Compression ratio (CR) is all the volume over the piston when the piston is down in the bore (V1) divided by the volume on top of the piston when it's at its top dead center (V2).

CR=V1/V2

Now in V2 you need to count together the volume of the piston bowl (or the dome in negative), the volume the piston comes above the deck height in negative, the volume of the head gasket and the volume of the combustion chamber.

V1 is all the above and the cylinder swept volume.