For sheathing and subfloor, area divided by sheet size plus ten percent is close enough. For anything where specific parts have to come out of the sheet, the area figure is only a starting point.
Nesting beats area
A project needing 60 square feet of parts does not need two sheets if the parts are awkward sizes. A part 25 inches wide uses up the full 48 inch width of a sheet on its own — the 23 inches left beside it will not take a second one.
Sketch the layout on paper or use a free cut-list optimiser before ordering. It routinely changes the answer by a full sheet or two.
Grain direction is not optional
Plywood face grain runs the length of the sheet and the panel is much stiffer in that direction. Structural sheathing goes with the grain across the supports.
In cabinet work the grain direction is visible on every exposed face, so parts have to be laid out with it in mind — which restricts nesting further.
Grades and what they mean
Letters describe the two faces. A is sanded and near-flawless, B has repairs, C and D have knots and voids. CDX is the common sheathing grade — C face, D back, exterior glue.
Exterior glue is not the same as exterior-rated plywood. CDX left in the rain for weeks will delaminate.
A cutting list beats a sheet count
Sheet goods are the one material where the total area tells you least. Twelve pieces measuring 30 by 30 inches is 75 square feet, which looks like two and a half sheets. It is actually three, because only four of them fit on a sheet.
Draw the layout before ordering. Graph paper is enough, and there are free cut-list programs that will do the nesting for you.
The same applies to grain direction, which removes rotation as an option and can turn a comfortable fit into a tight one.
Getting it cut and getting it home
A full sheet is 32 square feet and awkward rather than heavy, and it does not fit in most vehicles. Many suppliers will make a few straight cuts for a small fee or for nothing.
Their panel saw is more accurate than a circular saw on sawhorses, so it is worth using for the cuts that matter — but ask for the pieces slightly oversize and trim them yourself, because store cuts are not usually finish-grade.
Mark which face is the good face before any cutting. On a sheet with one good side, cutting with the good face in the wrong orientation produces tear-out exactly where it will show.