How many calories in popcorn? A cup of popcorn contains approximately 63 calories. Plain popcorn is high in carbohydrates and low in fat and protein. Plain popcorn forms a low fat snack while butter, caramel and other additives increase the calories. It’s a common snack item for diet conscious consumers as long as the additives such as butter are left out. Flavored popcorn has 6% fat, 3% cholesterol, 2% sodium, 1% potassium, 2% carbohydrates, 5% dietary fiber, 2% protein 3% vitamin A, 3% phosphorous, 3% magnesium, 2% zinc, 2% copper, 1% iron and 1% vitamin D.