International System of Units
Symbol  Defining constant  Exact value 

Δν_{Cs}  hyperfine transition frequency of Cs  9192631770 Hz 
c  speed of light  299792458 m/s 
h  Planck constant  6.62607015×10^{−34} J⋅s 
e  elementary charge  1.602176634×10^{−19} C 
k  Boltzmann constant  1.380649×10^{−23} J/K 
N_{A}  Avogadro constant  6.02214076×10^{23} mol^{−1} 
K_{cd}  luminous efficacy of 540 THz radiation  683 lm/W 
Symbol  Name  Quantity 

s  second  time 
m  metre  length 
kg  kilogram  mass 
A  ampere  electric current 
K  kelvin  thermodynamic temperature 
mol  mole  amount of substance 
cd  candela  luminous intensity 
The International System of Units, known by the international abbreviation SI^{[a]} in all languages^{[1]}^{: 125 }^{[2]}^{: iii }^{[3]} and sometimes pleonastically as the SI system,^{[b]} is the modern form^{[1]}^{: 117 }^{[6]}^{[7]} of the metric system^{[g]} and the world's most widely used system of measurement.^{[1]}^{: 123 }^{[9]}^{[10]} Established and maintained^{[11]} by the General Conference on Weights and Measures^{[j]} (CGPM^{[k]}), it is the only system of measurement with an official status^{[m]} in nearly every country in the world,^{[n]} employed in science, technology, industry, and everyday commerce.
The SI comprises a
The seven base units and the 22 coherent derived units with special names and symbols may be used in combination to express other coherent derived units.^{}[r] Since the sizes of coherent units will be convenient for only some applications and not for others, the SI provides twentyfour prefixes which, when added to the name and symbol of a coherent unit^{[s]} produce twentyfour additional (noncoherent) SI units for the same quantity; these noncoherent units are always decimal (i.e. poweroften) multiples and submultiples of the coherent unit.^{[t]}^{[u]} The SI is intended to be an evolving system; units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves.
Since 2019, the magnitudes of all SI units have been defined by declaring that seven defining constants have certain exact numerical values when expressed in terms of their SI units. These defining constants are the speed of light in vacuum c, the hyperfine transition frequency of caesium Δν_{Cs}, the Planck constant h, the elementary charge e, the Boltzmann constant k, the Avogadro constant N_{A}, and the luminous efficacy K_{cd}. The nature of the defining constants ranges from fundamental constants of nature such as c to the purely technical constant K_{cd}. Prior to 2019, h, e, k, and N_{A} were not defined a priori but were rather very precisely measured quantities. In 2019, their values were fixed by definition to their best estimates at the time, ensuring continuity with previous definitions of the base units.
The current way of defining the SI is a result of a decadeslong move towards increasingly abstract and idealised formulation in which the realisations of the units are separated conceptually from the definitions. A consequence is that as science and technologies develop, new and superior realisations may be introduced without the need to redefine the unit. One problem with artefacts is that they can be lost, damaged, or changed; another is that they introduce uncertainties that cannot be reduced by advancements in science and technology. The last artefact used by the SI was the International Prototype of the Kilogram, a cylinder of platinum–iridium.
The original motivation for the development of the SI was the diversity of units that had sprung up within the
Introduction
The International System of Units, or SI,^{}
The only other types of measurement system that still have widespread use across the world are the Imperial and US customary measurement systems,^{[z]} and they are legally defined in terms of the SI.^{[aa]} There are other, less widespread systems of measurement that are occasionally used in particular regions of the world. In addition, there are many individual nonSI units that don't belong to any comprehensive system of units, but that are nevertheless still regularly used in particular fields and regions. Both of these categories of unit are also typically defined legally in terms of SI units.^{[ab]}
Controlling body
The SI was established and is maintained by the
All the decisions and recommendations concerning units are collected in a brochure called The International System of Units (SI),^{[ad]} which is published by the International Bureau of Weights and Measures (BIPM^{[ae]}) and periodically updated.
Overview of the units
SI base units
The SI selects seven units to serve as base units, corresponding to seven base physical quantities.^{[af]}^{[ag]} They are the second, with the symbol s, which is the SI unit of the physical quantity of time; the metre, symbol m, the SI unit of length; kilogram (kg, the unit of mass); ampere (A, electric current); kelvin (K, thermodynamic temperature); mole (mol, amount of substance); and candela (cd, luminous intensity).^{[1]} All units in the SI can be expressed in terms of the base units, and the base units serve as a preferred set for expressing or analysing the relationships between units.
SI derived units
The system allows for an unlimited number of additional units, called derived units, which can always be represented as products of powers of the base units, possibly with a nontrivial numeric multiplier. When that multiplier is one, the unit is called a coherent derived unit.^{[ah]} The base and coherent derived units of the SI together form a coherent system of units (the set of coherent SI units).^{[ai]} Twentytwo coherent derived units have been provided with special names and symbols.^{[q]} The seven base units and the 22 derived units with special names and symbols may be used in combination to express other derived units,^{[r]} which are adopted to facilitate measurement of diverse quantities.
Why SI kept the distinction between base and derived units
Prior to its redefinition in 2019, the SI was defined through the seven base units from which the derived units were constructed as products of powers of the base units. After the redefinition, the SI is defined by fixing the numerical values of seven defining constants. This has the effect that the distinction between the base units and derived units is, in principle, not needed, since all units, base as well as derived, may be constructed directly from the defining constants. Nevertheless, the distinction is retained because 'it is useful and historically well established', and also because the ISO/IEC 80000 series of standards^{[aj]} specifies base and derived quantities that necessarily have the corresponding SI units.^{[1]}^{: 129 }
SI metric prefixes and the decimal nature of the SI
Like all metric systems, the SI uses metric prefixes to systematically construct, for the same physical quantity, a set of units that are decimal multiples of each other over a wide range.
