Small calories, big Calories
The calorie is the energy to warm one gram of water by one degree Celsius. Because that varies slightly with temperature, the thermochemical calorie was fixed at exactly 4.184 J and is the one used here. The “Calorie” on food labels (capital C, or kcal) is a thousand of them: 4,184 J or 4.184 kJ. Labels in Europe, Australia and elsewhere print kilojoules first, so a 2,000 kcal daily intake appears as about 8,400 kJ. Physics and chemistry use the small calorie or, more often now, the joule directly.
Rough conversion in your head
Divide kilojoules by 4.2 for food Calories: a 1,500 kJ snack is about 360 kcal (true 358). Multiply kcal by 4.2 the other way. For joules to small calories, divide by 4: 1,000 J → 250 cal (true 239). A watt-hour is 3,600 J or 860 cal; a kilowatt-hour is 860 kcal — roughly a third of a day's food energy.