Glycemic index or glycemic load..

Some good information:

Glycemic Load is the application of the glycemic index to a standard serving of food.
Glycemic index tests are not performed on typical portion sizes. So, by using the Glycemic Index alone, the glycemic effects of foods containing a small percentage of carbs are likely to be overstated, while the glycemic effects of foods containing a high percentage of carbs are likely to be understated. For example, foods that are mostly water or air will not cause a surge in your blood sugar levels even if their glycemic index is high.

This is why scientists developed the idea of Glycemic Load. It ranks foods according to actual carb content (eg. in a typical portion-size), not how fast a 50g amount of carbs raises blood sugar levels.