Calculating Your Insulin Sensitivity Factor (ISF) and Your Carb:Insulin Ratio

Posted on December 22, 2009



Your Diabetes Team may use any of the following methods to help find your ISF:

1. Insulin/Blood Glucose Formula

(One unit of rapid or short-acting  insulin for every 50 mg/dl increase or decrease in your blood glucose level).

2. Rule of 1500 – if you are using shortacting insulin (Regular).

3. Rule of 1700 – if you are using rapidacting insulin (Humalog®, Novalog®, Apidra®).

ISF Method I – Insulin/Blood

Glucose Formula

With ISF Method I, you take one unit of rapid or short-acting insulin for every 50 mg/dl increase or decrease in your blood glucose level.


Blood glucose target is 100 mg/dl.

John’s pre-lunch blood glucose is 180 mg/dl and his Insulin Sensitivity is 50.

His reading shows that he is above his target by 80 mg/dl.

[180 mg/dl – 100 mg/dl = 80 mg/dl]

He should take 1 (1.5 if you measure 1/2 units) extra units at lunch.

If his usual dose of rapid or short-acting insulin at lunch were 12 units, he would increase it by one and take 13 or 13.5 units.


Blood glucose target: 100 mg/dl

Susan’s pre-lunch blood glucose is 205 mg/dl.

Her reading shows that she is above her target by 105 mg/dl

[205 mg/dl – 100 mg/dl = 105 mg/dl]

Q – How many extra units should Susan take?

105 mg/dl / 50 = 2 units

A – She should take 2 extra units. In this

case she should take a total of 14 units.



A carb:insulin ratio is the amount of rapid or short acting insulin you need to match or “cover” the amount of carbohydrate you eat.

The Rule of 500:

Add up all the insulin given for 24 hours and divide it into 500. The answer is

your carb:insulin ratio.


Your total insulin dose is 50 units.

500 divided by 50 = 10

Your carb: insulin ratio is 10:1

Tom’s Calculations:

Insulin needed for carbohydrates 9 units (90 carbohydrates divided by 10)

Insulin needed to correct for high blood glucose (190-100=90) divided by 30 (ISF)=3

Total insulin for food and carb 9 + 3 = 12 units

Reduction for high intensity exercise (Step 5) 3 unit

TOTAL amount of pre-dinner insulin 9 units

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