Primary Unit - the main unit in the category. All conversion factors are specified in relation to this main unit. The primary units are as follows:

·	Category   Primary Unit

·	Length   Metres

·	Mass    Kilograms

·	Volume   Litres

·	Area      Square Metres

·	Density   Kilograms per Litre

·	Pressure   Pascals

·	Speed   Metres per Second

·	Energy   Joules

·	Force      Newtons

·	Power   Watts

·	Temperature   Celcius

The choice of primary unit has been derived from the System Internationalé standards (SI units), with the exception of Temperature where Celcius seemed a more logical choice than Kelvin.