British thermal unit

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 63^oF to 64^oF. It is often used to describe the heat value of fuels and heating and cooling system capacities.

1 Btu is approximately:

252.0 calories,

778 ft·lb,

1,055 joules

A unit called the quad is defined as 10¹⁵ BTUs, which is about 1.055 × 10¹⁸ joules.

The Btu should not be confused with the Board of Trade Unit (B.O.T.U.), which is a much larger quantity of energy.