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Carb Factoring

What is a Carb Factor?

A carb factor is a number that describes the carbohydrate content of a certain food, by specifying the percentage of that food which is carbohydrate.
 
For example, the carb 
factor of Kellogg’s Cornflakes is 0.83, which means that 83% of its weight comes from carbohydrates; the carb factor for fresh strawberries is 0.06 which means that carbohydrates make up 6% of its weight; the carb factor for apples is 0.13, which means apples are made up of 13% carbohydrate; the carb factor for our family’s recipe for pineapple fried rice is 0.16, so 16% of its weight is carbs.


tips from the trenches of type 1 diabetes
  I like to write the carb factor on cereal and cracker boxes so that when my son weighs out that food, he has a quick way of finding the carbs. I also calculate (and write down for quick future reference) the carb factors of all my favorite recipes. When I have baked an item, like banana bread, and left them on the counter for my kids, I simply write the carb factor on the container and then my son can cut any size he wishes and accurately know the carbs. ~Danielle


Weight or Volume?

Carb factors are typically calculated in terms of a food’s weight. However, it is also possible to calculate a carb factor in terms of a food’s volume. To illustrate this point:
  • a carb factor (CF) of 0.40 by weight means that 40% of the food’s weight is carbs. So in a 100 gram weight serving of that food, there is 40g of carbs.
  • a carb factor (CF) of 0.40 by volume means that 40% of the food’s volume is carbs. So in a 100 ml volume serving of that food, there is 40g of carbs.

Because of the built-in error that may occur when measuring by volume (i.e. using measuring cups), carb factoring is most useful when determined by weight. (The exception to this rule is liquids: carb factoring a liquid by volume is an accurate method of determining carb content.) This precision can be very important, especially in younger children, where a little bit of carb can have a big impact on blood sugar.


How Do I Calculate a Carb Factor?

It is easy to calculate the carb factor of a food. All you have to do is divide the carb content of the food by the total weight of the food, arriving at a percentage of the total weight which is carbohydrate.

The most important thing to remember is to always input the carbs FIRST into the calculator. Next, divide by the weight of that food in order to get the carb factor.
  

Example 1: Carb Factors by Weight - Mixed Recipes

You make a casserole, adding up the carb content of all the individual ingredients to find that there are 250g of carbs in the casserole (see Level1/Nutrition/Intro to Carb Counting for details on this process). You then weigh the finished product (minus the dish) and find that it weighs 500g. The carb factor for this particular casserole recipe is:

Carb Factor = Total Carbs in the Recipe
                           Total Weight of the Finished Product

=250
  500

=0.5

Thus, the Carb Factor (CF) of this casserole is 0.50.

You could write “CF: 0.50” on the recipe, and then every time you make this dish (assuming you are consistent in brands and amounts of ingredients), you can simply multiply the weight of the individual portion (in grams) by the carb factor in order to calculate the carbs in that portion.

For example, if your child’s serving size for a particular supper weighs 120g, the amount of carbs in that serving would be (120g) x (0.50) = 60g of carbs.

For a sample recipe (“Silly Greek Chili”) which includes an example of how to calculate the carbs in casseroles, salads, and other mixed recipes using a carb factor by weight, click here.
 
For an example of how to calculate the carbs in baked goods (without weight), check out the handout from the AB Children’s Hospital Diabetes Clinic, Adapting and Calculating Recipes.


calculating the carb factor of cereal


Example 2: Carb Factors by Weight - Packaged Foods


The nutritional panel on your child’s favourite (dry) breakfast cereal says the cereal contains 21g of available carbs per 1 cup (or 30g). 

Input the carb content (21) into the calculator. Next, divide by the weight of the serving size (30). 21 divided by 30 equals 0.70. Thus, the Carb Factor (CF) of this cereal is 0.70, and this is the carb factor that you would write on the box.

Whenever your child pours out a bowl of cereal in the future, all she has to do is weigh the portion and multiply it by the carb factor (0.70) to find out how many carbs are in that bowl of cereal. For example, if her serving size on this particular Saturday morning weighs 80g, the amount of carbs in that serving would be (80g) x (0.70) = 56g of carbs.

