Kilocalories to Kilojoules

The calorie on a US Nutrition Facts panel is really a kilocalorie, the energy to warm one kilogram of water by about one degree Celsius. Nutrition writing sometimes spells it Calorie with a capital C to show the difference, but most labels and apps simply say calories. So a 250-calorie snack bar contains 250 kcal, which is 1,046 kJ, not 1.046 kJ.

The factor 4.184 is the thermochemical calorie, the value nutrition science and most food authorities use. A slightly different international table calorie of 4.1868 J also exists in engineering, but the gap is under 0.1% and never visible on a label.

In the European Union, Regulation 1169/2011 requires energy in both kJ and kcal, usually printed as 1,046 kJ / 250 kcal per 100 g, and the reference intake for an average adult is 8,400 kJ or 2,000 kcal. Australia and New Zealand label primarily in kilojoules, and their average adult reference is 8,700 kJ, about 2,080 kcal. The United States and Canada print calories only.

Converting is useful when you count calories but buy food in Sydney or read a German recipe. Track in one unit and convert labels as you go; mixing units in a diary is the fastest way to double or quarter a day's total by accident.

Fitness trackers report active calories in kcal; sports scientists use kJ, especially for cycling, where power meters measure work directly. A ride averaging 200 watts for an hour does 720 kJ of mechanical work, and because the body is only about 20–25% efficient, it burns roughly as many kilocalories as kilojoules of work. For the opposite conversion, see kJ to kcal; household electricity is covered on kWh to joules.