ISO 31-4

ISO 31-4 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to heat. It is superseded by ISO 80000-5.

Its definitions include:

Quantity Unit Remarks
Name Symbol Name Symbol Definition
thermodynamic temperature T, (Θ) kelvin K The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. For practical measurements, the International Temperature Scale of 1990 defines several fixed points and interpolation procedures.
Celsius temperature t, φ degree Celsius °C The degree Celsius is a special name for the unit kelvin, for use in stating values of Celsius temperature. t = TT0, where T0 = 273.15 K
linear expansion coefficient αl reciprocal kelvin, kelvin to the power negative 1 K−1

Annex A of ISO 31-4 lists units of heat based on the foot, pound and second and some other units, including the degree Rankine, degree Fahrenheit, British thermal unit and others. Annex B lists conversion factors for three versions of the calorie.

British thermal unit

The British thermal unit (Btu or BTU) is a 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 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.


The calorie is a unit of energy. The Calorie is 1,000 calories. (The capitalization of this derived second unit's name is a standard, although mention, instead, that "kilocalories" are intended is common, in some consumer-directed contexts.)

The Calorie (large calorie or kilocalorie – symbols: Cal, kcal), also known as the food calorie, is defined as the heat energy involved in warming up one kilogram of water by just one degree Celsius. Note that where the context is clearly about food, nutrition and exercise the term often appears without the capital C.The small calorie (symbol: cal) was later defined as the heat energy to raise the temperature of one gram of water – rather than a kilogram – by the same amount. (See below for details of the definitions.)

Although both units relate to the metric system, they have been considered obsolete in science since the adoption of the SI system. (The SI unit of energy is the joule.) The small calorie is still often used for measurements in chemistry, although the amounts involved are typically recorded in kilocalories.

ISO/IEC 80000

ISO 80000 or IEC 80000 is an international standard promulgated jointly by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC).

The standard introduces the International System of Quantities (ISQ). It is a style guide for the use of physical quantities and units of measurement, formulas involving them, and their corresponding units, in scientific and educational documents for worldwide use. In most countries, the notations used in mathematics and science textbooks at schools and universities follow closely the guidelines in this standard.The ISO/IEC 80000 family of standards was completed with the publication of Part 1 in November 2009.

ISO 31

ISO 31 (Quantities and units, International Organization for Standardization, 1992) is a deprecated international standard for the use of physical quantities and units of measurement, and formulas involving them, in scientific and educational documents. It is superseded by ISO/IEC 80000.

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