A bag of microwave popcorn contains about 120 calories if the butter in the popcorn is fat-free. If the popcorn contains full-fat butter, one bag is around 170 calories.
One bag of microwave popcorn also contains about 300 milligrams of sodium and is free of cholesterol. There are about 6 grams of dietary fiber and 3 grams of protein in a bag of microwave popcorn. A bag of popcorn also contains about 23 grams of carbohydrates. Fat-free popcorn contains no calcium. However, popcorn made with full-fat butter provides about 2 percent of the daily recommended amount of calcium based on a 2,000-calorie diet.