Drywall Calculator

A 12 × 12 ft room with an 8 ft ceiling has 384 ft² of wall area (perimeter 48 ft × height 8 ft) and 144 ft² of ceiling area = 528 ft² (≈ 49 m²) of drywall. At 32 ft² per 4 × 8 sheet, that is 16.5 sheets before waste, or about 19–20 sheets with the typical 15 % waste allowance. Two 4 × 12 sheets cover the whole ceiling with one centre joint, so many pros switch to longer sheets for ceilings to cut down on taping — the calculator lets you compare by changing the sheet-size option.

Three things: cuts you cannot reuse (the offcut from a sheet trimmed around a window is usually too small to fit anywhere else on the job), breakage during handling (gypsum board is brittle along the cut edge, and stair-corners are where most damage happens), and miscuts. 15 % is the figure most US and UK drywall merchants build into their order calculators for a rectangular room with one door and a typical window. Drop it to 10 % only if the room is a clean rectangle with no openings and you have done drywall before. Bump it to 20 % if you have multiple windows, an alcove, sloped ceilings, or are doing your first job — running out of board mid-install means a second delivery charge that costs more than the 5 % of extra board.

For walls only (ceiling is already drywalled), divide the calculator's "total drywall area" output by your sheet area, multiply by (1 + waste %) and round up — or simply enter a very small ceiling height for the calc, no, easier: take the wall area figure (2 × (length + width) × height) and run those numbers through the math yourself. For ceiling only, the ceiling area is length × width — run that number through the sheet count yourself, or set the height very low (e.g. 0.1 m) and the ceiling dominates the total. A future update will add an "include ceiling" toggle to make this one-click.

For an estimate, no — the 15 % waste allowance roughly cancels out the area of one standard door (≈ 2 m²) and one window (≈ 1.5 m²) in a typical bedroom-sized room, because the offcuts around openings are mostly unusable anyway. For a precise material order on a larger job (room with multiple windows, double doors, full-height glazing), measure each opening, subtract its area from the calculator's total-area output, then divide by sheet area and apply waste. Note the perimeter framing around each opening still needs board on the surrounding wall, so the deduction is just the opening itself, not a buffer around it.

The calculator counts sheets and area, not thickness — pick the thickness at the merchant. For walls in a UK or US residential room, 12.5 mm (½ inch) is the standard. Ceilings use 12.5 mm at 16" o.c. joists or 15 mm (⅝ inch) at 24" o.c. to prevent sag. Bathrooms and kitchens use moisture-resistant board (green or blue face paper). Garages and stairwells in attached US dwellings need Type X fire-rated 15.9 mm (⅝") for the IRC R302.6 fire separation. Price per sheet varies by thickness and type — moisture-resistant and fire-rated cost roughly 30–50 % more than standard board.

For tape plus two finishing coats on installed board, USG specifies ~16.5 lb of all-purpose joint compound per 100 ft² of drywall. A standard 4.5-gallon (US) or 18.9-litre bucket of premix weighs about 51 lb, covering roughly 30 m² (320 ft²) — the figure used by this calculator. Paper joint tape coverage is ~370 linear ft per 1000 ft² (USG TDS), so a 250 ft roll covers about 670 ft² of drywall. The calculator does not output a tape figure because most home improvement stores sell paper tape in 250 ft or 500 ft rolls and one roll per small-room job is typically enough; for large jobs, plan on roughly 1.2 linear metres of tape per square metre of drywall.