I have the bore, stroke and deck. I looked on google but they confusing, thanks

 

You need a lot more than just those three. You also need piston dish or dome, head gasket thickness, and combustion chamber volume.
I prefer this one, but you do need to go gather a lot of info to use it:
Engine Compression Ratio (CR) Calculator

 

Its just homework for diesel tech so we dont need to know all of that yet

 

If you don`t need anything fancy then just take the volumn of the cylinder (stroke x area of piston) and divide that by the volume of the combustion chamber (usually a dish in the piston for diesel and dish in head for a gasser).

 

What's the point in asking for a formula if it's okay for the answer to be wrong?

 

When I teach it to my 10th grade small engine students, we do the math calculations for the cylinder first. Then we invert the head and use a syringe to measure how much water it takes to fill the oddly shaped combustion chamber and convert the cubic centimeters to cubic inches.
Add the cylinder displacement to the combustion chamber displacement for total displacement and divide by combustion chamber displacement to arrive at the compression ration.
Mind you, these engines have flat piston heads, so that makes calculating cylinder volume pretty straightforward.

 

Thats the same one I use. After I use that I then go and use this one here.

United Engine & Machine Co. Incorporated

It helps me to deside if the camshaft I am looking at will provide 8.5:1 or less dynamic compresion which is the max a engine can have and still run 93 octane pump gas. Ive been using it alot trying to plan a somewhat OEM rebuild for my 351W but trying to get 300 - 350 hp and 400 - 450 trq out of it with a powerband of idle up to 4,500 rpm (FMX shifts out of passing floored at 4500 @ 60 mph)

But both of them are great calculators though and work perfectly as far as I can tell.

 

That United Engine one is showing my ratio as being a good bit higher than the other one- The CSGnetwork one shows mine as 9.6, while the United Engine one shows the static as 9.98.

 

Strange both of them always comes out the same for me when imputting the same information and same values