Humans eat meals containing a combination of foods that contain varying amounts of carbohydrates, plus protein, fat, fiber, and other micronutrients. The Glycemic Index applies only to the carbohydrate content of a meal, but even if this carbohydrate content is divided across a number of different foods, the glycemic index of the meal can still be calculated. It is a cumbersome process to be sure and not realistic to consider doing for each meal. Nonetheless, it is factored as follows:
Add up the total grams of carbohydrates in the meal. An example would be 60 grams of carbohydrate total.
Then determine which foods are carbohydrates and what their glycemic indexes are. For example: bread's glycemic index is 70.
Then determine how many grams of each carbohydrate were consumed in the meal. For example perhaps two pieces of bread were eaten totaling 26 grams of carbohydrates.
Now the grams of each carbohydrate group are to be divided by the total gram carbohydrate load of the meal. This is called the percentage contribution of a carbohydrate group. So in this example: 26 is divided by 60 and this equals approximately 43.
Finally, the percentage contribution of the carbohydrate is multiplied by that carbohydrates glycemic index. In this example: 43% times 70, this equals approximately 30.
This process must be completed for all the carbohydrate groups. This will result in the glycemic index of the whole meal.
In formula form: Number of grams of a carbohydrate/Total meal carbohydrate, multiplied by the glycemic index of the carbohydrate. This value then gets added to all the other carbohydrates values and the sum is the whole meal glycemic index.