For example, while the coherent unit of length is the metre,^{[ak]} the SI provides a full range of smaller and larger units of length, any of which may be more convenient for any given application – for example, driving distances are normally given in kilometres (symbol km) rather than in metres. Here the metric prefix 'kilo' (symbol 'k') stands for a factor of 1000; thus, 1 km = 1000 m.^{[al]}
The current version of the SI provides twentyfour metric prefixes that signify decimal powers ranging from 10^{−30} to 10^{30}, the most recent being adopted in 2022.^{[1]}^{: 143–4 }^{[17]}^{[18]} Most prefixes correspond to integer powers of 1000; the only ones that do not are those for 10, 1/10, 100, and 1/100.
In general, given any coherent unit with a separate name and symbol,^{[am]} one forms a new unit by simply adding an appropriate metric prefix to the name of the coherent unit (and a corresponding prefix symbol to the coherent unit's symbol).^{[an]} Since the metric prefix signifies a particular power of ten, the new unit is always a poweroften multiple or submultiple of the coherent unit. Thus, the conversion between different SI units for one and the same physical quantity is always through a power of ten.^{[ao]} This is why the SI (and metric systems more generally) are called decimal systems of measurement units.^{[19]}^{[ap]}
The grouping formed by a prefix symbol attached to a unit symbol (e.g. 'km', 'cm') constitutes a new inseparable unit symbol. This new symbol can be raised to a positive or negative power and can be combined with other unit symbols to form compound unit symbols.^{[1]}^{: 143 } For example, g/cm^{3} is an SI unit of density, where cm^{3} is to be interpreted as (cm)^{3}.
Coherent and noncoherent SI units
When prefixes are used with the coherent SI units, the resulting units are no longer coherent, because the prefix introduces a numerical factor other than one.^{[1]}^{: 137 } The one exception is the kilogram, the only coherent SI unit whose name and symbol, for historical reasons, include a prefix.^{[an]}
The complete set of SI units consists of both the coherent set and the multiples and submultiples of coherent units formed by using the SI prefixes.^{}
Moreover, the metre is the only coherent SI unit of length. Every physical quantity has exactly one coherent SI unit, although this unit may be expressible in different forms by using some of the special names and symbols.^{[1]}^{: 140 } For example, the coherent SI unit of linear momentum may be written as either kg⋅m/s or as N⋅s, and both forms are in use (e.g. compare respectively here^{[20]}^{:205} and here^{[21]}^{:135}).
On the other hand, several different quantities may share the same coherent SI unit. For example, the joule per kelvin (symbol J/K) is the coherent SI unit for two distinct quantities: heat capacity and entropy; another example is the ampere, which is the coherent SI unit for both electric current and magnetomotive force. This is why it is important not to use the unit alone to specify the quantity.^{[aq]}
Furthermore, the same coherent SI unit may be a base unit in one context, but a coherent derived unit in another. For example, the ampere is a base unit when it is a unit of electric current, but a coherent derived unit when it is a unit of magnetomotive force.^{[1]}^{: 140 } As perhaps a more familiar example, consider rainfall, defined as volume of rain (measured in m^{3}) that fell per unit area (measured in m^{2}). Since m^{3}/m^{2} = m, it follows that the coherent derived SI unit of rainfall is the metre, even though the metre is also the base SI unit of length.^{[ar]}
Permitted nonSI units
There is a special group of units that are called "nonSI units that are accepted for use with the SI".^{[1]}^{: 145 } See NonSI units mentioned in the SI for a full list. Most of these, in order to be converted to the corresponding SI unit, require conversion factors that are not powers of ten. Some common examples of such units are the customary units of time, namely the minute (conversion factor of 60 s/min, since 1 min = 60 s), the hour (3600 s), and the day (86400 s); the degree (for measuring plane angles, 1° = π/180 rad); and the electronvolt (a unit of energy, 1 eV = 1.602176634×10^{−19} J).
New units
The SI is intended to be an evolving system; units^{[as]} and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves.
Defining magnitudes of units
Since 2019, the magnitudes of all SI units have been defined in an abstract way, which is conceptually separated from any practical realisation of them.^{[1]}^{: 126 }^{[at]} Namely, the SI units are defined by declaring that seven defining constants^{[1]}^{: 125–9 } have certain exact numerical values when expressed in terms of their SI units. Probably the most widely known of these constants is the speed of light in vacuum, c, which in the SI by definition has the exact value of c = 299792458 m/s. The other six constants are Δν_{Cs}, the hyperfine transition frequency of caesium; h, the Planck constant; e, the elementary charge; k, the Boltzmann constant; N_{A}, the Avogadro constant; and K_{cd}, the luminous efficacy of monochromatic radiation of frequency 540×10^{12} Hz.^{[au]} The nature of the defining constants ranges from fundamental constants of nature such as c to the purely technical constant K_{cd}.^{[1]}^{: 128–9 } Prior to 2019, h, e, k, and N_{A} were not defined a priori but were rather very precisely measured quantities. In 2019, their values were fixed by definition to their best estimates at the time, ensuring continuity with previous definitions of the base units.
As far as realisations, what are believed to be the current best practical realisations of units are described in the mises en pratique,^{[av]} which are also published by the BIPM.^{[24]} The abstract nature of the definitions of units is what makes it possible to improve and change the mises en pratique as science and technology develop without having to change the actual definitions themselves.^{[ay]}
In a sense, this way of defining the SI units is no more abstract than the way derived units are traditionally defined in terms of the base units. Consider a particular derived unit, for example, the joule, the unit of energy. Its definition in terms of the base units is kg⋅m^{2}/s^{2}. Even if the practical realisations of the metre, kilogram, and second are available, a practical realisation of the joule would require some sort of reference to the underlying physical definition of work or energy—some actual physical procedure for realising the energy in the amount of one joule such that it can be compared to other instances of energy (such as the energy content of gasoline put into a car or of electricity delivered to a household).
The situation with the defining constants and all of the SI units is analogous. In fact, purely mathematically speaking, the SI units are defined as if we declared that it is the defining constant's units that are now the base units, with all other SI units being derived units. To make this clearer, first note that each defining constant can be taken as determining the magnitude of that defining constant's unit of measurement;^{[1]}^{: 128 } for example, the definition of c defines the unit m/s as 1 m/s = c/299792458 ('the speed of one metre per second is equal to one 299792458th of the speed of light'). In this way, the defining constants directly define the following seven units:
