How to Use This Tool
Enter the room, choose a sheet length, and deduct the door and window area. The sheet count follows from area; the interesting column is the one counting butt joints.
Tapered edges and butt joints are not the same seam
A drywall sheet has a shallow recess along its two long edges. When two of those meet, the tape and compound sit in the recess and finish flush. That seam is easy.
The short ends are cut square with no recess. Where two ends meet you get a butt joint, and the tape has to be built up above the surface then feathered out 10 to 12 inches on each side to disguise it. Butt joints are the seams that reappear under a raking light once the compound dries and shrinks.
So the useful question is not how many sheets, it is how many butt joints the sheet length forces. A 12 ft sheet on a 12 ft wall creates none.
12 x 14 ft room, 8 ft ceiling, ceiling boarded walls 2 x (12 + 14) x 8 = 416 sq ft ceiling 12 x 14 = 168 sq ft total = 584 sq ft 4 x 8 sheets 584 / 32 = 18.25 -> 19 sheets, 24 sq ft over 4 x 12 sheets 584 / 48 = 12.2 -> 13 sheets, 40 sq ft over 4 x 16 sheets 584 / 64 = 9.1 -> 10 sheets, 56 sq ft over
Compound is three coats, and a third of it is sanded off
The tape coat embeds the tape, the fill coat covers it, and the skim coat feathers the edges. Each is thinner than the last, and a substantial part of what goes on comes back off as dust. That is why the sensible unit is gallons per square foot of board rather than per seam.
A common working figure is one gallon per 100 to 150 sq ft for all three coats combined, which is the default here. Hand-taping a room with many corners runs to the heavy end; machine tools use less.
Tape length is not seam length
You need tape for every seam, every inside corner and every outside corner bead, which works out at roughly 0.5 to 0.6 linear feet per square foot of board. A 584 sq ft room needs around 320 ft, so the 500 ft roll rather than the 250 ft one.
Ceilings are the part to reconsider
Boarding a ceiling is where sheet weight stops being an abstraction. A 4 x 12 sheet of half-inch board weighs around 70 lb and has to be held overhead while it is screwed off. If you are working alone, the shorter sheet is the right answer even though it makes more seams — or hire a panel lift.
What this does not include
Screws, corner bead, primer and the cost of your own sanding time. Primer matters more than people expect: bare compound and bare paper absorb paint very differently, and skipping primer is how a wall ends up with every seam visible through the finish coat.
