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 rule of thumb does not survive a data sheet

There is a figure of 0.05 gallons of joint compound per square foot in circulation. Run our 12 × 12 room through it and you get 26 gallons, which is nearly six pails. The figure from the manufacturers' own tables is 4.8 gallons — one pail, or two if you are heavy-handed and sand a lot back off. National Gypsum's 9 gal per 1,000 sq ft is 0.009, and USG's 9.4 is 0.0094, so the rule of thumb is more than five times either published rate (0.05 ÷ 0.0094 = 5.3×). This page uses the published rate, and that is the difference between one pail and six on the drywall aisle.

Match the sheet length to the wall

This is the advice that actually changes the job, and it does not fall out of a square-footage sum. 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.

What this count does not know about your walls

The sheet number is an area division — board area, times the waste allowance, divided by the area of one sheet. No sheet is ever laid out. So it cannot tell you where the joints land, whether they fall on a stud, or how much of the last board in each course gets ripped off and binned. On walls that divide evenly by the sheet length the 10% is generous; on a 13 ft wall boarded in 4 × 12 it is nowhere near enough, because every course leaves a 1 ft strip and that strip is only useful if another wall happens to want a 1 ft strip.

The same blindness runs through the finishing materials. Compound and tape are figured per 1,000 sq ft of board, which is how National Gypsum and USG publish them, so neither figure moves when you change the sheet size above. The butt joints the section on sheet length is about — the ones that cost you the feathering — are invisible to this estimate. Follow that advice for the work it saves you, not because the page will hand back a smaller number.

The room is a rectangle with four walls. Board area is 2 × (length + width) × height, plus length × width if the ceiling is on. An L-shaped room, a stairwell, the gable end over a vaulted ceiling, a soffit dropped above the cabinets and the inside of a closet are all outside that. Break the job into rectangles and run the page once for each. Leave the length or the width at zero and you get nothing back rather than a price for the two walls that arithmetic would otherwise leave standing on a floor plan with no depth. A ceiling height of zero is the one exception, and it is deliberate: with the ceiling switched on it boards the ceiling on its own, which is a job people really do.

There is no field for an opening. Charging the full wall area is deliberate — board is hung over the door and cut out afterwards — but it also means a wide cased opening, a garage door or a wall of glass is over-counted with no way to say so. Work that one out by hand: the opening area divided by the sheet area is the number of sheets to take off. It is only worth doing when the piece you cut out is big enough to use somewhere else.

Two more sit outside it, both about the materials rather than the walls:

• The screw spacing is for 1/2 in board on wood studs at 16 in on centre with no adhesive. Steel framing, studs at 24 in, 5/8 in Type X and any fire-rated assembly carry their own fastener schedules, and on a rated wall that schedule is not a suggestion.
• Corner bead, trim and a Level 5 skim coat are not in the compound figure. The manufacturers' number covers embedding, spotting and finishing the joints. Skimming the whole board is another coat over every square foot of it.