British thermal unit (Btu) Amount of heat required to raise the temperature of one pound of water (at or near 39.2 degrees Fahrenheit) by one degree Fahrenheit in practical terms, the amount of heat generated by one lighted stick of match. One Btu is equal to about 252 small calories or 0.252 kilocalories, 778.17 foot pounds, or 1055.06 joules.