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Calorie

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The calorie is a pre-SI metric unit of energy. The unit was first defined by Professor Nicolas Clément in 1824 as a unit of heat. This definition entered French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule. However, in many countries it remains in common use as a unit of food energy. The kilocalorie per mole remains in use in computational chemistry and molecular spectroscopy.

Definitions vary but are all based on the specific heat capacity of water. The gram calorie, approximately 4.2 J, is based on one gram of water. The kilogram calorie, equal to one thousand gram calories, is based on one kilogram of water. In the context of nutrition, and especially food labelling, a larger unit is used and referred to interchangeably by the terms calorie (or Calorie) and kilocalorie.

Definitions

Historically, the calorie has had two major alternative definitions differing by a factor of one thousand. In addition to these two major alternative definitions, minor variants of the definition of this unit also exist differing in the exact experimental conditions used, most notably the start temperature of the water.

Kilogram and gram calories

The original definition by Clément was based on the kilogram. Other definitions based on the gram have since been made. We thus have the two major variants: the kilogram calorie and the gram calorie. One thousand gram calories equal one kilogram calorie.

In the context of food energy the term calorie generally refers to the kilogram calorie. However, the term kilocalorie (kcal), referring to one thousand gram calories, is also in widespread use especially by professional nutritionists (when speaking in terms of calories rather than joules). To avoid confusion, the prefix kilo- is not used with the kilogram calorie.

Kilogram calorie

The kilogram calorie, large calorie, food calorie, Calorie (capital C) or just calorie (lowercase c) is the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius.

Gram calorie

The gram calorie, small calorie or calorie (cal) is the amount of energy required to raise the temperature of one gram of water by 1 °C. The gram calorie was once commonly used in chemistry and physics.

Pound calorie

The pound calorie is the amount of energy required to raise the temperature of one pound of water by 1 °C. The pound calorie is encountered in engineering usage for heat transfer calculations and is equivalent to 1.8 British thermal units (1.899 kJ). It is something of a hybrid unit, rarely used now, but may be found for example in older but still useful, engineering (and other) textbooks on heat transfer and heat exchanger design. A heat transfer coefficient may be given in the units lb-cal/(h·ft²·°F).

Variations

The energy needed to increase the temperature of a gram of water by 1 °C depends on the starting temperature and is difficult to measure precisely. Accordingly, there have been several definitions of the calorie. The two perhaps most popular definitions used in older literature are the 15 °C calorie and the thermochemical calorie.

The factors used to convert measurements in calories to their equivalents in joules are numerically equivalent to expressions of the specific heat capacity of water in joules per gram or kilojoules per kilogram.

Conversions

One gram calorie is approximately:

• 4.184 J (joules)
• 0.003964 BTU (British thermal units)
• 1.163×10⁻⁶ kW·h (kilowatt hours)
• 2.611×10¹⁹ eV (electron volts)

One kilogram calorie (food calorie) is approximately:

• 4.184 kJ
• 3.964 BTU
• 0.001163 kW·h
• 2.611×10²² eV

Notes and references

See also

• Food energy
• Basal metabolic rate
• Nutrition facts label