The amount of calories in a bag of light popcorn, or light butter popcorn, varies by brand, but is approximately 20 to 30 calories per cup of popped corn. A typical serving of popped corn is 4 cups, so the average serving has anywhere from 80 to 120 calories.
When popped, popcorn gets less than 10 percent of its calories from fat. In addition, popcorn serves as a source of sodium, with a typical serving having more than 10 percent of the recommended daily allowance per person. Popcorn is a source of carbohydrates, with a high amount of dietary fiber. Popcorn also provides a small amount of vitamin E and iron.