This is a list of units that are not defined as part of the International System of Units (SI) but are otherwise mentioned in the SI Brochure,listed as being accepted for use alongside SI-units, or for explanatory purposes.
|Name||Symbol||Quantity||Value in SI units|
|minute||min||time||1 min = 60 s|
|hour||h||time||1 h = 60 min = 3 600 s|
|day||d||time||1 d = 24 h = 1440 min = 86 400 s|
|astronomical unit||au||length||1 au = 149 597 870 700 m|
|degree||°||plane angle||1° = (π/180) rad|
|arcminute||′||plane angle||1′ = (1/60)° = (π/10 800) rad|
|arcsecond||″||plane angle||1″ = (1/60)′ = (1/3 600)° = (π/648 000) rad|
|hectare||ha||area||1 ha = 1 hm2 = 10 000 m2|
|litre||l, L||volume||1 l = 1 dm3 = 1 000 cm3 = 0.001 m3|
|tonne||t||mass||1 t = 1 Mg = 1 000 kg|
|dalton||Da||mass||1 Da = 1.66053906660(50)×10−27 kg|
|electronvolt||eV||energy||1 eV = 1.602176634×10−19 J|
|neper||Np||logarithmic ratio quantity||—|
|bel, decibel||B, dB||logarithmic ratio quantity||—|
The SI prefixes can be used with several of these units, but not, for example, with the non-SI units of time.
The following table lists units that are effectively defined in side- and footnotes in the 9th SI brochure. Units that are mentioned without a definition or that occur in historical material recorded in the appendices are not included.
|Name||Symbol||Quantity||Equivalent SI unit|
|gal||Gal||acceleration||1 Gal = 1 cm⋅s−2 = 0.01 m⋅s−2|
|unified atomic mass unit||u||mass||1 u = 1 Da|
|volt-ampere reactive||var||reactive power||1 var = 1 V⋅A|
With the publication of each edition of the SI brochure, the list of non-SI units listed in tables changed compared to the preceding SI brochures.The table below compares the status of each unit for which the status has changed between editions of the SI Brochure.
|Name||Symbol||1st–3rd SI Brochures||4–6th SI Brochures||7th SI Brochure||8th SI Brochure||9th SI Brochure|
|astronomical unit||au||accepted||not mentioned||accepted||accepted||accepted|
|parsec||pc||accepted||not mentioned||not mentioned||not mentioned||not mentioned|
|neper||Np||not mentioned||not mentioned||accepted||listed||accepted|
|bel||b||not mentioned||not mentioned||accepted||listed||accepted|
|decibel||db||not mentioned||not mentioned||not mentioned||listed||accepted|
|unified atomic mass unit||u||accepted||accepted||accepted||accepted||footnote only|
|dalton||Da||not mentioned||not mentioned||footnote only||accepted||accepted|
|natural unit (n.u.) of speed||c0||not mentioned||not mentioned||not mentioned||listed||not mentioned|
|n.u. of action||ħ||not mentioned||not mentioned||not mentioned||listed||not mentioned|
|n.u. of mass||me||not mentioned||not mentioned||not mentioned||listed||not mentioned|
|n.u. of time||ħ/(mec02)||not mentioned||not mentioned||not mentioned||listed||not mentioned|
|atomic unit (a.u.) of charge||e||not mentioned||not mentioned||not mentioned||listed||not mentioned|
|a.u. of mass||me||not mentioned||not mentioned||not mentioned||listed||not mentioned|
|a.u. of action||ħ||not mentioned||not mentioned||not mentioned||listed||not mentioned|
|a.u. of length||a0||not mentioned||not mentioned||not mentioned||listed||not mentioned|
|a.u. of energy||Eh||not mentioned||not mentioned||not mentioned||listed||not mentioned|
|a.u. of time||ħ/Eh||not mentioned||not mentioned||not mentioned||listed||not mentioned|
|nautical mile||M||temporarily accepted||temporarily accepted||temporarily accepted||listed||not mentioned|
|knot||kn||temporarily accepted||temporarily accepted||temporarily accepted||listed||not mentioned|
|ångström||Å||temporarily accepted||temporarily accepted||temporarily accepted||listed||not mentioned|
|are||a||temporarily accepted||temporarily accepted||temporarily accepted||not mentioned||not mentioned|
|hectare||ha||temporarily accepted||temporarily accepted||temporarily accepted||accepted||accepted|
|barn||b||temporarily accepted||temporarily accepted||temporarily accepted||listed||not mentioned|
|bar||bar||temporarily accepted||temporarily accepted||temporarily accepted||listed||not mentioned|
|standard atmosphere||atm||temporarily accepted||listed||listed||not mentioned||not mentioned|
|gal||Gal||temporarily accepted||temporarily accepted||listed||listed||footnote only|
|curie||Ci||temporarily accepted||temporarily accepted||listed||not mentioned||not mentioned|
|roentgen||R||temporarily accepted||temporarily accepted||listed||not mentioned||not mentioned|
|rad||rad||temporarily accepted||temporarily accepted||listed||not mentioned||not mentioned|
|rem||rem||not mentioned||temporarily accepted||listed||not mentioned||not mentioned|
|fermi||listed||listed||listed||not mentioned||not mentioned|
|metric carat||listed||listed||listed||not mentioned||not mentioned|
|torr||Torr||listed||listed||listed||not mentioned||not mentioned|
|kilogram-force||kgf||listed||listed||not mentioned||not mentioned||not mentioned|
|calorie||cal||listed||listed||listed||not mentioned||not mentioned|
|micron||μ||listed||listed||listed||not mentioned||not mentioned|
|x-unit||xu||listed||listed||not mentioned||not mentioned||not mentioned|
|stilb||sb||listed||listed||not mentioned||not mentioned||not mentioned|
|gamma (Mass)||γ||listed||listed||listed||not mentioned||not mentioned|
|γ (magnetic flux density)||γ||listed||listed||not mentioned||not mentioned||not mentioned|
|lambda||λ||listed||listed||not mentioned||not mentioned||not mentioned|
|jansky||Jy||not mentioned||not mentioned||listed||not mentioned||not mentioned|
|millimetre of mercury||mmHg||not mentioned||not mentioned||not mentioned||listed||not mentioned|
In this table, the status descriptions have the following meanings:
The ampere (, ; symbol: A), often shortened to amp, is the unit of electric current in the International System of Units (SI). One ampere is equal to 6.241509074×1018 electrons worth of charge moving past a point in a second. It is named after French mathematician and physicist André-Marie Ampère (1775–1836), considered the father of electromagnetism along with Danish physicist Hans Christian Ørsted.
