A calorie is a unit of measure of energy. One calorie is equal to the amount of energy necessary to warm one gram of water by one degree Celsius at sea level. This is equal to 4.184 Joules in the SI system. The "Calorie" on a nutrition label is actually different from the "calorie" or unit of measure for energy. The upper case "Calorie" is actually a kilocalorie, or 1,000 calories.