# Calorie

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The name calorie is used for two units of energy.

• The small calorie or gram calorie (symbol: cal) is the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.[1]
• The kilocalorie, also known as large calorie, kilogram calorie, food calorie, and similar names (symbols: kcal, Cal or C)[2] is 1000 small calories.[1]

Although these units are part of the metric system, they have been superseded in the International System of Units by the joule.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 calorie was first defined by Nicolas Clément in 1824 as a unit of heat,[3] and entered French and English dictionaries between 1841 and 1867. The word comes from Latin calor meaning "heat".

## Definitions

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 (101.325 kPa). The temperature increase can be expressed as one kelvin, which means the same as an increment of one degree Celsius.

Name Symbol Conversions Notes
Thermochemical calorie calth 4.184 J

≈ 0.003964 BTU ≈ 1.163×10−6 kWh ≈ 2.611×1019 eV

the amount of energy equal to exactly 4.184 joules [4][5][6][7]
4 °C calorie cal4 ≈ 4.204 J

≈ 0.003985 BTU ≈ 1.168×10−6 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

≈ 0.0039671 BTU ≈ 1.1626×10−6 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 J to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.[4]
20 °C calorie cal20 ≈ 4.182 J

≈ 0.003964 BTU ≈ 1.162×10−6 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

≈ 0.003971 BTU ≈ 1.164×10−6 kWh ≈ 2.615×1019 eV

1100 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

≈ 0.0039683 BTU ≈ 1.1630×10−6 kWh ≈ 2.6132×1019 eV

1860 international watt hours = 18043 international joules exactly.[note 1]
International Steam Table calorie (1956) calIT ≡ 4.1868 J

≈ 0.0039683 BTU ≈ 1.1630×10−6 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).[4]
1. The figure depends on the conversion factor between international joules and absolute (modern) joules. Using the mean international ohm and volt (1.00049 Ω, 1.00034 V[8]), the international joule is about 1.00019 J, using the US international ohm and volt (1.000495 Ω, 1.000330 V) it is about 1.000165 J, giving 4.18684 J and 4.18674 J, respectively.
2. The two definitions most common in older literature appear to be the 15 °C calorie and the thermochemical calorie.

## Usage

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

### Nutrition

In nutritional contexts, the kilojoule (kJ) is the SI unit of food energy, although the kilocalorie is still in common use.[9] In these contexts, confusingly, the word calorie is very often used for what is actually a kilocalorie of nutritional energy. Sometimes, in an attempt to avoid confusion, it is written Calorie (with a capital "C") in an attempt to make the distinction, although this is not universal, and is not widely understood.

In attempts to facilitate comparison, specific energy or energy density figures are often quoted, for example "calories per serving" or "kilocalories per 100 g". Nutritional requirements or intakes are often expressed in calories per day.

### Chemistry

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.[10] 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.[citation needed] However, its use is being superseded by the SI unit, the joule, and multiples thereof such as the kilojoule.