Units of the International System

UNITS OF THE INTERNATIONAL SYSTEM

The International System (abbreviated SI, for Systeme International, the French name for the system) was adopted in 1960 by the 11^th General Conference on Weights and Measures. An expanded and modified version of the metric system, the International System, addresses the need of modern science for additional and more accurate units of measurement. The key features of the International System are decimalization, system of prefixes and a standard defined in terms of an invariable physical measure.

BASE UNITS

The International System has base units from which all others in the system are derived. The standards for the base units, except for the kilogram, are defined by unchanging and reproducible physical occurrences. For example, the meter is defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second. The standard for the kilogram is a platinum-iridium cylinder kept at the International Bureau of Weights and Standards in Sèvres, France.

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 UNITS

The International System uses two supplementary units that are based on abstract geometrical concepts rather than physical standards.

UNIT	QUANTITY	SYMBOL
radian	plane angles	rad
steradian	solid angles	sr

DERIVED UNITS

Most of the units in the International System are a derived unit, that is units defined in terms of base units and supplementary units. Derived units can be divided into two groups – those that have a special name and symbol, and those that do not.

WITH 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

PREFIXES

A multiple of unit in the International System is formed by adding a prefix to the name of that unit. The prefixes change the magnitude of the unit by orders of 10 from_10¹⁸_to_10^-18. For example, a kilometer is 1000 meters, a hectoliter is 100 liters, a milligram is 1/1000 of a gram, a megawatt is 1,000,000 watt, etc.

PREFIX	SYMBOL	MULTIPLYING FACTOR
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 = 100
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

ADDITIONAL UNITS

Listed below are a few of the non-SI units that are commonly used with the International System:

UNIT	QUANITY	SYMBOL
angstrom (=10.10m)	length	Å
electron-volt (=0.160 aJ)	energy	eV
hectare (=10,000 m²)	land area	ha
liter (= 1.0 dm³)	volume or capacity	l
standard atmosphere (=101.3 kPa)	pressure	atm

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	fource	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