There are several different units of heat with the name of calorie: the International Steam Table calorie, the 15 degree C calorie, the 4 degree C calorie, the mean 1 to 100 degree C calorie, the thermiochemical calorie, and the nutritionists calorie (1000 15 deg C calories). The calorie is a unit from the now deprecated CGS system of units. The SI equivalent is the joule, which should be used instead.
Of these, the 15 deg. C calorie is what is most commonly meant by calorie in contemporary English text.
A 15 deg C. calorie is a unit of heat. 1 calorie is the amount of heat necessary to raise 1 gram of water from 14.5 to 15.5 oC. Typically nutritionists refer to a Calorie (with a capital C), which is really 1000 calories, or 1 kcal.
1 15 deg C. calorie is approximately the equivalent of 4.186 J or 3.968·10-3 Btu.
Am I the only person who thinks it funny that the U.S. insists on using a deprecated *metric* unit for nutritional purposes? If it was a customary unit one might understand, but insisting on using an out of date metric one?
We had a vote, and yes, you are.