The General Conference on Weights and Measures is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre Convention through which member states act together on matters related to measurement science and measurement standards. The CGPM is made up of delegates of the governments of the member states and observers from the Associates of the CGPM. Under its authority, the International Committee for Weights and Measures executes an exclusive direction and supervision of the BIPM.
The kilogram is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially. It means 'one thousand grams'.
The litre or liter is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3). A cubic decimetre occupies a volume of 10 cm × 10 cm × 10 cm and is thus equal to one-thousandth of a cubic metre.
The Metre Convention, also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations. The treaty created the International Bureau of Weights and Measures (BIPM), an intergovernmental organization under the authority of the General Conference on Weights and Measures (CGPM) and the supervision of the International Committee for Weights and Measures (CIPM), that coordinates international metrology and the development of the metric system.
The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and based on the metre as the unit of length and either the kilogram as the unit of mass or the kilogram-force as the unit of force.</ref> and the world's most widely used system of measurement. Established and maintained by the General Conference on Weights and Measures (CGPM), it is the only system of measurement with an official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.
The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived. The units and their physical quantities are the second for time, the metre for length or distance, the kilogram for mass, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. The SI base units are a fundamental part of modern metrology, and thus part of the foundation of modern science and technology.
The Avogadro constant, commonly denoted NA or L, is the proportionality factor that relates the number of constituent particles (usually molecules, atoms or ions) in a sample with the amount of substance in that sample. It is an SI defining constant with an exact value of 6.02214076×1023 reciprocal moles. It is named after the Italian scientist Amedeo Avogadro by Stanislao Cannizzaro, who explained this number four years after Avogadro's death while at the Karlsruhe Congress in 1860.
The dalton or unified atomic mass unit is a unit of mass widely used in physics and chemistry. It is defined as 1⁄12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. The atomic mass constant, denoted mu, is defined identically, giving mu = m(12C)/12 = 1 Da.
Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in France when a length standard taken from a natural source was proposed. This led to the creation of the decimal-based metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure conformity between the countries, the Bureau International des Poids et Mesures (BIPM) was established by the Metre Convention. This has evolved into the International System of Units (SI) as a result of a resolution at the 11th General Conference on Weights and Measures (CGPM) in 1960.
The newton is the unit of force in the International System of Units (SI). It is defined as 1 kg⋅m/s2, the force which gives a mass of 1 kilogram an acceleration of 1 metre per second per second. It is named after Isaac Newton in recognition of his work on classical mechanics, specifically Newton's second law of motion.
Myria- (symbol my) is a now obsolete decimal metric prefix denoting a factor of 104 (ten thousand). It originates from the Greek μύριοι (mýrioi) (myriad). The prefix was part of the original metric system adopted by France in 1795, but was not adopted when the SI prefixes were internationally adopted by the 11th CGPM conference in 1960.
Sound energy density or sound density is the sound energy per unit volume. The SI unit of sound energy density is the pascal (Pa), which is 1 kg⋅m−1⋅s−2 in SI base units or 1 joule per cubic metre (J/m3).
The molar mass constant, usually denoted by Mu, is a physical constant defined as one twelfth of the molar mass of carbon-12: Mu = M(12C)/12. The molar mass of any element or compound is its relative atomic mass multiplied by the molar mass constant.
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow based engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907). The Kelvin scale is an absolute thermodynamic temperature scale, meaning it uses absolute zero as its null (zero) point.
The angstrom or ångström is a metric unit of length equal to 10−10 m; that is, one ten-billionth (US) of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. Its symbol is Å, a letter of the Swedish alphabet. The unit is named after the Swedish physicist Anders Jonas Ångström (1814–1874).
In metrology, a standard is an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity. Standards are the fundamental reference for a system of weights and measures, against which all other measuring devices are compared. Historical standards for length, volume, and mass were defined by many different authorities, which resulted in confusion and inaccuracy of measurements. Modern measurements are defined in relationship to internationally standardized reference objects, which are used under carefully controlled laboratory conditions to define the units of length, mass, electrical potential, and other physical quantities.
In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram. Effective 20 May 2019, the 144th anniversary of the Metre Convention, the kilogram, ampere, kelvin, and mole are now defined by setting exact numerical values, when expressed in SI units, for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre, and candela had previously been redefined using physical constants. The four new definitions aimed to improve the SI without changing the value of any units, ensuring continuity with existing measurements. In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year after determining that previously agreed conditions for the change had been met. These conditions were satisfied by a series of experiments that measured the constants to high accuracy relative to the old SI definitions, and were the culmination of decades of research.
The history of the metric system began during the Age of Enlightenment with measures of length and weight derived from nature, along with their decimal multiples and fractions. The system became the standard of France and Europe within half a century. Other measures with unity ratios were added, and the system went on to be adopted across the world.