There is a difference between what a chemist calls a calorie and what a nutritionist calls a calorie.
The chemist’s calorie is the amount of heat energy required to raise the temperature of one gram of water by one degree Celsius, whereas the nutritionist’s calorie, the calorie you see in diet books and on food labels, is the amount of heat energy required to raise the temperature of one thousand grams (a kilogram) of water by one degree Celsius.
Obviously, then, a nutritionist’s calorie is a thousand times bigger than a chemist’s, and the chemist would call it a kilocalorie, or kcal.
I find myself in the awkward position of being a chemist writing about food for an audience that spans both camps. For consistency, and if my chemistry colleagues will forgive me, I will use the word calorie in the nutritionist’s sense unless otherwise noted.
In many cases, I use the word calories simply to mean an unspecified amount of heat energy, in which case the chemist/nutritionist dichotomy doesn’t matter.
For those chemists who are not appeased, you may have a supply of kilos to insert in front of the word calorie whenever you encounter it.
Note to users of the International System of units: One nutritional kilocalorie, kcal, is equal to 4.19 kiloJoules or kJ.