A calorie is a unit that is used to measure energy. The calorie you see on a food package is actually a kilocalorie, or 1,000 calories. A Calorie (kcal) is the amount of energy needed to raise the temperature of 1 kilogram of water 1 degree Celsius.
FACT: Until 1990 when the FDA began requiring nutrition labeling manufacturers of food would place food in a sealed container with water and “burn” it with electrical energy. The temperature of the water after all the food was burned determined the caloric content.
The standard measure used today is adding up the calories by the nutrition items or ingredients of the food items. So if you had a snack that had 5g of protein, 10g of carbohydrates, and 15g of fat the label on the package would then read 225 Calories.