How are calories determined?

Not sure if this is the right place to post this. How do companies determine how many calories are in food? What method is used? Are the calories listed in a bag of popcorn accurate for dieting purposes or just an estimate? It seems like I am eating much more than 275 calories, but maybe that is because of the fiber.

try to avoid double posting please. it's more difficult to navigate the threads and a bit overwhelming. to answer your question, yes it is an estimate because the weight in a packaged product varies slightly from what the package lists. The estimates aren't usually off by an obscene amount though, unless it's something like a packaged pastry or something where the calories can really add up. 

Original Post by mike61725:

Not sure if this is the right place to post this. How do companies determine how many calories are in food? What method is used? Are the calories listed in a bag of popcorn accurate for dieting purposes or just an estimate? It seems like I am eating much more than 275 calories, but maybe that is because of the fiber.

Calories, or actually Kilo Calories, are actually thermo units from physics so basically it's how much heat energy is available when you burn the food.