 the
 the inverse mole (mol^{−1}), a unit of a conversion constant between the amount of substance and the number of elementary entities (atoms, molecules, etc.);
 and the lumen per watt (lm/W), a unit of luminous efficacy
Further, one can show, using dimensional analysis, that every coherent SI unit (whether base or derived) can be written as a unique product of powers of the units of the SI defining constants (in complete analogy to the fact that every coherent derived SI unit can be written as a unique product of powers of the base SI units). For example, the kilogram can be written as kg = (Hz)(J⋅s)/(m/s)^{2}.^{[az]} Thus, the kilogram is defined in terms of the three defining constants Δν_{Cs}, c, and h because, on the one hand, these three defining constants respectively define the units Hz, m/s, and J⋅s,^{[ba]} while, on the other hand, the kilogram can be written in terms of these three units, namely, kg = (Hz)(J⋅s)/(m/s)^{2}.^{[bb]} While the question of how to actually realise the kilogram in practice would, at this point, still be open, that is not really different from the fact that the question of how to actually realise the joule in practice is still in principle open even once one has achieved the practical realisations of the metre, kilogram, and second.
Specifying fundamental constants vs. other methods of definition
The current way of defining the SI is the result of a decadeslong move towards increasingly abstract and idealised formulation in which the realisations of the units are separated conceptually from the definitions.^{[1]}^{: 126 }
The great advantage of doing it this way is that as science and technologies develop, new and superior realisations may be introduced without the need to redefine the units.^{[aw]} Units can now be realised with an accuracy that is ultimately limited only by the quantum structure of nature and our technical abilities but not by the definitions themselves.^{[ax]} Any valid equation of physics relating the defining constants to a unit can be used to realise the unit, thus creating opportunities for innovation... with increasing accuracy as technology proceeds.'^{[1]}^{: 122 } In practice, the CIPM Consultative Committees provide socalled "mises en pratique" (practical techniques),^{[24]} which are the descriptions of what are currently believed to be best experimental realisations of the units.^{[28]}
This system lacks the conceptual simplicity of using artefacts (referred to as prototypes) as realisations of units to define those units: with prototypes, the definition and the realisation are one and the same.^{}
In the past, there were also various other approaches to the definitions of some of the SI units. One made use of a specific physical state of a specific substance (the triple point of water, which was used in the definition of the kelvin^{[37]}^{: 113–4 }); others referred to idealised experimental prescriptions^{[1]}^{: 125 } (as in the case of the former SI definition of the ampere^{[37]}^{: 113 } and the former SI definition (originally enacted in 1979) of the candela^{[37]}^{: 115 }).
In the future, the set of defining constants used by the SI may be modified as more stable constants are found, or if it turns out that other constants can be more precisely measured.^{[bk]}
History
The original motivation for the development of the SI was the diversity of units that had sprung up within the
Adopted in 1889, use of the MKS system of units succeeded the centimetre–gram–second system of units (CGS) in commerce and engineering. The metre and kilogram system served as the basis for the development of the International System of Units (abbreviated SI), which now serves as the international standard. Because of this, the standards of the CGS system were gradually replaced with metric standards incorporated from the MKS system.^{[38]}
In 1901, Giovanni Giorgi proposed to the Associazione elettrotecnica italiana (AEI) that this system, extended with a fourth unit to be taken from the units of electromagnetism, be used as an international system.^{[39]} This system was strongly promoted by electrical engineer
The International System was published in 1960, based on the MKS units, as a result of an initiative that began in 1948.
Controlling authority
The SI is regulated and continually developed by three international organisations that were established in 1875 under the terms of the Metre Convention. They are the General Conference on Weights and Measures (CGPM^{[k]}), the International Committee for Weights and Measures (CIPM^{[ac]}), and the International Bureau of Weights and Measures (BIPM^{[ae]}). The ultimate authority rests with the CGPM, which is a plenary body through which its Member States^{[bl]} act together on matters related to measurement science and measurement standards; it usually convenes every four years.^{[12]} The CGPM elects the CIPM, which is an 18person committee of eminent scientists. The CIPM operates based on the advice of a number of its Consultative Committees, which bring together the world's experts in their specified fields as advisers on scientific and technical matters.^{[41]}^{[bm]} One of these committees is the Consultative Committee for Units (CCU), which is responsible for matters related to the development of the International System of Units (SI), preparation of successive editions of the SI brochure, and advice to the CIPM on matters concerning units of measurement.^{[42]} It is the CCU which considers in detail all new scientific and technological developments related to the definition of units and the SI. In practice, when it comes to the definition of the SI, the CGPM simply formally approves the recommendations of the CIPM, which, in turn, follows the advice of the CCU.
The CCU has the following as members:^{[43]}^{[44]} national laboratories of the Member States of the CGPM charged with establishing national standards;^{[bn]} relevant intergovernmental organisations and international bodies;^{[bo]} international commissions or committees;^{[bp]} scientific unions;^{[bq]} personal members;^{[br]} and, as an ex officio member of all Consultative Committees, the Director of the BIPM.
All the decisions and recommendations concerning units are collected in a brochure called The International System of Units (SI),^{[1]}^{[ad]} which is published by the BIPM and periodically updated.
Units and prefixes
The International System of Units consists of a set of
Derived units apply to derived quantities, which may by definition be expressed in terms of base quantities, and thus are not independent; for example,
Base units
The SI base units are the building blocks of the system and all the other units are derived from them.
Unit name  Unit symbol  Dimension symbol  Quantity name  Typical symbols  Definition 

