 = 45 mg/dL per unit.

When is the 1500 rule used?

The 1500 rule is generally used with regular (short-acting) insulin or for people who are more insulin resistant. It gives a smaller drop per unit than the 1800 rule for the same total daily dose.

What is the 100 rule for mmol/L?

In mmol/L, divide 100 by your total daily dose. It is the 1800 rule converted: 1800 ÷ 18 is 100. With 32 units a day, one unit lowers glucose by about 3 mmol/L.

What counts as my total daily dose?

Add up all the insulin you take in a typical day: long-acting (basal) plus every mealtime and correction dose. Averaging over several days gives a steadier figure.

Should I change my doses based on this?

No. These rules give a starting estimate that care teams then adjust from your readings, and your own factor can differ a lot. Use the result to understand your settings or discuss them, not to change doses on your own.

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