Carb Factor – The 2.6 Rule

The 2.6 Rule (formerly the 500 or 450 rule) is a great way to estimate how many grams of carbohydrate will be covered by one unit of Humalog, Novolog, or Apidra insulin. This is your insulin to carb ratio (I:C or ICR) or your carb factor (CarbF). Once you know this, you can count the grams of carb in the food you want to eat and divide by your carb factor to find how many units of bolus insulin are needed to cover the carbs. This allows flexibility in your food choices because any number of carbs can be covered with a matching dose of insulin.

The 2.6 Rule:

An accurate carb factor can be determined only after you’ve calculated an accurate TDD (all fast insulin taken before meals, plus all long-acting insulin used in a day). To find a starting CarbF, multiply your weight (lb) by 2.6 grams per pound and divide this number by your average TDD.

For lbs: CarbF = (2.6 x Weight(lb)) ÷ TDD
For kg: CarbF = (5.7 x Weight(kg)) ÷ TDD

Someone’s weight is 160 lbs and their 14 day average TDD is 40 units a day. Their carbF would be 2.6 x 160 ÷ 40. That equals 10.4 grams of carb per unit of insulin.

Testing and Adjusting Your Carb Factor

When your Insulin to Carb Ratio works well, your glucose reading 5 hours later will end up within 30 mg/dL of your starting glucose. To accurately test your CarbF, you should:

  1. Start the test with a glucose between 100 to 140 md/dl.
  2. Eat enough carbs to challange your Insulin to Carb Ratio, such as grams equal to half your weight in lbs.
  3. Enter the recommended carb bolus 20 minutes before you eat. The carb bolus can be calculated by dividing the grams of carb you will be eating by your carb factor: Grams of Carb ÷ ICR
  4. Check your glucose each hour for the next 5 hours. Stop the test and eat carbs if your glucose goes below 70 mg/dl.

If your glucose goes low during testing, falls more than 30 mg/dL, or remains more than 30 mg/dL above your starting glucose after 5 hours, change the ICR in small steps. Your ICR number is adjusted in the opposite direction of the glucose problem.

Change ICR in Small Steps
If current ICR is: Adjust up or down by:
Less than 5.0 gr/u +/- 0.2 to 0.3 grams/u
5-10 grams/u +/- 0.3 to 0.5 grams/u
10-15 grams/u +/- 1.0 grams/u
16-24 grams/u +/- 1 to 2 grams/u

If you frequently go low or frequently go high after meals:

  • Check your basal/carb bolus balance. If your basal rates make up more than 55% or 60% of your TDD, this may be the source for lows. If basals make up only 40% or 45% of your TDD, this may be a source for highs.
  • Change your CarbF: raise it for frequent lows, lower it for frequent highs.
  • Improve your carb counting, especially if your postmeal glucoses are erratic or you frequently go high.

Pumping Insulin provides much more pump information and is the most helpful book ever written on insulin use.

Using Insulin takes you step by step toward excellent control. Whether you use one injection and pills, or six injections, you’ll learn far more from this book than from any other!