The Compression ratio reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Compression ratio

Compression ratio (CR) is an engineering term used to describe internal-combustion engines. It describes the difference in volume from maximum to minimum as a piston engine operates.

The compression ratio is a single number that can be used to predict the performance of any internal-combustion engine.

To calculate the compression ratio;
b = cylinder bore (diameter)
s = piston stroke length
V_c = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated.

CR= (π/4 × b² × s + V_c) / V_c

Due to pinging the CR in a petrol or gas (LPG or CNG)-fueled engine will not exceed 10:1
In a turbo charged engine the CR wil be around 8,5:1
In a diesel engine the CR will be 20:1 and higher.

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