Calories are units of energy. Various definitions exist but fall into two broad categories. The first is the small calorie, or gram calorie (symbol: cal), is the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere. The second is the large calorie or kilogram calorie (symbol: Cal), also known as the food calorie and similar names,is defined in terms of the kilogram rather than the gram. It is equal to 1000 small calories or 1 kilocalorie (symbol: kcal)

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. The precise conversion factor depends on the definition adopted.