Tons of free calculators online.... http://www.summitracing.com/expertadviceandnews/calcsandtools/Compression-Calculator/

Using the actual math is cool, but the calculators should be fast and efficient. I have one loaded on my computer, have not actually used the one from Summit that I linked.

 

KB pistons has one of the better ones on the net. It will also calculate dynamic CR as well is you have the rod length and intake valve closing timing. Brooks Racing Components had a good one as well, but it has taken input for a couple years now. No idea why.

If it doesn't ask for the head gasket diameter separate from the cylinder bore diameter, its not that accurate.

Some go so far as to ask the piston clearance above the top ring, and the distance from the crown to the top ring to account for crevice volume.

If you are guessing at the combustion chamber volume, and/or deck clearance, you are playing darts in a dark room with a blind fold after being spun on a bar stool 20 times, after you drank 6 rum and cokes. That might work for picking a camshaft, but not for figuring compression ratio.

 

bore sq'd x.7854 x stroke gives you cu. in. x 16.387 gives you cc's . you need to know quench clearance + chamber vol. - dome vol. to know what you have at tdc . hope that helps .