SI units

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SI units Systéme International d'Unités: the international system of units now recommended for all scientific purposes. A coherent and rationalized system of units derived from the m.k.s. units (a metric system based on the metre, kilogram, and second), SI units have now replaced c.g.s. units and Imperial units. The system has seven base units and two dimensionless units (formerly called supplementary units), all other units being derived from these nine units. There are 18 derived units with special names. Each unit has an agreed symbol (a capital letter or an initial capital letter if it is named after a scientist, otherwise the symbol consists of one or two lower-case letters). Decimal multiples of the units are indicated by a set of prefixes; when possible a prefix representing 10 raised to a power that is a multiple of three should be used.

TABLE 1.1 Base and dimensionless SI units

Physical quantity	Name	Symbol
*dimensionless units
length	metre	m
mass	kilogram	kg
time	second	s
electric current	ampere	A
thermodynamic temperature	kelvin	K
luminous intensity	candela	cd
amount of substance	mole	mol
*plane angle	radian	rad
*solid angle	steradian	sr

TABLE 1.2 Derived SI units with special names

Physical quantity	Name of SI unit	Symbol of SI unit
frequency	hertz	Hz
energy	joule	J
force	newton	N
power	watt	W
pressure	pascal	Pa
electric	charge	coulomb	C
electric potential difference	volt	V
electric resistance	ohm	Ω
electric conductance	siemens	S
electric capacitance	farad	F
magnetic flux	weber	Wb
inductance	henry	H
magnetic flux density (magnetic induction)	tesla	T
luminous flux	lumen	lm
illuminance	lux	lx
absorbed dose	gray	Gy
activity	becquerel	Bq
dose equivalent	sievert	Sv

TABLE 1.3 Decimal multiples and submultiples to be used with SI units

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

TABLE 1.4 Conversion of units of SI units

From	To	Multiply by
in	m	2.54 × 10⁻²
ft	m	0.3048
sq. in	m²	6.4516 × 10⁻⁴
sq. ft	m²	9.2903 × 10⁻²
cu. in	m³	1.63871 × 10⁻⁵
cu. ft	m³	2.83168 × 10⁻²
l(itre)	m³	10⁻³
gal(lon)	l(itre)	4.546 09
miles/hr	m s⁻¹	0.477 04
km/hr	m s⁻¹	0.277 78
lb	kg	0.453 592
g cm⁻³	kg m⁻³	10³
lb/in³	kg m⁻³	2.767 99 × 10⁴
dyne	N	10⁻⁵
poundal	N	0.138 255
lbf	N	4.448 22
mmHg	Pa	133.322
atmosphere	Pa	1.013 25 × 10⁵
hp	W	745.7
erg	J	10⁻⁷
eV	J	1.602 10 × 10⁻¹⁹
kW h	J	3.6 × 10⁶
cal	J	4.1868

A Dictionary of Biology

SI units

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SI units (Système International d'Unités) pl. n. the internationally agreed system of units, based on the metre-kilogram-second system, now in use for all scientific purposes.

Table 11.1 Base and supplementary SI units

Physical quantity	Name of unit	Symbol for unit
length	metre	m
mass	kilogram	kg
time	second	s
electric current	ampere	A
thermodynamic temperature	kelvin	K
luminous intensity	candela	cd
amount of substance	mole	mol
*plane angle	radian	rad
*solid angle	steradian	sr
*supplementary units

Table 11.2 Derived SI units with special names

Physical quantity	Name of unit	Symbol for unit
frequency	hertz	Hz
energy	joule	J
force	newton	N
power	watt	W
pressure	pascal	Pa
electric charge	coulomb	C
electric potential difference	volt	V
electric resistance	ohm	Ω
electric conductance	siemens	S
electric capacitance	farad	F
magnetic flux	weber	Wb
inductance	henry	H
magnetic flux density (magnetic induction)	tesla	T
luminous flux	lumen	lm
illuminance (illumination)	lux	lx
absorbed dose	gray	Gy
activity	becquerel	Bq
dose equivalent	sievert	Sv

Table 11.3 Decimal multiples and submultiples to be used with SI units

Submultiple	Prefix	Symbol	Multiple	Prefix	Symbol
10⁻¹	deci	d	10¹	deca	da
10⁻²	centi	c	10²	hecto	h
10⁻³	milli	m	10³	kilo	k
10⁻⁶	micro	μ	10⁶	mega	M
10⁻⁹	nano	n	10⁹	giga	G
10⁻¹²	pico	p	10¹²	tera	T
10⁻¹⁵	femto	f	10¹⁵	peta	P
10⁻¹⁶	atto	a	10¹⁶	exa	E
10⁻²¹	zepto	z	10²¹	zetta	Z
10⁻²⁴	yocto	y	10²⁴	yotta	Y

