Units and Prefixes of the International System of Units (SI)

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SI units
SI prefixes
Units outside the SI
Use the International System of Units

(US) The United States uses different names for certain units and prefixes.

International Bureau of Weights and Measures (BIPM): SI
United States: National Institute of Standards and Technology: SI

SI units

SI base units

SI base unit
			Base quantity
m	metre; US: meter
kg	kilogram
s	second
A	ampere
			electrical current
K	kelvin
			thermodynamic temperature
mol	mole
			amount of substance
cd	candela
			luminous intensity

The mass is commonly called the weight in people's daily life and trade.
 (The word weight denotes a quantity of the same nature as a force.)

Examples of SI derived units expressed in terms of SI base units

SI derived unit
Symbol Name
					Derived quantity
m2     square metre
m3     cubic metre
m/s    metre per second
					speed, velocity
m/s2   metre per second squared
m-1    reciprocal metre
kg/m3  kilogram per cubic metre
					density, mass density
m3/kg  cubic metre per kilogram
					specific volume
A/m2   ampere per square metre
					current density
A/m    ampere per metre
					magnetic field strength
mol/m3 mole per cubic metre
					concentration (of amount of substance)
cd/m2  candela per square metre
1      (the number) one
					refractive index
m3/s   cubic metre per second
					volume flow rate
kg/s   kilogram per second
					mass flow rate
m2/s   square metre per second
					kinematic viscosity
m3     metre to the power of three
					section modulus
m3/mol cubic metre per mole
					molar volume

SI derived units with special names and symbols

SI derived unit
			Derived quantity
rad	radian
			plane angle
sr	steradian
			solid angle
Hz	hertz
N	newton
Pa	pascal
J	joule
			quantity of heat
W	watt
			radiant flux
C	coulomb
			electric charge,
			quantity of electricity
V	volt
			electric potential difference,
			electromotive force
F	farad
Ω	ohm
			electric resistance
S	siemens
			electric conductance
Wb	weber
			magnetic flux
T	tesla
			magnetic flux density
H	henry
°C	degree Celsius
			Celsius temperature
lm	lumen
			luminous flux
lx	lux
Bq	becquerel
			activity (referred to a radionuclide)
Gy	gray
			absorbed dose,
			specific energy (imparted),
Sv	sievert
			dose equivalent,
			ambient dose equivalent,
			directional dose equivalent,
			personal dose equivalent,
			organ equivalent dose
kat	katal
			catalytic activity

Examples of SI derived units whose names and symbols include SI derived units with special names and symbols

SI derived unit
Symbol      Name
					Derived quantity
Pa . s      pascal second
					dynamic viscosity
N . m       newton metre
					moment of force
N/m         newton per metre
					surface tension
rad/s       radian per second
					angular velocity
rad/s2      radian per second squared
					angular acceleration
W/m2        watt per square metre
					heat flux density,
J/K         joule per kelvin
					heat capacity,
J/(kg . K)  joule per kilogram kelvin
					specific heat capacity,
					specific entropy
J/kg        joule per kilogram
					specific energy
W/(m . K)   watt per metre kelvin
					thermal conductivity
J/m3        joule per cubic metre
					energy density
V/m         volt per metre
					electric field strength
C/m3        coulomb per cubic metre
					electric charge density
C/m2        coulomb per square metre
					electric flux density
F/m         farad per metre
H/m         henry per metre
J/mol       joule per mole
					molar energy
J/(mol . K) joule per mole kelvin
					molar entropy,
					molar heat capacity
C/kg        coulomb per kilogram
					exposure (x and γ rays)	
Gy/s        gray per second
					absorbed dose rate
W/sr        watt per steradian
					radiant intensity
W/(m2 . sr) watt per square metre steradian
kat/m3      katal per cubic metre
					catalytic (activity) concentration
C/(kg . s)  coulomb per kilogram second
					exposure rate
Sv/s        sievert per second
					dose equivalent rate
S/m         siemens per metre
					electrical conductivity
S . m2/mol  siemens square metre per mole
					molar conductivity

SI prefixes


1024	Y	yotta
1021	Z	zetta
1018	E	exa
1015	P	peta
1012	T	tera
109	G	giga
106	M	mega
103	k	kilo
102	h	hecto
101	da	deca; US: deka
10-1	d	deci
10-2	c	centi
10-3	m	milli
10-6	µ	micro
10-9	n	nano
10-12	p	pico
10-15	f	femto
10-18	a	atto
10-21	z	zepto
10-24	y	yocto

Units outside the SI


Non-SI units accepted for use with the International System

min	minute
h	hour
d	day
°	degree
'	minute
"	second
l, L	litre; US: liter
t	tonne; US: metric ton
Np	neper
B	bel
dB	decibel

Non-SI units accepted for use with the SI, whose values in SI units are obtained experimentally

eV	electronvolt
u	unified atomic mass unit
ua	astronomical unit

Other non-SI units currently accepted for use with the International System

	nautical mile
a	are
ha	hectare
bar	bar
Å	ångstöm
b	barn

Other non-SI units

erg	erg
dyn	dyne
P	poise
St	stokes
G	gauss
Oe	oersted
Mx	maxwell
sb	stilb
ph	phot
Gal	gal
Ci	curie
R	röntgen
rad	rad
rem	rem
	X unit
γ	gamma
Jy	jansky
	metric carat
Torr	torr
atm	standard atmosphere
cal	calorie
µ	micron

Use the International System of Units

The International System of Units (SI) is the modern metric system and adopted by the International Organization for Standardization (ISO) as International Standard ISO 1000.

The United States signed the Convention of the Metre (Meter Convention in American English) in 1875, but limited use of the SI and overuse of the inch-pound units cause obstacles. The United States is not yet seriously committed to metrication and some resistances exist. The International System of Units is the only official name of the modern metric system, but the inch-pound units are called variously. Paul TRUSTEN, a registered pharmacist in Texas in the United States, has devised an acronym WOMBAT (Way Of Measuring Badly in America Today) for the inch-pound units and the like. Another interpretation not from him is Waste Of Money, Brain, And Time.

The United Kingdom is widely metricated, but resistance exists and metric road signs are usually illegal. Metric traffic signs in the United States are legal but not required unless a state law says otherwise, though the inch-pound units still prevail. Canada is mostly metricated, but its metrication is incomplete in a few cases.

The SI is easy to use, but many people in the United States and the United Kingdom claim that they do not understand the SI. Most people know the decimal relations of common units of the SI, but even people against the SI may not know the chaotic relations of the inch-pound units. They may not know how many yards or feet in a mile, how many acres in a square mile, how many cubic inches in a gallon, or how many pounds in a ton.

Some people are hostile toward the SI. Many news media against the SI convert original values of the SI to the inch-pound units. Some people attempt to smuggle the inch-pound units to countries and areas where they are illegal.

Globalization requires the SI as the uniform system. People against the SI who claim denial of freedom of choice must consider how hard it is to handle the inch-pound units. Freedom must have a limit or conflicts will occur.

External links

FR France: Louis JOURDAN: La grande métrication

GB United Kingdom (UK):
UK Metric Association
KryssTal: The Metric System
A Dictionary of Measures, Units and Conversions by Frank TAPSON

US United States:
U.S. Metric Association
SI Navigator
James R. FRYSINGER: SI Guide
Metric 4 US
Tom J. PRICE: Metric, Once and For All
Russ ROWLETT: A Dictionary of Units of Measurement
Make the Metric System Standard In the USA Petition
Yahoo! Groups: metricamerica
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