We say an apple has 50 calories but we write it 50 kcal (kilocalories). That's strange, isn't it? You don't understand it either?
But explanation is quite simple. The word calorie represents different definitions for energy. Very simplified it goes like this:
There are two different types of calories:
- "small" calorie (cal) is the approximate amount of energy needed to raise the temperature of 1 gram of water by 1°C.
- "big" calorie (Cal - capital C), this is the one usually used with food, is the approximate amount of energy needed to raise the temperature of 1 kilogram of water by 1°C. Therefore, 1 Cal = 1000 kcal = 1 kcal.
Because of all the confusion with calories the internationally proposed unit is Joule (J). This is also the reason why you see the energy information on food products in kcal and in kilojoules (kJ).
And how do you get joules from calories? Simply, 1 cal = 4.184 J or as it says on food products 1 kcal = 4.184 kJ.