second ^{[n 1]} 
s  T  time  The duration of 9192631770 periods of the radiation corresponding to the transition between the two caesium133 atom.
 
metre  m  L  length  , , , , , , , etc.^{[n 2]}  The distance travelled by light in a vacuum in 1/299792458 seconds. 
kilogram ^{[n 3]} 
kg  M  mass  The kilogram is defined by setting the Planck constant h exactly to 6.62607015×10^{−34} J⋅s (J = kg⋅m^{2}⋅s^{−2}), given the definitions of the metre and the second.^{[35]}  
ampere  A  I  electric current  The flow of exactly 1/1.602176634×10^{−19} times the elementary charge e per second.
Equalling approximately 6.2415090744×10^{18} elementary charges per second.  
kelvin  K  Θ  thermodynamic temperature 
The kelvin is defined by setting the fixed numerical value of the Boltzmann constant k to 1.380649×10^{−23} J⋅K^{−1}, (J = kg⋅m^{2}⋅s^{−2}), given the definition of the kilogram, the metre, and the second.  
mole  mol  N  amount of substance  The amount of substance of exactly 6.02214076×10^{23} elementary entities.^{[n 4]} This number is the fixed numerical value of the Avogadro constant, N_{A}, when expressed in the unit mol^{−1}.  
candela  cd  J  luminous intensity  The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 5.4×10^{14} hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.  

