The unit that is used to measure energy in food is the kilocalorie, or kcal. In food, kilocalories are usually known as simply "calories," sometimes as "Calories" with a capital "C."
One kilocalorie equals 1,000 calories, and one calorie is defined as the energy needed to raise the temperature of 1 gram of water by 1 degree Celsius, or 1.8 degrees Fahrenheit. The calorie counts listed on food packaging refer to the amount of energy that is produced when the food is burned completely, not the exact amount of energy that the human body can extract. These calories are divided by their source, such as carbohydrates, sugars and fats.