## Introduction to SI Units

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.