kilocalorie
Energyexact by definition
The kilocalorie is exactly 1000 thermochemical calories, which is exactly 4184 joules. The relation is exact because the thermochemical calorie is exactly 4.184 J. This is the unit food labels call a Calorie with a capital C, and it is also the working unit of older European heating calculations.
Watch out: One kilocalorie equals one food Calorie. A label reading "2000 Calories" means 2000 kcal, which is 8.368 MJ, not 2000 cal. The lowercase calorie is 1000 times smaller, and the two are routinely conflated in casual writing.
| 1 kcal | 4,184 J |
About the kilocalorie
The kilocalorie is the unit almost everyone means when they say "calorie" in a nutritional context. One kcal, one food Calorie, and 4184 joules are three names for the same quantity. The lowercase calorie of chemistry is a thousandth of it, which is why a chemistry paper reporting a bond energy of 100 kcal/mol and a food label reporting a 100 Calorie snack are describing quantities that happen to be numerically equal by coincidence of the capital letter.
The unit also survives in older heating practice, especially in continental Europe, where boiler outputs were once given in kcal/h. The conversion is worth having: 1 kcal/h is about 1.163 W, so a 20 000 kcal/h boiler is about 23.3 kW. Refrigeration capacity in the same tradition was quoted in "frigories" per hour, which is a kcal of heat removed rather than added, and is numerically the same size.