Derived units
The derived units in the SI are formed by powers, products, or quotients of the base units and are potentially unlimited in number.^{[37]}^{: 103 }^{[2]}^{: 14, 16 } Derived units are associated with derived quantities; for example, velocity is a quantity that is derived from the base quantities of time and length, and thus the SI derived unit is metre per second (symbol m/s). The dimensions of derived units can be expressed in terms of the dimensions of the base units.
Combinations of base and derived units may be used to express other derived units. For example, the SI unit of force is the newton (N), the SI unit of pressure is the pascal (Pa)—and the pascal can be defined as one newton per square metre (N/m^{2}).^{[49]}
Name  Symbol  Quantity  In SI base units  In other SI units 

radian^{[N 1]}  rad  plane angle  m/m  1 
steradian^{[N 1]}  sr  solid angle  m^{2}/m^{2}  1 
hertz  Hz  frequency  s^{−1}  
newton  N  force, weight  kg⋅m⋅s^{−2}  
pascal  Pa  stress

kg⋅m^{−1}⋅s^{−2}  N/m^{2} 
joule  J  work, heat

kg⋅m^{2}⋅s^{−2}  N⋅m = Pa⋅m^{3} 
watt  W  power, radiant flux  kg⋅m^{2}⋅s^{−3}  J/s 
coulomb  C  electric charge  s⋅A  
volt  V  electric potential, voltage, emf  kg⋅m^{2}⋅s^{−3}⋅A^{−1}  W/A = J/C 
farad  F  capacitance  kg^{−1}⋅m^{−2}⋅s^{4}⋅A^{2}  C/V = C^{2}/J 
ohm

Ω  reactance

kg⋅m^{2}⋅s^{−3}⋅A^{−2}  V/A = J⋅s/C^{2} 
siemens  S  electrical conductance

kg^{−1}⋅m^{−2}⋅s^{3}⋅A^{2}  Ω^{−1} 
weber  Wb  magnetic flux  kg⋅m^{2}⋅s^{−2}⋅A^{−1}  V⋅s 
tesla  T  magnetic flux density

kg⋅s^{−2}⋅A^{−1}  Wb/m^{2} 
henry  H  inductance  kg⋅m^{2}⋅s^{−2}⋅A^{−2}  Wb/A 
degree Celsius

°C  temperature relative to 273.15 K  K  
lumen  lm  luminous flux  cd⋅sr  cd⋅sr 
lux  lx  illuminance  cd⋅sr⋅m^{−2}  lm/m^{2} 
becquerel  Bq  activity referred to a radionuclide (decays per unit time)  s^{−1}  
gray  Gy  ionising radiation )

m^{2}⋅s^{−2}  J/kg 
sievert  Sv  ionising radiation )

m^{2}⋅s^{−2}  J/kg 
katal  kat  catalytic activity

mol⋅s^{−1}  
Notes 
Name  Symbol  Derived quantity  Typical symbol 

square metre  m^{2}  area  A 
cubic metre  m^{3}  volume  V 
metre per second  m/s  speed, velocity  v 
metre per second squared  m/s^{2}  acceleration  a 
reciprocal metre

m^{−1}  wavenumber  σ, ṽ 
vergence (optics)  V, 1/f  
kilogram per cubic metre  kg/m^{3}  density  ρ 
kilogram per square metre  kg/m^{2}  surface density

ρ_{A} 
cubic metre per kilogram  m^{3}/kg  specific volume  v 
ampere per square metre  A/m^{2}  current density  j 
ampere per metre

