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# The SI System

Any physical quantity is the product of a numerical value and a unit. Since 1964, the International System of Units (SI system) has gradually been adopted worldwide, with the exception of Liberia, Myanmar and the United States.

Basic information can be found in standard ISO 31, which is under revision and will be superseded by ISO/IEC 80000: Quantities and Units. Units are divided into four different classes :

• Base units
• Supplementary units
• Derived units

Base units, supplementary units and derived units are called SI units. The additional units are not SI units, although they are accepted for use with SI units. Base units are any of the established, independent units in which all other units can be expressed.

 Length meter m Mass kilogram kilogram kg Time second s Electrical current ampere A Temperature kelvin K Luminous intensity candela cd Amount of substance mole mol

Derived units are formed as a power or product of powers of one or more base units and/or supplementary units according to the physical laws for the relationship between these different units. Additional units: A limited number of units outside the SI system cannot be eliminated for different reasons, and continue to be used along with the SI as additional units.

 Quantity Unit Symbol Expressed in other SI units frequency hertz Hz s^-1 force newton N kg x m x s^-2 pressure / mechanical stress pascal Pa N / m² energy / work joule J N x m power watt W J / s electric quantity / charge coulomb C A x s electric voltage volt V W / A capacitance farad F C / V resistance ohm Ω V / A conductivity siemens S A / V magnetic flux weber Wb V x s magnetic flux density tesla T Wb / m² inductance henry H Wb / A luminous flux lumen lm Cd x sr light lux lx Lm / m² angle radian rad m / m solid angle steradian sr m² / m²
 Quantity Unit Symbol Remark volume liter l 1 l = 1 dm³ time minute min 1 min = 60 s time hour h 1 h = 60 min mass metric ton t 1 t = 1.000 kg pressure bar bar 1 bar = 105 Pa plane angle degree .° 1° = π/180 rad plane angle minute .' 1' = 1°/60 plane angle second ." 1" = 1'/60

Prefixes may be added to a unit to produce a multiple of the original unit. All of these multiples are integer powers of ten, for example: kilo-denotes a multiple of thousand (10³) milli-denotes a multiple of one thousandth (10-3)

 Power Prefix Designation Prefix Symbol Example Symbol 1012 tera T 1 terajoule 1 TJ 109 giga G 1 gigahertz 1 GHz 106 mega M 1 megawatt 1 MW 103 kilo k 1 kilometer 1 km 102 hecto h 1 hectoliter 1 hl 101 deca da 1 decalumen 1 dalm 10-1 deci d 1 decibel 1 dB 10-2 centi c 1 centimeter 1 cm 10-3 milli m 1 milligram 1 mg 10-6 micro µ 1 micrometer 1 µm 10-9 nano N 1 nanohenry 1 nH 10-12 pico p 1 picofarad 1 pF 10-15 femto f 1 femtometer 1 fm 10-18 atto a 1 attosecond 1 as

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