Table 11.4 Conversion of units to and from SI units

From	To	Multiply by
Temperature conversion
°C (Celsius) = 5/9(°F − 32)
°F (Fahrenheit) = (9/5 × °C) + 32
in	m	0.0254
ft	m	0.3048
sq in	m²	0.00064516
sq ft	m²	0.092903
cu in	m³	0.0000164
cu ft	m³	0.0283168
l(itre)	m³	0.001
gal(lon)	m³	0.0045609
gal(lon)	litres	4.5609
lb	kg	0.453592
g cm⁻³	kg m⁻³	1000
lb/in³	kg m⁻³	27679.9
mmHg	Pa	133.322
cal	J	4.1868
m	in	39.3701
cm	in	0.393701
cm²	sq in	0.155
m²	sq in	1550
m²	sq ft	10.7639
m³	cu in	61023.6
m³	cu ft	35.3146
m³	l(itre)	1000
m³	gal(lon)	219.969
kg	lb	2.20462
kg m⁻³	g cm⁻³	0.001
kg m⁻³	lb/in³	0.0000363
Pa	mmHg	0.0075006
J	cal	0.238846

A Dictionary of Nursing

SI units

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SI units (Système International d'Unités)

Quantity	Name of unit	Symbol	Equivalent	Reciprocal
length	metre	m	3.281 feet	1 ft = 0.3048 m
mass	kilogram	kg	2.2 pounds	1 lb = 0.454 kg
time	second	s
electric current	ampere	A
thermodynamic temperature	kelvin	K	1°C = 1.8°F	1°C = 1 K
luminous intensity	candela	cd
amount of substance	mole	mol

Supplementary units

Quantity	Unit	Symbol
plane angle	radian	rad
solid angle	steradian	sr

Derived SI units

Quantity	Name of unit	Symbol	Equivalent	Reciprocal
frequency	hertz	Hz
energy	joule	J	0.2388 calories	1 cal = 4.1868 J
force	newton	N	0.225 pounds force	1 lbf = 4.448 N
power	watt	W	0.00134 horse power	1 hp = 745.7 W
pressure	pascal	Pa	0.00689 pounds force/sq.inch	1 lbf/sq.in = 145 Pa
electric charge	coulomb	C
electric potential difference	volt	V
electric resistance	ohm	Ω
electric conductance	siemens	S
electric capacitance	farad	F
magnetic flux	weber	Wb
inductance	henry	H
magnetic flux density	tesla	T
luminous flux	lumen	lm
illuminance	lux	lx
absorbed dose	gray	Gy
activity	becquerel	Bq
dose equivalent	sievert	Sv

Multiples used with SI units

Name of multiple	Symbol	Value (multiply by)
atto	a	10⁻¹⁸
femto	f	10⁻¹⁵
pico	p	10⁻¹²
nano	n	10⁻⁹
micro	μ	10⁻⁶
milli	m	10⁻³
centi	c	10⁻²
deci	d	10⁻¹
deca	da	10
hecto	h	10²
kilo	k	10³
mega	M	10⁶
giga	G	10⁹
tera	T	10¹²
peta	P	10¹⁵
exa	E	10¹⁸

A Dictionary of Plant Sciences MICHAEL ALLABY

SI units

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SI units (Système International d'Unités)

Quantity	Name of unit	Symbol	Equivalent	Reciprocal
length	metre	m	3.281 feet	1 ft = 0.3048 m
mass	kilogram	kg	2.2 pounds	1 lb = 0.454 kg
time	second	s
electric current	ampere	A
thermodynamic temperature	kelvin	K	1°C = 1.8°F	1°C = 1 K
luminous intensity	candela	cd
amount of substance	mole	mol

Supplementary units

Quantity	Unit	Symbol
plane angle	radian	rad
solid angle	steradian	sr

Derived SI units

Quantity	Name of unit	Symbol	Equivalent	Reciprocal
frequency	hertz	Hz
energy	joule	J	0.2388 calories	1 cal = 4.1868 J
force	newton	N	0.225 pounds force	1 lbf = 4.448 N
power	watt	W	0.00134 horse power	1 hp = 745.7 W
pressure	pascal	Pa	0.00689 pounds force/sq.inch	1 lbf/sq.in = 145 Pa
electric charge	coulomb	C
electric potential difference	volt	V
electric resistance	ohm	Ω
electric conductance	siemens	S
electric capacitance	farad	F
magnetic flux	weber	Wb
inductance	henry	H
magnetic flux density	tesla	T
luminous flux	lumen	lm
illuminous	lux	lx
absorbed dose	gray	Gy
activity	becquerel	Bq
dose equivalent	sievert	Sv

