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Questions such as how to quantify the amount of heat given off by a candle led to the first measurements of heat. Heat is recognized as one of the many forms of energy. The joule is the standard unit within the International System of Units (SI) for measuring energy. It is defined as the energy transferred (or "work done") by a force of one newton over a distance of one meter. In everyday terms, this is approximately the amount energy require to lift an apple one meter.

A calorie is a unit of measurement used to quantify energy. While its most familiar application is in the field of nutrition, calories can also measure heat energy in sources such as coal and natural gas, or the energy expenditure generated by specific physical activities. From a scientific standpoint, there are two different types of calories, informally known as small calories and large calories. A small calorie represents the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. It is sometimes called the fifteen-degree calorie, as its most precise application increases the temperature of one gram of water across the fifteen-degree threshold, from 14.5 to 15.5 degrees Celsius.