The calorie is a unit of measurement for energy. Unfortunately, the evolution of its use in the twentieth century has produced several different meanings for the term, producing a lot of confusion.

Definition

The calorie (cal) (with a small C) is the CGS unit of heat energy. This calorie (also called a gram calorie or small calorie) is the amount of heat required at a pressure of one atmosphere to raise the temperature of one gram of water by 1 °C.

This quantity of heat, however, varies with the temperature of the water, so it is necessary to specify which degree Celsius is meant. A traditional choice was the degree from 14.5°C to 15.5°C; raising the temperature of water through this range requires 4.1858 joules, a quantity called the 15° calorie. Another choice produces the thermochemical calorie, equal to exactly 4.184 joules. More common today is the international steam table calorie, or IT calorie for short, defined by an international conference in 1956 to equal exactly 4.1868 joules.

The Calorie (with a capital C) is the common name for the MKS unit of heat energy. This unit is properly called the ‘’’kilocalorie’’’; it is also called the kilogram calorie (kcal)or large calorie (Cal). It often is distinguished from the small calorie by capitalizing its name and symbol. The large calorie, or rather kilocalorie, is the amount of heat required at a pressure of one atmosphere to raise the temperature of one kilogram of water by 1 °C. Since this is 1,000 times as much water as mentioned in the definition of the small calorie, the kilocalorie equals 1,000 small calories.

In many scientific applications, such as measuring energy use, the name "calorie" refers strictly to the gram calorie, and this unit has the symbol cal. SI prefixes are used with this name and symbol, so that the kilogram calorie is known as the "kilocalorie" and has the symbol kcal. In nutrition and food labelling, the term "calorie" usually refers to the kilogram calorie.

History

The name of the unit comes from the Latin calor, heat. The French chemist Nicholas Clement was perhaps the first to use the term between 1819 and 1824 in a series of lectures on heat engines. In 1863, the word entered the English language through translation of Adolphe Ganot's popular French physics text, which defined a Calorie as the heat needed to raise the temperature of 1 kilogram of water from 0 to 1 degrees °C.

Conversions

The following conversions are based on the IT calorie.

Further reading

• How Many? A Dictionary of Units of Measurement, Russ Rowlett, Center for Mathematics and Science Education, University of North Carolina at Chapel Hill
• SI base units and SI derived units, the Physics Laboratory, National Institute of Standards and Technology
• Energy conversion tool at unitconversion.org