A/m  magnetic field strength

H 
mole per cubic metre  mol/m^{3}  concentration  c 
kilogram per cubic metre  kg/m^{3}  mass concentration  ρ, γ 
candela per square metre  cd/m^{2}  luminance  L_{v} 
Name  Symbol  Quantity  In SI base units 

pascalsecond

Pa⋅s  dynamic viscosity

m^{−1}⋅kg⋅s^{−1} 
newtonmetre  N⋅m  moment of force

m^{2}⋅kg⋅s^{−2} 
newton per metre  N/m  surface tension  kg⋅s^{−2} 
radian per second  rad/s  angular velocity, angular frequency  s^{−1} 
radian per second squared

rad/s^{2}  angular acceleration  s^{−2} 
watt per square metre

W/m^{2}  heat flux density, irradiance  kg⋅s^{−3} 
joule per kelvin  J/K  entropy, heat capacity  m^{2}⋅kg⋅s^{−2}⋅K^{−1} 
joule per kilogramkelvin  J/(kg⋅K)  specific entropy

m^{2}⋅s^{−2}⋅K^{−1} 
joule per kilogram  J/kg  specific energy  m^{2}⋅s^{−2} 
watt per metrekelvin  W/(m⋅K)  thermal conductivity  m⋅kg⋅s^{−3}⋅K^{−1} 
joule per cubic metre  J/m^{3}  energy density  m^{−1}⋅kg⋅s^{−2} 
volt per metre  V/m  electric field strength

m⋅kg⋅s^{−3}⋅A^{−1} 
coulomb per cubic metre  C/m^{3}  electric charge density

m^{−3}⋅s⋅A 
coulomb per square metre  C/m^{2}  electric displacement

m^{−2}⋅s⋅A 
farad per metre  F/m  permittivity  m^{−3}⋅kg^{−1}⋅s^{4}⋅A^{2} 
henry per metre  H/m  permeability  m⋅kg⋅s^{−2}⋅A^{−2} 
joule per mole  J/mol  molar energy  m^{2}⋅kg⋅s^{−2}⋅mol^{−1} 
joule per molekelvin  J/(mol⋅K)  molar entropy, molar heat capacity

m^{2}⋅kg⋅s^{−2}⋅K^{−1}⋅mol^{−1} 
coulomb per kilogram  C/kg  exposure (x and γrays)  kg^{−1}⋅s⋅A 
gray per second  Gy/s  absorbed dose rate

m^{2}⋅s^{−3} 
watt per steradian  W/sr  radiant intensity  m^{2}⋅kg⋅s^{−3} 
watt per square metresteradian  W/(m^{2}⋅sr)  radiance  kg⋅s^{−3} 
katal per cubic metre  kat/m^{3}  catalytic activity concentration

m^{−3}⋅s^{−1}⋅mol 
Prefixes
Prefixes are added to unit names to produce multiples and
The BIPM specifies 24 prefixes for the International System of Units (SI):
Prefix  Base 10  Decimal  English word  Adoption ^{[nb 1]}^{[52]}  

Name  Symbol  Short scale  Long scale  
quetta  Q  10^{30}  1000000000000000000000000000000  nonillion  quintillion  2022  
ronna  R  10^{27}  1000000000000000000000000000  octillion  quadrilliard  2022  
yotta  Y  10^{24}  1000000000000000000000000  septillion  quadrillion  1991  
zetta  Z  10^{21}  1000000000000000000000  sextillion  trilliard  1991  
exa  E  10^{18}  1000000000000000000  quintillion  trillion  1975  
peta  P  10^{15}  1000000000000000  quadrillion  billiard  1975  
tera  T  10^{12}  1000000000000  trillion  billion  1960  
giga  G  10^{9}  1000000000  billion  milliard  1960  
mega  M  10^{6}  1000000  million  1873  
kilo  k  10^{3}  1000  thousand  1795  
hecto  h  10^{2}  100  hundred  1795  
deca  da  10^{1}  10  ten  1795  
10^{0}  1  one  –  
deci  d  10^{−1}  0.1  tenth  1795  
centi  c  10^{−2}  0.01  hundredth  1795  
milli  m  10^{−3}  0.001  thousandth  1795  
micro  μ  10^{−6}  0.000001  millionth  1873  
nano  n  10^{−9}  0.000000001  billionth  milliardth  1960  
pico  p  10^{−12}  0.000000000001  trillionth  billionth  1960  
femto  f  10^{−15}  0.000000000000001  quadrillionth  billiardth  1964  
atto  a  10^{−18}  0.000000000000000001  quintillionth  trillionth  1964  
zepto  z  10^{−21}  0.000000000000000000001  sextillionth  trilliardth  1991  
yocto  y  10^{−24}  0.000000000000000000000001  septillionth  quadrillionth  1991  
ronto  r  10^{−27}  0.000000000000000000000000001  octillionth  quadrilliardth  2022  
quecto  q  10^{−30}  0.000000000000000000000000000001  nonillionth  quintillionth  2022  

