Drywall Calculator
Work out how many drywall sheets a wall or ceiling needs, plus screws, joint compound and cost.
How to use this calculator
- 1Measure the area. For an irregular space, split it into rectangles, work each out, and add them.
- 2Set the unit you measured in — the calculator converts everything for you.
- 3Check the coverage figure against the bag or tin you are actually buying; it varies between brands.
- 4Leave the waste allowance on. Running short mid-job costs far more than one spare unit.
How the calculation works
Units = ⌈ (Area × (1 + waste)) / Area of one piece ⌉- Area
- Surface area of the job
- waste
- Allowance for cuts, breakage and spillage
- ⌈ ⌉
- Round up — you cannot buy a fraction of a unit
Every dimension is converted to metres internally, so feet, yards and metres can be mixed with inches or centimetres without error.
The default piece size is 48 × 96 in — the standard 4 × 8 ft sheet. A 4 × 12 ft sheet is 48 × 144 in, and metric plasterboard is commonly 1200 × 2400 mm.
Longer sheets cover the same area with fewer joints to tape and finish, which is why 12 ft sheets are preferred on long walls despite being harder to handle.
Worked example
A 20 × 12 ft area
- 1.20 ft × 12 ft = 240 sq ft, which is 22.30 m².
- 2.Divided by the coverage per sheet, that gives the raw requirement.
- 3.A 10% waste allowance is added, then the figure is rounded up to whole sheets.
Result: See the result panel — it updates live as you change the numbers.
How drywall is sized and sold
Drywall — also sold as sheetrock, plasterboard or gypsum board — is a gypsum core sandwiched between paper facings, produced in standard sheet sizes rather than cut to order. In North America the near-universal width is 4 ft, with lengths of 8, 10 or 12 ft; metric markets typically use 1200 mm wide boards in 2400 or 3000 mm lengths. Because the sheets come in fixed sizes, estimating a job is fundamentally a packing problem: total area divided by sheet area, rounded up, with an allowance for the offcuts that awkward walls inevitably produce.
Thickness, and why it varies by location
Sheet thickness is chosen for the structural and fire situation, not for preference.
- 1/4 in (6 mm) — a thin overlay board, used to skim over an existing damaged surface or to bend around a curve. Not used as a primary wall board.
- 3/8 in (9.5 mm) — mainly a repair and remodelling thickness, matching older construction.
- 1/2 in (12.7 mm) — the standard for interior walls and, on closely spaced joists, ceilings. The default assumption for most residential work.
- 5/8 in (15.9 mm) — used for ceilings (it sags less between joists), for fire-rated assemblies, and where better sound isolation is wanted. Often labelled "Type X" when fire-rated.
What else the order needs beyond sheets
Sheet count is the easy part of a drywall order; the finishing materials are what people forget. Every sheet needs fasteners — roughly 32 screws for a standard 4 × 8 sheet on 16 in centres. Every joint between sheets needs paper or mesh tape and at least three coats of joint compound, with each coat wider than the last. External corners need metal or plastic corner bead. As a rough planning figure, budget around 400 linear feet of tape and a few gallons of compound per 1,000 square feet of board, then adjust for how many joints your particular layout creates.
Reducing waste and joints
A little planning at the ordering stage cuts both cost and finishing work.
- 1Hang sheets horizontally on walls — in most residential work this produces fewer linear feet of joint than vertical hanging and puts the seam at a comfortable working height.
- 2Choose the longest sheet you can handle — fewer butt joints means less taping and a flatter finished wall, since butt joints are not tapered and are the hardest to hide.
- 3Stagger the joints — offsetting seams between rows, rather than letting them line up, avoids a continuous weak line across the wall.
- 4Keep large offcuts — pieces cut around windows and doors are usually big enough to fill in above and below openings, which is where much of the theoretical waste is actually recovered.
What this assumes, and where it stops
Assumptions
- The area is flat and regular as described. Slopes, steps and obstructions change the quantity.
- Coverage figures are the manufacturer’s stated values, which assume good conditions and an even surface.
- The area entered is the total surface to be boarded — walls, ceiling, or both added together.
Limitations
- This estimates material only — not labour, tools, delivery, hire or disposal.
- Manufacturer coverage varies between brands and batches. Always check the packaging.
- Does not subtract door and window openings. On a normal room that acts as a useful waste margin; on a wall that is mostly glazing, measure the boarded area directly.
- Screws, joint tape, corner bead and joint compound are estimated as guidance in the notes rather than ordered precisely — they vary with sheet layout and finish level.
Common questions
How many drywall sheets do I need for a 12 × 12 ft room?
With 8 ft ceilings the four walls come to about 384 sq ft, which is 12 standard 4 × 8 sheets before waste, or about 13–14 with a 10% allowance. Add the ceiling — another 144 sq ft, or 4.5 sheets — if you are boarding that too.
Should I use 4 × 8 or 4 × 12 sheets?
Longer sheets mean fewer butt joints, and butt joints are the hardest kind to finish invisibly because they are not tapered. On walls longer than 8 ft, 12 ft sheets usually give a better result — the trade-off is that they are heavy and awkward for one person to handle.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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