The International System of Units is abbreviated “SI” from the French version of the name. It is the modern metric system of measurement and it’s the most widely used system in everyday commerce and in science. The SI is maintained by a small agency in Paris, the International Bureau of Weights and Measures. The older metric system included several groups of units and was formerly called the meter-kilogram-second (MKS) system. The key features of the International System are a system of prefixes, decimalization, and a standard defined in terms of an invariable physical measure.

Base Units

The International System has seven base units from which all other units in the system are derived.

Unit	Quantity	Symbol
meter	length	m
kilogram	mass	kg
second	time	s
ampere	electric current	A
kelvin	temperature	K
mole	amount of matter	mol
candela	luminous intensity	cd

Supplementary Dimensionless Units

The International System uses two supplementary dimensionless units that are based on abstract geometrical concepts rather than physical standards. They consist of the radian and steradian.

Unit	Quantity	Symbol
radian	plane angles	rad
steradian	solid angles	sr

Prefixes

A multiple of a unit in the International System is formed by adding a prefix to the name of that unit. The prefixes are used to change the magnitude of the unit by orders of ten from 10²⁴ to 10^-24

Prefix	Symbol	Multiplying Factor
yotta-	Y	1024 = 1,000,000,000,000,000,000,000,000
zetta-	Z	1021 = 1,000,000,000,000,000,000,000
exa-	E	1018 = 1,000,000,000,000,000,000
peta-	P	1015 = 1,000,000,000,000,000
tera-	T	1012 = 1,000,000,000,000
giga-	G	109 = 1,000,000,000
mega-	M	106 = 1,000,000
kilo-	K	103 = 1,000
hecto-	h	102 = 1000
deca-	da	10 = 10
deci-	d	10-1 = 0.1
centi-	c	10-2 = 0.01
milli-	m	10-3 = 0.001
micro-	µ	10-6 = 0.000,001
nano-	n	10-9 = 0.000,000,001
pico-	p	10-12 = 0.000,000,000,001
femto-	f	10-15 = 0.000,000,000,000,001
atto-	a	10-18 = 0.000,000,000,000,000,001
zepto-	z	10-21 = 0.000,000,000,000,000,000,001
yocto-	y	10-24 = 0.000,000,000,000,000,000,000,001

Additional Units

Other units outside the International System that are currently accepted, subject to further review.

Unit	Quantity	Symbol
angstrom (= 10-10m)	length	Å
electron-volt (= 0.160 aJ)	energy	eV
hectare (= 10,000 m2)	land area	ha
liter (= 1.0dm3)	volume or capacity	l
standard atmosphere (= 101.3 kPa)	pressure	atm

Derived Units

Most of the units in the International System are actually derived units. In the below tables are examples of derived units containing names and symbols and those without names and symbols.

With Names and Symbols Unit Measure of Symbol Derivation coulomb electric charge C A·s farad electric capacitance F A·s/V henry inductance H V·s/A hertz frequency Hz cycles/s joule quantity of energy J N·m lumen flux of light lm cd·sr lux illumination lx lm/m2 newton force N kg·m/s2 ohm electric resistance Ω V/A pascal pressure Pa N/m2 tesla magnetic flux density T Wb/m2 volt voltage V W/A watt power W J/s weber magnetic flux Wb V·s

Without Names and Symbols Measure of Derivation acceleration m/s2 angular acceleration rad/s2 angular velocity rad/s density kg/m3 electric field strength V/m luminance cd/m2 magnetic field strength A/m velocity m/s

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