To calculate the static compression ratio of an engine, you need to measure the bore (the width of the cylinder), the stroke (how far the piston moves up and down), the deck height (the distance from the top of the cylinder to the top of the piston when it's all the way up), the thickness of the head gasket (a layer between the cylinder and the cylinder head), the volume at the top of the piston (which changes if the piston has a dome or a dish), and the volume inside the combustion chamber (the area above the piston).

You then calculate how much space these parts take up and add them together. The compression ratio is the total volume when the piston is down divided by the volume of when the piston is up.