The number of cups in a microwavable bag of popcorn depends on the size of the bag. To find the answer, consult the label and multiply the serving size times the number of servings in the bag.
For example, ACT II 94 percent fat-free microwaveable popcorn comes in bags weighing 2.71 ounces. The label lists the nutritional details of a single serving of 6.5 cups, with two servings in the entire container. That means there are 13 cups of popcorn in the bag.
If reading a label for a box that includes more than one bag, other calculations may be necessary. For example, a box containing 12 bags of equal size may say that there are 24 servings of 6.5 cups each in the box. In this case, to calculate the number of cups in one bag, the cups should still be multiplied by two, since there are two servings in each of the 12 bags.