NonSI units accepted for use with SI
Many nonSI units continue to be used in the scientific, technical, and commercial literature. Some units are deeply embedded in history and culture, and their use has not been entirely replaced by their SI alternatives. The CIPM recognised and acknowledged such traditions by compiling a list of
Some units of time, angle, and legacy nonSI units have a long history of use. Most societies have used the solar day and its nondecimal subdivisions as a basis of time and, unlike the foot or the pound, these were the same regardless of where they were being measured. The radian, being 1/2π of a revolution, has mathematical advantages but is rarely used for navigation. Further, the units used in navigation around the world are similar. The tonne, litre, and hectare were adopted by the CGPM in 1879 and have been retained as units that may be used alongside SI units, having been given unique symbols. The catalogued units are given below:
Quantity  Name  Symbol  Value in SI units 

time  minute  min  1 min = 60 s 
hour  h  1 h = 60 min = 3600 s  
day  d  1 d = 24 h = 86400 s  
length  astronomical unit  au  1 au = 149597870700 m 
plane and phase angle 
degree  °  1° = π/180 rad 
arcminute  ′  1′ = 1/60° = π/10800 rad  
arcsecond  ″  1″ = 1/60′ = π/648000 rad  
area  hectare  ha  1 ha = 1 hm^{2} = 10^{4} m^{2} 
volume  litre  l, L  1 l = 1 L = 1 dm^{3} = 10^{3} cm^{3} = 10^{−3} m^{3} 
mass  tonne (metric ton)  t  1 t = 1 Mg = 10^{3} kg 
dalton  Da  1 Da = 1.660539040(20)×10^{−27} kg  
energy  electronvolt  eV  1 eV = 1.602176634×10^{−19} J 
logarithmic ratio quantities 
neper  Np  In using these units it is important that the nature of the quantity be specified and that any reference value used be specified. 
bel  B  
decibel  dB 
These units are used in combination with SI units in common units such as the kilowatthour (1 kW⋅h = 3.6 MJ).
Common notions of the metric units
The basic units of the metric system, as originally defined, represented common quantities or relationships in nature. They still do – the modern precisely defined quantities are refinements of definition and methodology, but still with the same magnitudes. In cases where laboratory precision may not be required or available, or where approximations are good enough, the original definitions may suffice.^{[bu]}
 A second is 1/60 of a minute, which is 1/60 of an hour, which is 1/24 of a day, so a second is 1/86400 of a day (the use of base 60 dates back to Babylonian times); a second is the time it takes a dense object to freely fall 4.9 metres from rest.^{[bv]}
 The length of the equator is close to 40000000 m (more precisely 40075014.2 m).^{[53]} In fact, the dimensions of our planet were used by the French Academy in the original definition of the metre.^{[54]}
 The metre is close to the length of a pendulum that has a period of 2 seconds;^{[bw]} most dining tabletops are about 0.75 metres high;^{[55]} a very tall human (basketball forward) is about 2 metres tall.^{[56]}
 The kilogram is the mass of a litre of cold water; a cubic centimetre or millilitre of water has a mass of one gram; a 1euro coin weighs 7.5 g;^{[57]} a Sacagawea US 1dollar coin weighs 8.1 g;^{[58]} a UK 50pence coin weighs 8.0 g.^{[59]}
 A candela is about the luminous intensity of a moderately bright candle, or 1 candle power; a 60 W tungstenfilament incandescent light bulb has a luminous intensity of about 64 candelas.^{[bx]}
 A mole of a substance has a mass that is its molecular mass expressed in units of grams; the mass of a mole of carbon is 12.0 g, and the mass of a mole of table salt is 58.4 g.
 Since all gases have the same volume per mole at a given temperature and pressure far from their points of liquefaction and solidification (see Perfect gas), and air is about 1/5 oxygen (molecular mass 32) and 4/5 nitrogen (molecular mass 28), the density of any nearperfect gas relative to air can be obtained to a good approximation by dividing its molecular mass by 29 (because 4/5 × 28 + 1/5 × 32 = 28.8 ≈ 29). For example, carbon monoxide (molecular mass 28) has almost the same density as air.
 A temperature difference of one kelvin is the same as one degree Celsius: 1/100 of the temperature differential between the freezing and boiling points of water at sea level; the absolute temperature in kelvins is the temperature in degrees Celsius plus about 273; human body temperature is about 37 °C or 310 K.
 A 60 W incandescent light bulb rated at 120 V (US mains voltage) consumes 0.5 A at this voltage. A 60 W bulb rated at 230 V (European mains voltage) consumes 0.26 A at this voltage.^{[by]}
Lexicographic conventions
Unit names
According to the SI Brochure,^{}
