How much drywall for a 20 × 14 ft ceiling?

Ceiling boards

10× ½″ 4 × 8 ft

280 ft² ceiling plus 12% — framing at 16″ o.c.

Boards12% waste — ceilings cut around fixtures	10½″ 4 × 8 ft
Ceiling area20 × 14 ft	280ft²
Sag check½″ 4 × 8 ft is rated for framing at 16″ o.c.	OK
Screws12″ spacing on ceilings, not 16″ — about 1 lb box	392screws
Joint compound	0.64.5 gal buckets
Paper tape	128linear ft
Weight per boardoverhead — a panel lift is worth the hire, and it is not optional on ⅝″ 12 ft boards	51lb
Fixture cut-outscut after the board is up, from the joist positions marked on the floor	0holes

Notes

½″ 4 × 8 ft is rated for 16″ framing. Note it is the board that carries the rating, not the paint or the texture over it.
Screws every 12 inches on a ceiling, not 16. A ceiling board fights gravity for its whole life; a wall board rests on the plate. That difference is why ceiling screw patterns are tighter, and why a ceiling that was screwed to wall spacing shows fastener pops first.
Run the boards perpendicular to the joists. A board bridging several joists is far stiffer than one running along a single bay, and it spreads the load across the joints instead of concentrating it.
Each board weighs 51 lb overhead. Two people can manage it, but a panel lift makes the joints tighter because nobody is fighting the weight while positioning.
Mark the joist positions on the floor before you lift a board. Once it is up you cannot see them, and hunting for framing with a screw gun leaves a ceiling full of holes.
12% waste rather than the 10% used on walls — ceilings are cut around 0 lights, plus any fan boxes, vents and the perimeter.

A ceiling is not a wall lying down

Three things change, and all three are in the standards rather than in preference.

Thickness depends on framing spacing. ½″ standard board sags between joists at 24″ on center — it looks fine for a season and then develops a slow wave that no skim coat corrects. At 24″ you want ⅝″, or ½″ sag-resistant board rated for it.

Sag check for this ceiling: OK.

Screws every 12 inches, not 16. A ceiling board fights gravity for its whole life; a wall board rests on the plate. That is 392 screws here.

Boards run perpendicular to the joists. A board bridging several joists is far stiffer than one running along a single bay, and it spreads load across the joints instead of concentrating it.

What to buy

10 boards of ½″ 4 × 8, 0.6 buckets of compound, 128 linear ft of tape.

Waste is 12% rather than the 10% used on walls, because a ceiling gets cut around 0 light fitting0 plus vents and the perimeter.

Each board weighs 51 lb, overhead

A panel lift is worth the hire. On the heavier boards it is not a convenience — it is the difference between doing the job and dropping a board on somebody.

Mark the joists on the floor first

Once a board is up you cannot see the framing, and hunting for it with a screw gun leaves a ceiling full of holes that all have to be filled.

Nearby sizes

Popcorn Ceiling Removal Cost — Scrape, Skim, or CoverScraping is the cheap part. Skimming the ceiling flat afterwards is the job, and it is where quotes that looked similar stop being similar.
Drywall Screw Spacing for Ceilings — 12 Inches, and Why a Ceiling Is Not a WallA wall board rests on the bottom plate. A ceiling board fights gravity for its whole life, and every rule that differs comes from that.
Cost to Repair a Drywall Ceiling: Patch, Section, or Take It DownA twelve-inch patch and a whole ceiling are not on the same price curve. The patch is three visits and a texture match; the ceiling is one day and no matching at all.
Drywall Installation Cost by State — From BLS Installer PayDrywall has the widest state spread of any trade priced here except painting — 1.78x — and the reason is that hanging and finishing is almost entirely hours.