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General Information, Facts & Tables
[Metric Measurement System]

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Overview

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The metric system is an attempt to standardise a system of measurement units world wide.

See Also: Unit Conversions Index, Units named after people, Old Weight Measurements.
 

Prefixes

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Unit

Symbol

Value (Multiplier)

Name

yotta

Y

1 000 000 000 000 000 000 000 000

septillion

zetta

Z

1 000 000 000 000 000 000 000

sextillion

exa

E

1 000 000 000 000 000 000

quintillion

peta

P

1 000 000 000 000 000

quadrillion

tera

T

1 000 000 000 000

trillion

giga

G

1 000 000 000

billion

mega

M

1 000 000

million

myria *

my*

* 10 000

ten thousand *

kilo

k

1 000

thousand

hecta

h

100

hundred

deka

da

10

ten

Base Unit

unit

1

unit

deca

d

0.1

tenth

centi

c

0.01

hundredth

milli

m

0.001

thousandth

micro

µ (mu)

0.000 001

millionth

nano

n

0.000 000 001

billionth

pico

p

0.000 000 000 001

trillionth

femto

f

0.000 000 000 000 001

quadrillionth

atto

a

0.000 000 000 000 000 001

quintillionth

zepto

z

0 000 000 000 000 000 000 001

sextillionth

yocto

y

0.000 000 000 000 000 000 000 001

septillionth

Prefixes proposed for use with computing 

yobi (yotta binary) *

Yi

* 1 208 925 819 614 629 174 706 176

=280 *

zebi (zeta binary) *

Zi

* 1 180 591 620 717 411 303 424

=270 *

exbi (exa binary)

Ei

1 152 921 504 606 846 976

=260

pebi (peta binary)

Pi

1 125 899 906 842 624

=250

tebi (tera binary)

Ti

1 099 511 627 776

=240

gibi (giga binary)

Gi

1 073 741 824

=230

mebi (mega binary) 

Mi

1 048 576

=220

kibi (kilo binary) 

ki

1 024

=210

Notes
* This prefix is no longer used.
* Not currently under consideration for inclusion by SI, however as data storage capacity increases they will logically be included.
The units metre and litre are spelt "meter" and "liter" in the USA.

 

SI Base Units

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Quantity; unit (symbol)
Definition

Amount of substance; mole (mol)
1 mole (mol) = the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. 

Electric current; ampere (A)
1 ampere (A) = that constant which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 metre apart in a vacuum, would produce between those conductors a force equal to 2 x 10-7 newton metre of length.

Length; metre (m)
1 metre (m) = the length of light travelled by light in a vacuum during a time interval of 1/299 792 458 of a second. 

Luminous intensity; candela (cd)
1 candela (cd) =  the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency of 540 x 1012 hertz and that has a radiant intensity in that direction of (1/683) watt per steradian.

Mass; kilogram (kg)
1 kilogram (kg) = the mass of the international prototype of the kilogram. 

Thermodynamic temperature; kelvin (K)
1 Kelvin (K) = the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Time; second (s)
1 second = the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom when exited.
 

Supplementary Base Units

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Plane Angle; radian (rad)
1 radian (rad) = the plane angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius.

Solid Angle; steradian (sr)
1 steradian (sr) = the solid angle that, having its vertex in the centre of a sphere, cuts of an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.
 

Derived Units

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Derived quantity

Name

Symbol

Expression from base

absorbed dose rate

gray per second

Gy / s

m2 · s -3

angular velocity

radian per second

rad / s

m · m -1 · s -1 = s -1

angular acceleration

radian per second squared

rad / s2

m · m -1 · s -2 = s -2

dynamic viscosity

pascal second

Pa·s

m -1 · kg · s -1

electric field strength

volt per metre

V / m

m · kg · s-3 · A -1

electric charge density

coulomb per cubic metre

C / m3

m -3 · s · A

electric flux density

coulomb per square metre

C / m2

m -2 · s · A

energy density

joule per cubic metre

J / m3

m -1 · kg · s -2

exposure

coulomb per kilogram

C / kg

kg -1 · s · A

heat capacity / entropy

joule per kelvin

J / K

m2 · kg · s -2 · K -1

heat flux density / irradiance

watt per square metre

W / m2

kg · s -3

permittivity

farad per metre

F / m

m -3 · kg -1 · s4 · A2

permeability

henry per metre

H / m

m · kg · s -2 · A -2

molar energy

joule per mole

J / mol

m2 · kg · s -2 · mol -1

molar entropy

joule per mole kelvin

J / (mol·K)

m2 · kg · s -2 · K -1 · mol -1

moment of force

newton metre

N·m

m2 · kg · s -2

radiant intensity

watt per steradian

W / sr

m2 · kg · s -3 · sr -1

radiance

watt per square metre steradian

W / (m2·sr)

kg · s -3 · sr -1

specific heat capacity

joule per kilogram kelvin

J / (kg·K)

m2 · s -2 · K -1

specific energy

joule per kilogram

J / kg

m2 · s -2

surface tension

newton per metre

N / m

kg · s -2

thermal conductivity

watt per metre kelvin

W/(m·K)

m · kg · s -3 · K -1

 

Special Derived Units

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Derived quantity

Name

Symbol  

Expression from SI derived

Expression from SI base

Celsius temperature

degree Celsius

°C

 

K

electric charge

coulomb

C

 

s · A

electric potential

volt

W / A

m2 · kg · s -2 · A

electric resistance

ohm

W

V / A

m2 · kg · s -3 · A -2

electric capacitance

farad

F

C / V

m -2 · kg -1 · s4 · A2

electric conductance

siemens

S

A / V

m -2 · kg -1 · s3 · A2

energy / work

joule

J

N · m

m2 · kg · s -2

force

newton

N

 

m · kg · s -2

frequency

hertz

Hz

 

s -1

illuminance

lux

lx

lm / m2

m -2 · cd · sr

inductance

henry

H

Wb / A

m2 · kg · s -2 · A -2

luminous flux

lumen

lm

cd · sr

cd · sr

magnetic flux

weber

Wb

V · s

m2 · kg · s -2 · A -1

magnetic flux density

tesla

T

Wb / m2

kg · s -2 · A -1

plane angle

radian

rad

 

m · m -1 = 1

power / radiant flux

watt

W

J / s

m2 · kg · s -3

pressure / stress

pascal

Pa

N / m2

m -1 · kg · s -2

solid angle

steradian

sr

 

m2 · m -2 = 1

 

Medical Units

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Base quantity

Name

Symbol

Expression from derived

Expression from base

activity of a radionuclide

becquerel

Bq

-

s-1

absorbed dose

gray

Gy

J / kg

m2·s-2

dose equivalent

sievert

Sv

J / kg

m2·s-2

 

SI Quantities

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Derived quantity

Name

Symbol

acceleration

metre per second squared

m / s2

amount of substance concentration

mole per cubic metre

mol / m3

area

square meter

m2

current density

ampere per square metre

A / m2

luminance

candela per square metre

cd / m2

magnetic field strength

ampere per metre

A / m

mass density

kilogram per cubic metre

kg / m3

specific volume

cubic metre per kilogram

m3 / kg

speed / velocity

metre per second

m/s

volume

cubic metre

m3

wave number

reciprocal metre

m-1 (or 1/m)

 

Notes

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The expressions on this page have spaces between the variable names, and math symbols to improve their readability. In normal practice there are no spaces.