The English spelling and even names for certain SI units and metric prefixes depend on the variety of English used.
Unit symbols and the values of quantities
Symbols of SI units are intended to be unique and universal, independent of the context language.^{}[37]^{: 130–135 } The SI Brochure has specific rules for writing them.^{[37]}^{: 130–135 } The guideline produced by the National Institute of Standards and Technology (NIST)^{[61]} clarifies languagespecific details for American English that were left unclear by the SI Brochure, but is otherwise identical to the SI Brochure.^{[62]}
General rules
General rules^{[ca]} for writing SI units and quantities apply to text that is either handwritten or produced using an automated process:
 The value of a quantity is written as a number followed by a space (representing a multiplication sign) and a unit symbol; e.g., 2.21 kg, 7.3×10^{2} m^{2}, 22 K. This rule explicitly includes the percent sign (%)^{[37]}^{: 134 } and the symbol for degrees Celsius (°C).^{[37]}^{: 133 } Exceptions are the symbols for plane angular degrees, minutes, and seconds (°, ′, and ″, respectively), which are placed immediately after the number with no intervening space.
 Symbols are mathematical entities, not abbreviations, and as such do not have an appended period/full stop (.), unless the rules of grammar demand one for another reason, such as denoting the end of a sentence.
 A prefix is part of the unit, and its symbol is prepended to a unit symbol without a separator (e.g., k in km, M in MPa, G in GHz, μ in μg). Compound prefixes are not allowed. A prefixed unit is atomic in expressions (e.g., km^{2} is equivalent to (km)^{2}).
 Unit symbols are written using roman (upright) type, regardless of the type used in the surrounding text.
 Symbols for derived units formed by multiplication are joined with a centre dot(⋅) or a nonbreaking space; e.g., N⋅m or N m.
 Symbols for derived units formed by division are joined with a exponent. E.g., the "metre per second" can be written m/s, m s^{−1}, m⋅s^{−1}, or m/s. In cases where a solidus is followed by a centre dot (or space), or more than one solidus is present, parentheses must be used to avoid ambiguity; e.g., kg/(m⋅s^{2}), kg⋅m^{−1}⋅s^{−2}, and (kg/m)/s^{2} are acceptable, but kg/m/s^{2} and kg/m⋅s^{2} are ambiguous and unacceptable.
 The first letter of symbols for units derived from the name of a person is written in may exceptionally be written using either an uppercase "L" or a lowercase "l", a decision prompted by the similarity of the lowercase letter "l" to the numeral "1", especially with certain typefaces or Englishstyle handwriting. The American NIST recommends that within the United States "L" be used rather than "l".
 Symbols do not have a plural form, e.g., 25 kg, not 25 kgs.
 Uppercase and lowercase prefixes are not interchangeable. E.g., the quantities 1 mW and 1 MW represent two different quantities (milliwatt and megawatt).
 The symbol for the decimal marker is either a point or comma on the line. In practice, the decimal point is used in most Englishspeaking countries and most of Asia, and the comma in most of Latin America and in continental European countries.^{[63]}
 Spaces should be used as a thousands separator(1000000) in contrast to commas or periods (1,000,000 or 1.000.000) to reduce confusion resulting from the variation between these forms in different countries.
 Any linebreak inside a number, inside a compound unit, or between number and unit should be avoided. Where this is not possible, line breaks should coincide with thousands separators.
 Because the value of "billion" and "trillion" varies between languages, the dimensionless terms "ppb" (parts per billion) and "ppt" (parts per trillion) should be avoided. The SI Brochure does not suggest alternatives.
Printing SI symbols
The rules covering printing of quantities and units are part of ISO 800001:2009.^{[64]}
Further rules^{[ca]} are specified in respect of production of text using printing presses, word processors, typewriters, and the like.
International System of Quantities
 SI Brochure
The CGPM publishes a brochure that defines and presents the SI.^{}[37] Its official version is in French, in line with the Metre Convention.^{[37]}^{: 102 } It leaves some scope for local variations, particularly regarding unit names and terms in different languages.^{[cb]}^{[2]}
The writing and maintenance of the CGPM brochure is carried out by one of the committees of the
The quantities and equations that provide the context in which the SI units are defined are now referred to as the International System of Quantities (ISQ). The ISQ is based on the