The calorie is a unit of energy.

There are actually two units with the name that have been widely used. The small calorie or gram calorie (usually denoted cal) is the amount of heat energy needed to raise the temperature of one gram of water by one degree Celsius (or one kelvin). The large calorie, food calorie, or kilocalorie (Cal or kcal) is the amount of heat needed to cause the same increase on one kilogram of water.^[1] Thus, 1 kcal = 1000 cal.

The large calorie is sometimes written Calorie (with a capital C) to distinguish from the other unit. However, this convention cannot be relied upon, since it is often ignored – especially in non-technical publications.^[2]

Although both units are related to the metric system, they have been considered obsolete, or deprecated, in scientific usage, since the adoption of the SI system.^[3] The SI unit of energy is the joule, with symbol "J" (since 1889); one small calorie is now defined as exactly 4.184 J, and one large calorie is 4184 J.

However, the two units are still used occasionally in technical work, and the large calorie is still widely used in nutrition. In most countries, the labels of industrialized food products are required to indicate the nutritional energy value in (large) calories per serving or per weight.

The calorie was first introduced by Nicolas Clément in lectures during the period of 1819–1824 as a unit of heat energy (the "large" calorie, viz. modern kcal).^[3][4] It entered French and English dictionaries between 1841 and 1867. The word comes from Latin calor, meaning 'heat'. The "small" calorie (modern cal) was introduced by Pierre Antoine Favre (Chemist) and Johann T. Silbermann (Physicist) in 1852. In 1879, Marcellin Berthelot distinguished between g-calorie (modern cal) and kg-calorie (modern kcal).^[4] Berthelot also introduced the convention of capitalizing the kg-calorie, as Calorie. The use of the kg-calorie (kcal) for nutrition was introduced to the American public by Wilbur Olin Atwater, a professor at Wesleyan University, in 1887.^[3] The modern calorie (cal) was first recognized as a unit of the cm-g-s system (cgs), in 1896,^[4] alongside the existing cgs unit of energy, the erg, first suggested by Clausius in 1864 (under the name ergon), and officially adopted in 1882. The joule was adopted as equivalent to 107 erg in 1889. Use of the calorie was officially deprecated by ninth General Conference on Weights and Measures, in 1948.^[5]

The alternate spelling calory is archaic.

The (small) calorie was intended to be the amount energy needed to increase the temperature of 1 gram of water by 1 °C (or 1 K, which is the same increment). That amount however depends on the atmospheric pressure and the starting temperature. Accordingly, several different precise definitions of the calorie have been used.

The two definitions most common in older literature appear to be the 15 °C calorie and the thermochemical calorie. Until 1948, the latter was defined as 4.1833 international joules; the current standard of 4.184 J was chosen to have the new thermochemical calorie represent the same quantity of energy as before.^[7]

The calorie was first defined specifically to measure energy in the form of heat, especially in experimental calorimetry.^[11]

In a nutritional context, the kilojoule (kJ) is the SI unit of food energy, although the kilocalorie is still in common use.^[12] The word calorie is popularly used with the number of kilocalories of nutritional energy measured. To avoid confusion, it is sometimes written Calorie (with a capital "C") to make the distinction, although this is not widely understood (in part because capitalization contravenes the SI rule that the initial letter of a unit name or its derivative shall be lower case in English).^[13]

To facilitate comparison, specific energy or energy density figures are often quoted as "calories per serving" or "kilocalories per 100 g". A nutritional requirement or consumption is often expressed in calories per day. One gram of fat in food contains nine calories, while a gram of either a carbohydrate or a protein contains approximately four calories.^[14] Alcohol in a food contains seven calories per gram.^[15]

In other scientific contexts, the term calorie almost always refers to the small calorie. Even though it is not an SI unit, it is still used in chemistry. For example, the energy released in a chemical reaction per mole of reagent is occasionally expressed in kilocalories per mole.^[16] Typically, this use was largely due to the ease with which it could be calculated in laboratory reactions, especially in aqueous solution: a volume of reagent dissolved in water forming a solution, with concentration expressed in moles per litre (1 litre weighing 1 kg), will induce a temperature change in degrees Celsius in the total volume of water solvent, and these quantities (volume, molar concentration and temperature change) can then be used to calculate energy per mole. It is also occasionally used to specify energy quantities that relate to reaction energy, such as enthalpy of formation and the size of activation barriers.^[17] However, its use is being superseded by the SI unit, the joule, and multiples thereof such as the kilojoule.

In the past, a bomb calorimeter was used to determine the energy content of food by burning a sample and measuring a temperature change in the surrounding water. Today, this method is not commonly used in the USA and has been succeeded by calculating the energy content indirectly from adding up the energy provided by energy-containing nutrients of food (such as protein, carbohydrates, and fats). The fibre content is also subtracted to account for the fact that fibre is not digested by the body.^[14]