the british thermal unit (btu or btu) is a non-si, traditional unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree fahrenheit. it is also part of the united states customary units. its counterpart in the old metric system is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one degree celsius. heat is now known to be equivalent to energy, for which the si unit is the joule; one btu is about 1055 joules. while units of heat are often supplanted by energy units in scientific work, they are still used in many fields. as examples, in the united states the price of natural gas is quoted in dollars per million btus.