British thermal unit (BTU): unit of heat and energy

Definition and conversions

By definition a BTU is the amount of heat required to raise the temperature of one avoirdupois pound of water by one degree Fahrenheit under specified conditions. In modern terms it is approximately 1,055 joules, or a small fraction of a kilowatt-hour. Slight differences in reference temperature and specific heat tables produce small variant values quoted in different sources.

History and geographic use

The BTU developed in the 19th century with industrial steam and heating practice. Although metrication and the International System (SI) use the joule, the BTU remains in common use in the United Kingdom, the United States and Canada for appliance labeling, building services and some energy markets: see regional practice in United Kingdom, United States and Canada.

Common applications

BTUs appear in ratings for furnaces, boilers and air conditioners (often quoted as BTU per hour when indicating power or capacity). Fuel energy contents and commodity trading for products such as natural gas may be expressed with BTU-based measures; industry terms such as MMBtu (million BTU) and the therm are also used in commercial practice.

Practical notes and distinctions

• Energy vs power: A BTU is a quantity of energy; when expressed as BTU/h it denotes a rate of heat transfer (power).
• Relation to SI: The joule is the SI unit of energy; conversions are straightforward but users should note which BTU definition a specification uses.
• Variants and standards: Thermochemical and older table definitions differ by only a small fraction; consult authoritative references for precise conversion factors and industry conventions.

For technical work and equipment sizing, convert BTU figures to SI units so that performance, energy consumption and costs can be compared consistently. For general background on heat and units, see heat unit details.

Further reading and standards guidance are available from regional energy authorities and technical guides: US practice, energy concepts, Canadian guidance, UK guidance, and materials on fuel pricing and measurement such as those for natural gas.