# Calories

Calories are units of energy. Various definitions exist but fall into two broad categories.

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 calorie was first defined by Nicolas Clément in 1824 as a unit of heat energy, and entered French and English dictionaries between 1841 and 1867. The word comes from Latin calor meaning "heat".

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.

≈ 0.003964 BTU ≈ 1.162×10−6 kWh ≈ 2.611×1019 eV

Name Symbol Conversions Notes
Thermochemical calorie calth 4.184 J

≈ 0.003964 BTU ≈ 1.162×10−6 kWh ≈ 2.611×1019 eV

the amount of energy equal to exactly 4.184 joules
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 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

≈ 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.
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).

• 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.
• The large calorie or kilogram calorie (symbol: Cal), also known as the food calorie and similar names, is defined in terms of the kilogram rather than the gram. It is equal to 1000 small calories or 1 kilocalorie (symbol: kcal).
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