Drywall for a 18 × 18 ft ceiling: ⅝″ 4 × 8 on 24″ joists

Ceiling boards

12× ⅝″ 4 × 8 ft

324 ft² ceiling plus 12% — framing at 24″ o.c.

Boards12% waste — ceilings cut around fixtures	12⅝″ 4 × 8 ft
Ceiling area18 × 18 ft	324ft²
Sag check⅝″ 4 × 8 ft is rated for framing at 24″ o.c.	OK
Screws12″ spacing on ceilings, not 16″ — about 1 lb box	454screws
Joint compound	0.84.5 gal buckets
Paper tape	154linear ft
Weight per boardoverhead — a panel lift is worth the hire, and it is not optional on ⅝″ 12 ft boards	70lb
Fixture cut-outscut after the board is up, from the joist positions marked on the floor	4holes

Length: 18 ft
Width: 18 ft
Board: ⅝″ 4 × 8
Joist spacing: 24 in o.c.
Light fittings: 4

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 454 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

12 boards of ⅝″ 4 × 8, 0.8 buckets of compound, 154 linear ft of tape.

Waste is 12% rather than the 10% used on walls, because a ceiling gets cut around 4 light fitting4 plus vents and the perimeter.

Each board weighs 70 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.

Notes

⅝″ 4 × 8 ft is rated for 24″ 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 70 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 4 lights, plus any fan boxes, vents and the perimeter.

Nearby sizes

How Much Joint Compound — by Board Area, Not by Joint LengthCompound goes by the area of board, not the length of the joints, and the finish level moves the figure by 40 percent.
Drywall Finish Levels 0 to 5 — What Each Costs and Where Each BelongsLevel 4 is the standard and it shows every joint on a wall that gets raking light. That one condition is the whole Level 5 argument.
Drywall Cost per Sheet Installed — Board, Hang, Finish, and the Level You ChoseThe board is about a fifth of the bill. The finish level you specify moves the total more than any material decision.
How Many Sheets of Drywall — and Why Sheet Length Changes the Job More Than the CountSheet count is arithmetic. Sheet length is a decision, and it changes how the finished wall looks under raking light.