Drywall Calculators
The board is the cheap part. Everything expensive about drywall happens after it is on the wall.
The calculators
A sheet of gypsum board costs less than the compound, tape, corner bead and labour that turn it into a wall. Which makes drywall the one trade here where a material quantity answers the smallest part of the question, and where the things that drive cost are not things you can count off a drawing.
Finish levels, and why no page here quotes a compound figure
There are six levels of gypsum board finish, 0 through 5, and they are defined in GA-214. Level 1 is tape embedded and nothing else, which is fine above a suspended ceiling. Level 4 is what most rooms get. Level 5 is a skim coat over the entire surface, and it exists because gloss paint and raking side light will find every joint that Level 4 leaves.
The compound between Level 1 and Level 5 differs by several coats over the whole area rather than over the joints, so it is not one number with a tolerance, it is different jobs. GA-214 is not a free document and no figure for it appears on this site.
Butt joints are the enemy, and sheet length is how you fight them
A tapered edge gives the tape somewhere to sit below the surface. A butt joint does not, so it has to be feathered wide to disappear, and on a long wall in side light it usually does not disappear at all.
Longer sheets remove butt joints entirely rather than reducing them. That is why professionals fight stairwells with 12 ft boards instead of taking the easy 8s, and it is a judgement about finishing rather than about material.
Weight, which decides who can do the job
A 4 by 12 sheet of 5/8 board is around 100 lb. A ceiling of them is not a one-person task with a friend, it is a two-person task with a lift, and that constraint shapes the sheet size long before the code does.
Where the code does bite
IRC Table R702.3.5 pairs board thickness with framing spacing, prescribes sheet orientation at the wider spacings, and sets fastener spacing. One row in it, for a Type X garage ceiling under a habitable room, calls for six inches for both nails and screws where every other ceiling gets twelve. That row is a fire separation, and it is the most commonly missed line in the chapter.