The calorie is a pre-SI unit of energy, synonymous with heat.[source?] A calorie is defined as the amount of energy required to raise the temperature of one gram of water (specific heat capacity = 4,184 J kg-1 K-1) by one K (= °C + 273).
On food labels, calorie refers to kilocalories (kilo- = 103), which equals to 1000 calories. As such, the word calorie is usually written with a capital C.[source?]
In simple words, calories are a way to measure energy. Our bodies get calories from the food we eat. If we eat more calories than we use, the body stores the extra energy as fat. How the body uses a calorie can depend on whether it comes from fat, protein, or carbohydrates.