The SI system has 7 base units (see the table ‘SI base units and their symbols’), from which others have been derived (see the table ‘SI derived units that have been given special names’).
SI base units and their symbols
| SI unita |
Symbola |
Quantity |
|
ampere
|
A
|
electric current
|
|
candela
|
cd
|
luminous intensity
|
|
kelvin
|
K
|
thermodynamic temperature
|
|
kilogram
|
kg
|
mass
|
|
metre
|
m
|
length
|
|
mole
|
mol
|
amount of substance
|
|
second
|
s
|
time
|
a Shows correct capitalisation
SI derived units that have been given special names
| SI derived unita |
Symbola |
Derived quantity |
|
becquerel
|
Bq
|
activity (radionuclide)
|
|
coulomb
|
C
|
electric charge, quantity of electricity
|
|
degree Celsius
|
°C
|
Celsius temperature
|
|
farad
|
F
|
capacitance
|
|
gray
|
Gy
|
absorbed dose, specific energy imparted
|
|
henry
|
H
|
inductance
|
|
hertz
|
Hz
|
frequency
|
|
joule
|
J
|
energy, work, quantity of heat
|
|
katal
|
kat
|
catalytic activity
|
|
lumen
|
lm
|
luminous flux
|
|
lux
|
lx
|
illuminance
|
|
newton
|
N
|
force
|
|
ohm
|
Ω
|
electric resistance
|
|
pascal
|
Pa
|
pressure, stress
|
|
radian
|
rad
|
plane angle
|
|
siemens
|
S
|
electric conductance
|
|
sievert
|
Sv
|
dose equivalent
|
|
volt
|
V
|
electric potential difference
|
|
watt
|
W
|
power, radiant flux
|
|
weber
|
Wb
|
magnetic flux
|
a Shows correct capitalisation
Many other derived units have not been given special names (eg square metre [m2], amperes per metre [A/m], metres per second [m/s]).
When written out in full, units with a proper noun do not have an initial capital. However, the symbols for these units do have an initial capital:
pascal [symbol Pa] newton [symbol N] watt [symbol W]
Caution! Exception: In the unit ‘degrees Celsius’, the capital C is maintained.
SI units can be used with standard prefixes to show larger or smaller quantities. Standard prefixes and their values are shown in the table below. Take great care to use the correct lower-case or capital letter for the symbol representing a prefix.
Beware the transfer of the μ (micro) symbol (character code 181 in Microsoft Word) between computer programs, to avoid inadvertent conversion to m.
Standard SI prefixes
| Prefix |
Symbola |
Value |
|
yotta
|
Y
|
1024
|
|
zetta
|
Z
|
1021
|
|
exa
|
E
|
1018
|
|
peta
|
P
|
1015
|
|
tera
|
T
|
1012
|
|
giga
|
G
|
109
|
|
mega
|
M
|
106
|
|
kilo
|
k
|
103
|
|
hecto
|
h
|
102
|
|
deca
|
da
|
101
|
|
deci
|
d
|
10–1
|
|
centi
|
c
|
10–2
|
|
milli
|
m
|
10–3
|
|
micro
|
µ
|
10–6
|
|
nano
|
n
|
10–9
|
|
pico
|
p
|
10–12
|
|
femto
|
f
|
10–15
|
|
atto
|
a
|
10–18
|
|
zepto
|
z
|
10–21
|
|
yocto
|
y
|
10–24
|
a Shows correct capitalisation
Caution! Be careful to check that the μ symbol used in units is not accidentally converted to m between computer programs.