Multiples used with SI units

Name of multiple	Symbol	Value (multiply by)
atto	a	10⁻¹⁸
femto	f	10⁻¹⁵
pico	p	10⁻¹²
nano	n	10⁻⁹
micro	μ	10⁻⁶
milli	m	10⁻³
centi	c	10⁻²
deci	d	10⁻¹
deca	da	10
hecto	h	10²
kilo	k	10³
mega	M	10⁶
giga	G	10⁹
tera	T	10¹²
peta	P	10¹⁵
exa	E	10¹⁸

A Dictionary of Zoology MICHAEL ALLABY

SI units

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SI units (Système International d'Unites) Internationally agreed system of units, derived from the mks (metre, kilogram, and second) system. SI units are now used for many scientific purposes, and have replaced the fps (foot, pound, and second) and cgs (centimetre, gram, and second) systems. The seven basic units are: the metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electrical current, Kelvin (K) for temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity.

Measurement	Unit	Symbol
Basic units
Length	metre	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Thermodynamic temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd
Supplementary units
Plane angle	radian	rad
Solid angle	steradian	sr
Derived units
Frequency	hertz	Hz
Force	newton	N
Pressure, stress	pascal	Pa
Work (energy, heat)	joule	J
Power	watt	W
Electric charge	coulomb	C
Electromotive force	volt	V
Electric resistance	ohm	1/2
Electric conductance	siemens	S
Electric capacitance	farad	F
Inductance	henry	H
Magnetic flux	weber	Wb
Magnetic flux density	tesla	T
Illuminance	lux	lx
Luminous flux	lumen	lm
Radiation exposure	röntgen	r
Radiation activity	becquerel	Bq
Radiation absorbed dose	gray	Gy
Radiation dose equivalent	sievert	Sv
Celsius temperature	°Celsius	°C

World Encyclopedia

SI units

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SI units (Système International d'Unités) The international system of units of measurement which evolved over a number of years from the m.k.s. system (based on the metre, kilogram, and second) and was finally adopted in 1960 by the 11th Conférence Générale des Poids et Mesures. The system is widely used in trade and is recommended for all scientific work. There are seven basic units: metre (m, length); kilogram (kg, mass); second (s, time); ampere (A, electric current); kelvin (K, thermodynamic temperature); candela (cd, luminous intensity); and mole (mol, amount of substance). There are also two supplementary units: radian (rad, plane angle); and steradian (sr, solid angle). From these, a further 18 units are derived: hertz (Hz, frequency); joule (J, energy); newton (N, force); watt (W, power); pascal (Pa, pressure); coulomb (C, electric charge); volt (V, electric potential); ohm (Ω, electric resistance); siemens (S, electric conductance); farad (F, electric capacitance); weber (Wb, magnetic flux); tesla (T, magnetic induction); lumen (lm, luminous flux); lux (lx, illuminance); gray (Gy, absorbed radiation dose); becquerel (Bq, radioactivity); and sievert (Sv, radiation dose equivalent). See Derived SI units.

SI Units (Système international d'unités)

Quantity	Name of unit	Symbol	Equivalent	Reciprocal
length	metre	m	3.281 feet	1 ft = 0.3048 m
mass	kilogram	kg	2.2 pounds	1 lb = 0.454 kg
time	second	s
electric current	ampere	A
thermodynamic temperature	kelvin	K	1°C = 1.8°F	1°C = 1 K
luminous intensity	candela	cd
amount of substance	mole	mol

Supplementary units

Quantity	Unit	Symbol
plane angle	radian	rad
solid angle	steradian	sr

A Dictionary of Ecology MICHAEL ALLABY

SI units

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SI units The system of units of measurement adopted internationally for scientific and technical use under the Système International d'Unités. There are seven fundamental units – the meter, second, kilogram, ampere, kelvin, candela, and mole – of seven dimensionally independent physical quantities. Other units are derived algebraically from these base units without the use of numerical factors, or are dimensionless (like the radian). The symbols for all units are standardized, as are the prefixes (and their symbols) that represent decimal multiples (e.g. nano-, micro-, milli-, kilo-, mega-, giga-) of the units.

A Dictionary of Computing JOHN DAINTITH