What’s the difference between a calorie, a kilocalorie, and a kilojoule?

500 kilocalories, 500 Calories, 500000 calories or 2100 kilojoules (sketch courtesy sketchory.com - CC+)

500 kilocalories or 500 Calories or 2100 kilojoules (sketch courtesy sketchory.com - CC+)

In older diet books the energy content of foods is stated in Calories, and yet when we read product labels we may find kilocalories and kilojoules. The prefix “kilo” means a thousand, so what gives?

When Nicolas Clément devised the Calorie in 1824, he defined it as the energy required to heat one kilogram of water by one degree Celsius. However, in thermochemistry the calorie was defined as the energy required to heat one gram of water by one degree Celsius.

The two kinds of calorie have caused much confusion. The small calorie can be described as the “gram calorie”, and the large calorie as the “kilogram calorie”. The kilogram calorie is sometimes spelled with an initial capital, as Calorie, but this usage is not universal. The kilogram calorie is often called the kilocalorie.

Thermochemists now use the SI System (the “metric system”), and use the Joule as their unit of energy. One Joule is the energy expended by a power of one watt over one second. The thermochemical calorie is exactly 4.184 Joules, and the kilogram calorie, or kilocalorie, is 4184 Joules.

If the traditional gram calorie is used outside of a thermodynamic context, its exact definition varies a little according to the temperature of the water being used to measure the heat required to raise its temperature. When used in the context of diet or nutrition, the gram calorie is defined by the International Union of Nutritional Sciences as exactly 4.182 Joules, making the nutritional kilocalorie 4182 Joules or 4.182 kilojoules.

The daily calorific requirement of an adult male could be, for example, 2500 or 4000 Calories (depending on age and activity). But those are dietary Calories—kilocalories—and correspond to around 10500 or 16800 kilojoules per day.

To convert kilocalories or dietary Calories to kilojoules, multiply by 4.182. To convert kilojoules to kilocalories or dietary Calories, divide by 4.182. To convert thermochemical calories to Joules, multiply by 4.184. To convert Joules to thermochemical calories, divide by 4.184.

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