To calculate area, multiply the length of the blank by the width. To convert Square inches to Square feet, divide by 144. But this only gets you pay of the way there.
Leather dealers sell cuts, which are proceed by the Square foot: Shoulders, bellies, backs, sides, butts etc. You need a piece which is long enough from which to cut two blanks. Presuming these are for adult men, you might be able to squeeze them out of a double shoulder, but you would have to cut them the "wrong" way so they might end up a bit too stretchy. To cut them the "right" way you will need a butt, side or back. These are big pieces of leather, and not cheap -- but they do work out well per usable Square foot if you need it all.