<P> A calorie or calory (archaic) is a unit of energy . Various definitions exist but fall into two broad categories . The first, the small calorie, or gram calorie (symbol: cal), is defined as the amount of heat energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere . The second, the large calorie or kilogram calorie (symbols: Cal, kcal), also known as the food calorie and similar names, is defined as the heat energy required to raise the temperature of one kilogram (rather than a gram) of water by one degree Celsius . It is equal to 7003100000000000000 ♠ 1,000 small calories . </P> <P> Although these units relate to the metric system, all of them have been considered obsolete in science since the adoption of the SI system . 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, for kilocalories). The precise conversion factor depends on the definition adopted . </P> <P> 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 . </P>

How many joules is equal to one calorie