The International System of Units, internationally known by the abbreviation SI (from French Système international d'unités), is the modern form of the metric system and the world's most widely used system of measurement. Coordinated by the International Bureau of Weights and Measures (abbreviated BIPM from French: Bureau international des poids et mesures) 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.

SI base units

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 SI comprises a coherent system of units of measurement starting with seven base units, which are the second (symbol s, the unit of time), metre (m, length), kilogram (kg, mass), ampere (A, electric current), kelvin (K, thermodynamic temperature), mole (mol, amount of substance), and candela (cd, luminous intensity). The system can accommodate coherent units for an unlimited number of additional quantities. These are called coherent derived units, which can always be represented as products of powers of the base units. Twenty-two coherent derived units have been provided with special names and symbols.

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. Since the sizes of coherent units will be convenient for only some applications and not for others, the SI provides twenty-four prefixes which, when added to the name and symbol of a coherent unit produce twenty-four additional (non-coherent) SI units for the same quantity; these non-coherent units are always decimal (i.e. power-of-ten) multiples and sub-multiples of the coherent unit.

The current way of defining the SI is a result of a decades-long 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 original motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second (CGS) systems (specifically the inconsistency between the systems of electrostatic units and electromagnetic units) and the lack of coordination between the various disciplines that used them. The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presenting measurements. The system was published in 1960 as a result of an initiative that began in 1948, and is based on the metre–kilogram–second system of units (MKS) combined with ideas from the development of the CGS system.

Definition

The International System of Units consists of a set of defining constants with corresponding base units, derived units, and a set of decimal-based multipliers that are used as prefixes.^[1]: 125

SI defining constants

SI defining constants

Symbol	Defining constant	Exact value
ΔνCs	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
NA	Avogadro constant	6.02214076×1023 mol−1
Kcd	luminous efficacy of 540 THz radiation	683 lm/W

The seven defining constants are the most fundamental feature of the definition of the system of units.^[1]: 125 The magnitudes of all SI units are defined by declaring that seven 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. The values assigned to these constants were fixed to ensure continuity with previous definitions of the base units.^[1]: 128

SI base units

The SI selects seven units to serve as base units, corresponding to seven base physical quantities. 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] The base units are defined in terms of the defining constants. For example, the kilogram is defined by taking the Planck constant h to be 6.62607015×10⁻³⁴ J s, giving the expression in terms of the defining constants^[1]: 131

1 kg = (299792458)²/(6.62607015×10⁻³⁴)(9192631770)h Δν_Cs/c².

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. The choice of which and even how many quantities to use as base quantities is not fundamental or even unique – it is a matter of convention.^[1]: 126

SI base units^[1]: 136

Unit name	Unit symbol	Dimension symbol	Quantity name	Typical symbols	Definition
second	s	${\displaystyle {\mathsf {T}}}$	time	${\displaystyle t}$	The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
metre	m	${\displaystyle {\mathsf {L}}}$	length	${\displaystyle l}$, ${\displaystyle x}$, ${\displaystyle r}$, etc.	The distance travelled by light in vacuum in 1/299792458 second.
kilogram [n 1]	kg	${\displaystyle {\mathsf {M}}}$	mass	${\displaystyle m}$	The kilogram is defined by setting the Planck constant h to 6.62607015×10−34 J⋅s (J = kg⋅m2⋅s−2), given the definitions of the metre and the second.[2]
ampere	A	${\displaystyle {\mathsf {I}}}$	electric current	${\displaystyle I,\;i}$	The flow of 1/1.602176634×10−19 times the elementary charge e per second, which is approximately 6.2415090744×1018 elementary charges per second.
kelvin	K	${\displaystyle {\mathsf {\Theta }}}$	thermodynamic temperature	${\displaystyle T}$	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⋅m2⋅s−2), given the definition of the kilogram, the metre, and the second.
mole	mol	${\displaystyle {\mathsf {N}}}$	amount of substance	${\displaystyle n}$	The amount of substance of 6.02214076×1023 elementary entities.[n 2] This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol−1.
candela	cd	${\displaystyle {\mathsf {J}}}$	luminous intensity	${\displaystyle I_{\rm {v}}}$	The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 5.4×1014 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
Notes ^ Despite the prefix "kilo-", the kilogram is the coherent base unit of mass, and is used in the definitions of derived units. Nonetheless, prefixes for the unit of mass are determined as if the gram were the base unit. ^ When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

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. For example, the coherent derived SI unit unit of velocity is the metre per second, with the symbol m/s.^[1]: 139 The base and coherent derived units of the SI together form a coherent system of units (the set of coherent SI units). A useful property of a coherent system is that when the numerical values of physical quantities are expressed in terms of the units of the system, then the equations between the numerical values have exactly the same form, including numerical factors, as the corresponding equations between the physical quantities.^[3]: 6

Twenty-two coherent derived units have been provided with special names and symbols as shown in the table below. The radian and steradian have no base units but are treated as derived units for historical reasons.^[1]: 137

Zdroj:https://en.wikipedia.org?pojem=International_System_of_Units
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The 22 SI derived units with special names and symbols^[4]: 15

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	m2/m2	1
hertz	Hz	frequency	s−1
newton	N	force, weight	kg⋅m⋅s−2
pascal	Pa	pressure, stress	kg⋅m−1⋅s−2	N/m2 = J/m3
joule	J	energy, work, heat	kg⋅m2⋅s−2	N⋅m = Pa⋅m3
watt	W	power, radiant flux	kg⋅m2⋅s−3	J/s
coulomb	C	electric charge	s⋅A
volt	V	electric potential, voltage, emf	kg⋅m2⋅s−3⋅A−1	W/A = J/C
farad	F	capacitance	kg−1⋅m−2⋅s4⋅A2	C/V = C2/J
ohm	Ω	resistance, impedance, reactance	kg⋅m2⋅s−3⋅A−2