Although these units relate to the metric system all forms of the calorie were deemed obsolete in science after the SI system was adopted in the 1950s. The unit of energy in the International System of Units is the joule. One small calorie is approximately 4.2 joules (so one large calorie is about 4.2 kilojoules). The factor used to convert calories to joules at a given temperature is numerically equivalent to the specific heat capacity of water expressed in joules per kelvin per gram or per kilogram. The precise conversion factor depends on the definition adopted.
In spite of its non-official status, the large calorie is still widely used as a unit of food energy. The small calorie is also often used for measurements in chemistry, although the amounts involved are typically recorded in kilocalories.
The energy needed to increase the temperature of a given mass of water by 1 °C depends on the atmospheric pressure and the starting temperature. Accordingly, several different precise definitions of the calorie have been used.
The pressure is usually taken to be the standard atmospheric pressure (). The temperature increase can be expressed as one 101.325 kPakelvin, which means the same as an increment of one degree Celsius.
|Thermochemical calorie||calth||≡ J 4.184||the amount of energy equal to exactly 4.184 joules |
|4 °C calorie||cal4||≈ 4.204 J
≈ 985 BTU ≈ 1.168 0.003×10−6 kWh ≈ 2.624×1019 eV
|the amount of energy required to warm one gram of air-free water from 3.5 to 4.5 °C at standard atmospheric pressure.|
|15 °C calorie||cal15||≈ 4.1855 J
≈ 9671 BTU ≈ 1.1626 0.003×10−6 kWh ≈ 2.6124×1019 eV
|the amount of energy required to warm one gram of air-free water from 14.5 to 15.5 °C at standard atmospheric pressure. Experimental values of this calorie ranged from 4.1852 to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.|
|20 °C calorie||cal20||≈ 4.182 J
≈ 964 BTU ≈ 1.162 0.003×10−6 kWh ≈ 2.610×1019 eV
|the amount of energy required to warm one gram of air-free water from 19.5 to 20.5 °C at standard atmospheric pressure.|
|Mean calorie||calmean||≈ 4.190 J
≈ 971 BTU ≈ 1.164 0.003×10−6 kWh ≈ 2.615×1019 eV
|1⁄100 of the amount of energy required to warm one gram of air-free water from 0 to 100 °C at standard atmospheric pressure.|
|International Steam table calorie (1929)||≈ 4.1868 J
≈ 9683 BTU ≈ 1.1630 0.003×10−6 kWh ≈ 2.6132×1019 eV
|1⁄860 international watt hours = 180⁄43 international joules exactly.[note 1]|
|International Steam Table calorie (1956)||calIT||≡ 4.1868 J
≈ 9683 BTU ≈ 1.1630 0.003×10−6 kWh ≈ 2.6132×1019 eV
|1.163 mW·h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).|
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.
The calorie was first defined specifically to measure energy in the form of heat, especially in experimental calorimetry.
In a nutritional context, the kilojoule (kJ) is the SI unit of food energy, although the kilocalorie is still in common use. The word calorie is popularly used with the number of kilocalories of nutritional energy measured. As if to avoid confusion, it is sometimes written Calorie (with a capital "C") in an attempt to make the distinction, although this is not widely understood. Capitalization contravenes the rule that the initial letter of a unit name or its derivative shall be lower case in English.
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 kilocalories, while a gram of either a carbohydrate or a protein contains approximately four kilocalories. Alcohol in a food contains seven kilocalories per gram.
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. 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 liter (1 liter 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. However, its use is being superseded by the SI unit, the joule, and multiples thereof such as the kilojoule.
The 'Thermochemical calorie' was defined by Rossini simply as 4.1833 international joules in order to avoid the difficulties associated with uncertainties about the heat capacity of water (it has been redefined as 4.1840 J exactly).
both the IT calorie and the thermochemical calorie are completely independent of the heat capacity of water.