British thermal unit

The British thermal unit is a nonmetric unit of heat, used in the United States and the UK. The metric unit is the joule, which is used by most other countries. 1 Btu is defined by the amount of heat required to raise the temperature of one pound of water from 63^oF to 64^oF.

1 Btu is approximately 252.0 calories, 778 ft·lb, or 1055 J.