What Is a Calorie?

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.

More commonly, the term calorie is used to define the amount of energy a certain food source supplies, referred to as a food calorie and is equal to 1 kilocalorie (kcal), or 1,000 gram calories.