British thermal unit, or Btu, is a unit used to measure heat in the inch-pound system of measurement customarily used in the United States. One Btu raises the temperature of 1 pound (0.454 kilogram) of water 1 Fahrenheit degree (0.556 Celsius degree). Raising the temperature of 5 pounds of water 20 Fahrenheit degrees requires 100 Btu’s (5 X 20). Burning fuels release various amounts of heat. For example, a cubic foot of natural gas releases about 1,000 Btu’s of heat. But a cubic foot of manufactured gas releases only about 550 Btu’s. In the metric system, heat is measured in calories or in joules. One Btu equals 251.996 calories or 1,054.350 joules. See also Calorie ; Joule .
