Skip to main content
Select Source:

SI units

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


Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"SI units." A Dictionary of Biology. . Encyclopedia.com. 20 Jul. 2017 <http://www.encyclopedia.com>.

"SI units." A Dictionary of Biology. . Encyclopedia.com. (July 20, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/si-units-2

"SI units." A Dictionary of Biology. . Retrieved July 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/si-units-2

SI units

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


Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"SI units." A Dictionary of Nursing. . Encyclopedia.com. 20 Jul. 2017 <http://www.encyclopedia.com>.

"SI units." A Dictionary of Nursing. . Encyclopedia.com. (July 20, 2017). http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/si-units

"SI units." A Dictionary of Nursing. . Retrieved July 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/si-units

SI units

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


Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"SI units." A Dictionary of Ecology. . Encyclopedia.com. 20 Jul. 2017 <http://www.encyclopedia.com>.

"SI units." A Dictionary of Ecology. . Encyclopedia.com. (July 20, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/si-units

"SI units." A Dictionary of Ecology. . Retrieved July 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/si-units

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

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


Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"SI units." A Dictionary of Plant Sciences. . Encyclopedia.com. 20 Jul. 2017 <http://www.encyclopedia.com>.

"SI units." A Dictionary of Plant Sciences. . Encyclopedia.com. (July 20, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/si-units-0

"SI units." A Dictionary of Plant Sciences. . Retrieved July 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/si-units-0

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

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


Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"SI units." A Dictionary of Zoology. . Encyclopedia.com. 20 Jul. 2017 <http://www.encyclopedia.com>.

"SI units." A Dictionary of Zoology. . Encyclopedia.com. (July 20, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/si-units-1

"SI units." A Dictionary of Zoology. . Retrieved July 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/si-units-1

SI units

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


Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"SI units." World Encyclopedia. . Encyclopedia.com. 20 Jul. 2017 <http://www.encyclopedia.com>.

"SI units." World Encyclopedia. . Encyclopedia.com. (July 20, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/si-units

"SI units." World Encyclopedia. . Retrieved July 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/si-units

SI units

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.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"SI units." A Dictionary of Computing. . Encyclopedia.com. 20 Jul. 2017 <http://www.encyclopedia.com>.

"SI units." A Dictionary of Computing. . Encyclopedia.com. (July 20, 2017). http://www.encyclopedia.com/computing/dictionaries-thesauruses-pictures-and-press-releases/si-units

"SI units." A Dictionary of Computing. . Retrieved July 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/computing/dictionaries-thesauruses-pictures-and-press-releases/si-units