Example 3: Carb Factors by Volume - Packaged Foods

The available carbs (carbs minus fiber) cited on your can of mushrooms is 4g, but the serving size is quoted as a volume, 125 ml (rather than as a weight in grams, as in the previous example). The same procedure works in this case: simply input the carbs first and divide by the serving size in ml: 4 divided by 125 equals 0.03. This is the carb factor by volume for those mushrooms. 

To figure out how many carbs are in the whole can to use in your homemade pasta sauce, multiply 0.03 by the total volume of the can (284 ml) to arrive at 9 grams of carbs.

Example 4: Carb Factors by Volume – Liquids
Calculating Carbs in fruit juice
The nutrition information on your brand of apple juice says there are 24g of available carbs per 200ml of juice. Input the carb content (24) into the calculator. Next, divide by the volume of the serving size (200). 24 divided by 200 equals 0.12. Thus, the Carb Factor (CF) of this juice is 0.12, and this is the carb factor that you would write on the tetrapack.

Then, when your child pours a glass of juice, she simply measures the volume in that glass (in ml, since that was the unit you used to create the carb factor) and multiplies it by the carb factor (0.12) to find out how many carbs are in that particular glass of juice.  For example, if her serving size of juice is 350ml, the amount of carbs in that serving would be (350ml) x (0.12) = 42g of carbs.


Example 4: Carb Factors by Volume – Mixed Recipes

For a sample recipe (“Silly Greek Chili”) which includes an example of how to calculate the carbs in a mixed recipe using a carb factor by volume, click here.



Is a High Carb Factor a Bad Thing?

Not necessarily. A high carb factor does suggest that there is a high content of carbohydrates in that food, relative to other same-weight foods with a lower carb factor. But that doesn’t necessarily mean that it’s “worse” for post-meal blood glucose. For example, a food like spaghetti is higher in carbs than the same weight of, say, a boiled potato. But the pasta is low glycemic, so it will result in less of a post-meal spike than the potatoes, which have a lower carb factor and a high glycemic index.

(For more information on glycemic index, see Level 1/Nutrition/Intro to Glycemic Index)


To predict how a food will impact blood glucose, it is best to consider a number of factors, including things like:
  • Carb content, or carb factor, of the food.
  • Where a food ranks on the glycemic index.
  • The timing of insulin.
  • The amount of food consumed.
  • The combination of foods consumed at the same meal.


Why Are Carb Factors Handy?
 

  • You can save time and effort by writing the carb factor for a given packaged food right on the recipe or the package (of cereal, or crackers, or yogurt, just to name a few) then you don’t have to calculate the carb content per serving every time your child eats that food. You simply weigh a portion of that food and multiply by the carb factor to determine the carb content.
calculate carbs using a carb factor

  • Your child has the flexibility to eat any size portion of a given food while still accurately determining the carb content; she doesn’t have to eat exactly 1/20 of that banana bread, or servings of cereal in 1 cup increments – she can dish out whatever portion fits her appetite and calculate carbs from there. 


  • Carb factors allow you to “reverse engineer” part of the meal. When my son is nearing the end of his meal, and he still needs 8g of carbs for the amount of insulin he was already given (maybe he didn’t eat all of his sandwich, or I purposely gave him extra insulin for dessert, without yet knowing what that dessert would be), I can divide 8g by the carb factor for blueberries (0.12) and know that he needs 67g weight of blueberries for dessert (8 divided by 0.12 equals 67). Then I simply measure out a 67-gram-weight of blueberries, and he is “even” for the insulin he has received.



Any questions? Comments? Feel free to Contact Us.


The above information was reviewed for content accuracy by clinical staff of the Alberta Children’s Hospital Diabetes Clinic.

This material has been developed from sources that we believe are accurate, however, as the field of medicine (in particular as it applies to diabetes) is rapidly evolving, the information should not be relied upon, as it is designed for informational purposes only. It should not be used in place of medical advice, instruction and/or treatment. If you have specific questions, please consult your doctor or appropriate health care professional. 

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