Hello. I'm not sure if I understand the question, but a cubic foot of air
displaces a set amount of water, depending on how compressed the air is. In
other words, a cubic foot of water ( which doesn't really compress much ) weighs
62.4 pounds. As a starting point, calculate how much water your sub displaces,
weigh the sub with all its parts and occupants and " stuff " ( again,
calculate this rather than building one and putting it on a scale ) and what's
left over is the positive buoyancy.
There are conversion charts on the psub site with weights, measures, etc.
Down load and print and it will give you a rough idea of how big/heavy a thing
is.
Frank D.