Drywall calculator

How the number is worked out

Board area is the four walls plus the ceiling, with no deduction for doors and windows — you hang the board straight over the opening and cut it out afterwards, and that off-cut is rarely big enough to use elsewhere. A 12 × 12 room with an 8 ft ceiling is 384 sq ft of wall plus 144 sq ft of ceiling = 528 sq ft, which is 16.5 sheets of 4 × 8 before any waste.

The finishing materials come from the manufacturers' published estimating figures, and the two big names agree. National Gypsum states 123–140 lb, or 9 gallons, of ProForm all-purpose joint compound per 1,000 sq ft of board plus about 375 ft of paper tape; USG's figures are about 9.4 gallons and 370 linear ft for the same 1,000 sq ft. Both are for the complete job — embedding the tape, spotting the screws and two finish coats — not for one coat.

That works out at 479–500 sq ft per 4.5 gallon pail as the makers state it. Plan on 450–475 sq ft per pail in practice: the published number does not include what dries on the hawk, sets in the pan or gets sanded back off the wall.

The 0.05 gal/sq ft number you will see everywhere is wrong

Several popular calculators use 0.05 gallons of joint compound per square foot. Run our 12 × 12 room through it and you get 26 gallons, which is nearly six pails. The real answer from the manufacturers' own tables is 4.8 gallons — one pail, or two if you are heavy-handed and sand a lot back off. The figure in circulation is more than five times the truth (0.05 ÷ 0.0094 = 5.3×), and it is the single most expensive mistake on the drywall aisle.

Match the sheet length to the wall

This is the advice that actually changes the job, and almost nobody writing a drywall calculator gives it. Tapered edges are on the long sides of the sheet, so a joint between two long edges sinks into the taper and disappears. A butt joint — the cut ends meeting — sits proud of the board, and hiding it means feathering compound out 18 inches or more on both sides.

On a 12 ft wall, two 4 × 12 sheets hung horizontally give you one tapered joint at 4 ft and no butt joints at all. The same wall in 4 × 8 sheets gives you a butt joint in the middle of each course. The sheets cost the same per square foot; the finishing work is halved.

Screws

The spacing comes from IRC Table R702.3.5: for 1/2 in board on wood framing without adhesive, screws go a maximum of 16 in apart on walls and 12 in apart on ceilings, with framing at 16 in on centre. That works out at about 28 screws per 4 × 8 sheet on a wall and 36 on a ceiling — the ceiling takes more because gravity is pulling on it.

Buy 1-1/4 in coarse-thread screws for wood studs. A 1 lb box is roughly 180 screws, a 5 lb box roughly 900.