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 unit^{a} 
Symbol^{a} 
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 unit^{a} 
Symbol^{a} 
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 [m^{2}], 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 lowercase 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 
Symbol^{a} 
Value 
yotta

Y

10^{24}

zetta

Z

10^{21}

exa

E

10^{18}

peta

P

10^{15}

tera

T

10^{12}

giga

G

10^{9}

mega

M

10^{6}

kilo

k

10^{3}

hecto

h

10^{2}

deca

da

10^{1}

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.