Popcorn is often touted as a low-calorie snack that fills you up before you can eat too much. Add something to it, though, such as oil for popping or flavoring, and you increase the calorie count, sometimes substantially.
Plain popcorn, popped without oil or any flavorings, can have as little as 30 calories in 1 cup. Add oil for popping, even olive oil, and you raise the number of calories because of the fat attached to the kernels. Preflavored popcorn can be especially high in calories. For example, one brand-name butter-flavored popcorn using coconut oil has 190 calories in 2 cups.
Fats have the same amount of calories whether they're in solid or liquid form. It generally is 120 calories per 1 tbsp., according to Clemson Cooperative Extension. The type of fat is what varies when you change the cooking oil or fatty flavoring used with popcorn. Olive oil is mostly monounsaturated fat, which can have a beneficial effect on cholesterol levels, while butter is half saturated fat.
Olive oil's high monounsaturated fat content means if you use that instead of another oil or butter to pop and flavor the popcorn, you cut down on the number of calories you get from saturated fat. Your overall caloric intake will vary only if you change the amount of oil you use.