The British thermal unit (Btu) is a nonmetric unit of heat, used in the United States and, to a certain extent, the UK. The SI unit is the joule (J), which is used by most other countries. 1 Btu is defined by the amount of heat required to raise the temperature of one pound avoirdupois of water from 63oF to 64oF. It is often used to describe the heat value of fuels and heating and cooling system capacities.
1 Btu is approximately:
The Btu should not be confused with the Board of Trade Unit (B.O.T.U.), which is a much larger quantity of energy.