# International System of Units

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The International System of Units, internationally known by the abbreviation SI (for Système International),[lower-alpha 1][1]:125[2]:iii[3] [lower-alpha 2] is the modern form[1]:117[6][7] of the metric system[lower-alpha 7] 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[lower-alpha 10] (CGPM[lower-alpha 11]), it is the only system of measurement with an official status[lower-alpha 13] in nearly every country in the world,[lower-alpha 14] employed in science, technology, industry, and everyday commerce.

Table info: Symbol, Name, Quantity, s, m...
SI base units
Symbol Name Quantity
s secondtime
m metrelength
kg kilogrammass
A ampereelectric current
K kelvinthermodynamic temperature
mol moleamount of substance
cd candelaluminous intensity
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The SI comprises a coherent[lower-alpha 15] 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.[lower-alpha 16] Twenty-two coherent derived units have been provided with special names and symbols.[lower-alpha 17]

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.[lower-alpha 18] 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[lower-alpha 19] 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.[lower-alpha 20][lower-alpha 21] 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.

Table info: Symbol, Defining constant, Exact value, ΔνCs,...
SI defining constants
Symbol Defining constant Exact value
ΔνCs hyperfine transition frequency of Cs9192631770 Hz
c speed of light299792458 m/s
h Planck constant6.62607015×10−34 Js
e elementary charge1.602176634×10−19 C
k Boltzmann constant1.380649×10−23 J/K