SI Units (Systéme International d'Unites) were adopted worldwide in 1960 to replace the many differing national systems. Although in the United Kingdom some imperial measures can be found, SI Units are used in Nursing.
The standard SI Units are:
Quantity

SI Unit

Symbol

Length 
metre 
m

Mass 
kilogram 
kg

Volume 
litre 
l

Temperature 
Celsius 
°C

Energy 
Joule 
J

This list is not exhaustive, and others may be found.
Units of length are not always expressed in metres. Consider a finger nail which is 0.002 metres thick. This could be written as 0.002m or 0.2cm, thereby making the numbers more manageable. But what exactly does this 'cm' mean?
There is a standard list of prefixes which may be used to alter the standard units as shown below:
Prefix

Symbol

Number

Value


mega 
M

million 
1 000 000


kilo 
k

thousand 
1 000


hecto 
h

hundred 
100


deca 
da

ten 
10


deci 
d

tenths 
1


10


0.1

centi 
c

hundredth 
1


100


0.01

milli 
m

thousandth 
1


1 000


0.001

micro 
µ

millionth 
1


1 000 000


0.000 001

nano 
n

thousand millionth 
1


1 000 000 000


0.000 000 001

Because the metric system is a base ten system, decimal numbers, rather than vulgar fractions, are used to express quantities. The decimal system also facilitates the use of computers and calculators.