Leaflet No. 11
March 1993
International System of Units
The Système International d`Unités, or International System of Units, (SI) was adopted by
international agreement in 1960. It is a simplified version of the original French metric
system, based on 3 units of length, mass and time. Several variations of the metric system
were used before 1960; the best known being the CGS (based on the centimetre, gram and
second) and the MKS (metre, kilogram and second). The SI is based on seven base units and
two supplementary units.
Almost all countries in the world have now adopted SI as their legal system of measurement
and changes have been made from each country's indigenous system of measurement to the
new SI system.
In Australia metrication began in 1970 with the establishment of the Metric Conversion Board
to co-ordinate the conversion process. By the time the Board was dissolved in 1984 their work
could be regarded as complete, apart from a few areas that for various reasons had not been
converted. These exceptions included the engineering industry which still required nonmetric spare parts and the real estate industry.
Responsibility for the completion of metrication was transferred to the National Standards
Commission in 1984. The Commission is the Commonwealth Statutory Authority responsible
for co-ordinating the National Measurement System. It was established in 1950 and now
operates under the National Measurement Act 1960 (the Act).
One of the objectives of the Act is to bring about the use of the metric system of measurement
as the sole system of measurement of physical quantities in Australia. Regulations made
under the Act may prescribe Australian legal units of measurement of any physical quantity.
Such units are the sole legal units of measurement of those physical quantities, with the
exception of certain additional units which have been prescribed for particular purposes.
The following tables show the base, supplementary and selected derived units of the SI, the
prefixes that form part of the SI, non-SI units that are of such practical importance that they
are still prescribed (eg the day) and the units that are prescribed for particular purposes only.
Further information on the Australian legal units of measurement may be obtained from the
National Standards Commission, 888 3922 or for enquiries from outside Sydney (008) 251 942.
SI Base Units of Measurement
Ite
Quantity
Name
m
1
mass
kilogram
Symbo
l
kg
2
amount of
substance
mole
mol
3
length
metre
m
4
time
second
s
5
luminous
intensity
candela
cd
6
thermodynamic
temperature
kelvin
K
7
electric current
ampere
A
Definition
The mass of the cylinder:
(a) deposited in the International
Bureau of Weights and Measures;
and
(b) declared to be the International
Prototype Kilogram by the First
General Conference on Weights
and Measures held in Paris in
1889.
The amount of substance of a system
which contains as many elementary
entities as there are atoms in 0.012 kg
of carbon-12.
The length of the path travelled by
light in a vacuum during a time of
1/299 792 458 of a second.
The duration of 9 192 631 770 periods
of the radiation corresponding to the
transition between the 2 hyperfine
levels of the ground state of the
caesium-133 atom.
The luminous intensity, in a given
direction, of a source that emits
monochromatic radiation of the
frequency 540 × 1012 hertz and has a
radiant intensity in that direction of
1/683 watt per steradian.
The fraction 1/273.16 of the
thermodynamic temperature of the
triple point of water.
The unvarying electric current that,
when flowing in each of two parallel
straight conductors of infinite length
of negligible cross-section and
separated by a distance of 1 metre
from each other in free space,
produces between those conductors a
force equal to 0.2 × 10-6 newtons per
metre length of conductor.
SI Supplementary Units of Measurement
Ite
Quantity
Name
Symbo Definition
m
l
1
plane angle
radian
rad
The radian is the plane angle between
two radii of a circle that cut off on the
circumference an arc equal in length
to the radius.
2
solid angle
SI Prefixes
Ite
Factor
m
1
1024
2
1021
3
1018
4
1015
5
1012
6
109
7
106
8
103
9
102
10
101
11
10-1
12
10-2
13
10-3
14
10-6
15
10-9
16
10-12
17
10-15
18
10-18
19
10-21
20
10-24
steradian
Name
Symbol
yotta
zetta
exa
peta
tera
giga
mega
kilo
hecto
deka
deci
centi
milli
micro
nano
pico
femto
atto
zepto
yocto
Y
Z
E
P
T
G
M
k
h
da
d
c
m
µ
n
p
f
a
z
y
sr
The steradian is the solid angle that
has its vertex in the centre of a sphere
and cuts off 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.
