Here we give definitions and some explanation of metric units. They are divided
into what are know as the base units, derived units and those
which are approved for use with SI but not part of the core system.
Metric prefixes
Here we explain the metric prefixes in two parts. First we concentrate on those
in common use for everyday purposes and then give a comprehensive list for the whole range.
What is a metric prefix?
A metric prefix is a convenient way of expressing mulitiples and subdivisions (larger
and smaller) of any defined unit. In metric that means multiplying or dividing by
10, 100, 1000 etc. It consists of a partial word like "kilo" , "centi" etc.
The prefix is placed before the name or symbol for the unit to indicate a multiple
or subdivision. For example:
"kilo" means 1000, so kilometre means 1000 metres.
"centi" means one hundredth, so centimetre means one hundreth of a metre,
or in other words 100 centimetre = 1 metre.
In symbols these two examples are written km (symbol for kilo = k, symbol for metre
= m) and cm (symbol for centi = c) respectively.
These rules apply to all units and their symbols. So for example:
1 kW = 1000 W, 1 kg = 1000 g, 1 km = 1000 m, and so on.
Commonly used metric prefixes
|
name |
symbol |
value of multiple or subdivision |
example |
|
nano |
n |
billionth (0.000 000 001)
|
1000 000 000 nanometre = 1 metre (1000 000 000 nm = 1 m)
Wavelength of green light = 550 nm
|
|
micro |
µ |
millionth (0.000 001)
|
1000 000 microgram = 1 gram (1000 000 µg = 1 g)
Often seen in drug doses
|
|
milli |
m |
thousandth (0.001)
|
1000 millimetre = 1 metre (1000 mm = 1 m)
Very handy for small or precision measurements.
|
|
centi |
c |
hundredth (0.01)
|
100 centilitre = 1 litre (100 cL = 1 L)
Common for wine and spririts
|
|
deci |
d |
tenth (0.1)
|
1 dm = 0.1 m
Not seen a great deal but can be useful (note 1 dm3 = 1 L)
|
|
deca |
da |
10 |
1 decagram = 10 gram
Seldom used but shown for completeness
|
|
hecto |
h |
100 |
1 hectolitre = 100 litre (1 hL = 100 L)
See hectare for another example below.
|
|
kilo |
k |
1000 |
See examples above. Very common.
|
|
mega |
M |
1000 000 |
1 megawatt = 1000 000 watt = 1000 kilowatt (1 MW = 1000 000 W
= 1000 kW)
|
|
giga |
G |
1000 000 000 |
1 gigahertz = 1000 000 000 hertz
(1 GHz = 1000 000 000 Hz)
Seen in PC specs, e.g. 2.7 GHz processor.
|
The remainder of the prefixes are shown here. They cover a very wide range and are
not generally encountered outside science and engineering applications. Note that
scientific notation is used for convenience.
Other SI prefixes
| name |
symbol |
value |
name |
symbol |
value |
|
yotta |
Y |
1024 |
pico |
p |
10-12 |
|
zetta |
Z |
1021 |
femto |
f |
10-15 |
|
exa |
E |
1018 |
atto |
a |
10-18 |
|
peta |
P |
1015 |
zepto |
z |
10-21 |
|
tera |
T |
1012 |
yocto |
y |
10-24 |