Its a ratio, therefore doesn't have a unit. It's as Gareth pointed out above. As long as the units are the same top and bottom of the equation, i.e psi or bar. Either way you will end up with the ratio. This is what is known as a dimensionless number

So at 2.00, the pressure coming out of the compressor is double that of what is going in.

i.e runnning 14psi will be

14psi boost + 14psi atmospheric pressure(roughly)
------------------------------------------------------------- = 2.00
14 psi atmospheric pressure

The variable is the boost. Stick different no's in and the ratio will change