Noun

Wikipedia calorie (plural calories)

1. (physics) a non-SI unit of energy, equivalent to approximately 4.2 joules. This unit was widely used in chemistry and physics, being the amount of heat needed to raise the temperature of 1 gram of water by 1°C.
2. (nutrition) the kilocalorie, kilogram calorie or large calorie. A unit of energy 1,000 times larger than the normal calorie. It is equivalent to approximately 4.2 kilojoules.

The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering 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 In most fields its use is archaic, having been replaced by the SI unit of energy, the joule.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.

Definitions of a calorie fall into two classes:

• The small calorie or gram calorie (symbol: cal) approximates the energy needed to increase the temperature of 1 gram of water by 1 °C. This is about 4.2 joules.
• The large calorie, kilogram calorie or food calorie (symbol: Cal) approximates the energy needed to increase the temperature of 1 kilogram of water by 1 °C. This is exactly 1000 small calories or about 4.2 kilojoules.

In scientific contexts, the name "calorie" refers to the gram calorie, and this unit has the symbol cal. Metric prefixes are used with this name and symbol, so that the kilogram calorie is known as the "kilocalorie" and has the symbol kcal. In other contexts, the kilocalorie is often referred to as a Calorie (capital "C"), or just a calorie, and it has to be inferred from the context that the small calorie is not intended. When referring to food energy, calorie refers to the large calorie.

In other words, 1 kcal = 1000 cal = 1 Cal

The conversion factor between calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units).