O.K.
There's 2 ways you can measure compression.
1. Atmospheric pressure is roughly 14.7 PSI. So if you divide your 205 by 14.7, you get a 13.9:1 compression...... As someone else said 'Is this a diesel'? This method is a little bit inaccurate as you loose a slight amount of your air pressure through your piston rings. Also this all changes when you add a Turbo. You have to work of inlet pressure, which of course changes with a Turbo.
2. The right way - Measure the volume of the combustion chamber at bottom dead centre & at top dead centre. Divide the 1st number by the second & you have your ratio. Most guys do this using liquid of some sort, as combustion chambers can be all shapes & sizes.... Difficult to calculate volume.