SI Derived Units of Measurement having Special Names
Ite
Quantity
Name
Symbol Definition
m
1
frequency
hertz
Hz
1 Hz = 1 1/s
2
force
newton
N
1 N = 1 kg.m/s2
3
pressure
pascal
Pa
1 Pa = 1 N/m2
4
energy, work
joule
J
1 J = 1 N.m
5
power
watt
W
1 W = 1 J/s
6
electric charge
coulomb
C
1 C = 1 A.s
7
potential difference,
volt
V
1 V = 1 W/A
electromotive force
8
electric capacitance
farad
F
1 F = 1 C/V
9
electric conductance
siemens
S
1 S = 1/ W
10
electric inductance
henry
H
1 H = V.s/A
11
electric resistance
ohm
1 W = 1 V/A
Ω
12
magnetic flux
weber
Wb
1 Wb = 1 V.s
13
magnetic flux density
tesla
T
1 T = Wb/m2
14
15
16
17
18
luminous flux
illuminance
activity of a radionuclide
absorbed dose,
absorbed dose
index, kerma, specific
energy imparted
dose equivalent
lumen
lux
becquerel
gray
lm
lx
Bq
Gy
1 lm = 1 cd.sr
1 lx = 1 lm/m2
1 Bq = 1/s
1 Gy = 1 J/kg
sievert
Sv
1 Sv = 1 J/kg
Units of Measurement Legal for Particular Purposes
Ite
Quantity
Name
Symbol
Definition
m
1
length
inch
in
0.9144/36 m
2
length
foot
ft
0.9144/3 m
3
mass
troy ounce
oz tr
480(0.453 592 37)/7000 k
g
4
power
horsepower
hp
745.7 W
5
pressure
millibar
mb or mbar
100 Pa
6
pressure
millimetre of
mmHg
133.322 19 Pa
mercury
7
velocity
foot per minute ft/min
0.3048/60 m/s
8
work and
kilocalorie
kcal
4.1868 × 103 J
energy
Non-SI Units of Measurement Used with SI Units of Measurement
Ite
Quantity
Name
Symbol Definition
m
1
area
hectare
ha
104 m2
2
energy
electronvolt
eV
The kinetic energy acquired by an
electron in passing through a
potential difference of 1 volt in a
vacuum.
1 eV ≅ 1· 602 19 × 10-19 J
3
length
nautical mile
n mile
1852 m
4
length
astronomical
AU
1 AU = 149 600 × 106 m
unit
5
length
parsec
pc
The distance at which 1 astronomical
unit subtends an angle of 1 second of
arc.
1 pc ≅ 30 857 × 1012 m
6
mass
tonne
t
103 kg
7
mass
atomic mass
u
The fraction 1/12 of the mass of an
unit
atom of the nuclide 12 C.
1 u ≅ 1· 660 53 × 10-27 kg
8
mass
metric carat
CM or 0· 2 × 10-3 kg
ct
9
plane angle
degree
°
p/180 rad
10
plane angle
minute
´
1/60 × p/180 rad
11
plane angle
second
´´
1/3600 × p/180 rad
12
13
14
15
time
time
time
temperature
16
17
18
velocity
volume
sound power
level
sound pressur
e level
sound intensit
y level
19
20
day
hour
minute
degree
Celsius
knot
litre
decibel
d
h
min
°C
86 400 s
3 600 s
60 s
1K
kn
L or l
dB
1852/3600 m/s
10-3 m3
1 dB = 10 log (W/10-12 )
decibel
dB
1 dB = 20 log (Parms / (2×10-5 ))
decibel
dB
1 dB = 10 log ((W/m2 ) / 10-12 )