Calorie is a unit that is used to measure heat energy in the metric system of measurement. A calorie is the amount of energy that is needed to raise the temperature of one gram of water by one Celsius degree. The word calorie comes from a Latin word that means heat. A kilocalorie, also called a kilogram calorie, is equal to 1,000 calories.

Many chemical reactions (changes) produce heat. Scientists measure the amount of heat produced with an instrument called a calorimeter. One of the most important uses of the calorimeter is to measure the amount of heat given off by different foods when they burn. This measurement tells how much energy a certain food yields when it is completely used by the body. Food scientists measure the heat produced in the calorimeter in kilogram calories, but they report the measurements as calories.

Another metric unit that is used to measure heat energy is the joule. One joule equals 0.239 calorie. The British thermal unit (Btu) is used to measure heat in the inch-pound system of measurement, which is customarily used in the United States. One Btu equals 251.996 calories.

Heating engineers make their estimates in calories or Btu's when designing furnaces, boilers, steam turbines, and other machinery. Air-conditioning and refrigeration engineers also use calories or Btu's when designing cooling systems.

Gregory Benford, Ph.D., Professor of Physics, University of California, Irvine. -