What the drywall calculator does
Drywall (called plasterboard in the UK and Australia, and also known as gypsum board) is sold in full sheets. The calculator works out the area of the walls and optional ceiling, removes door and window openings, adds a waste allowance and divides by the area of one sheet, rounding up.
It covers the common US sheet sizes (4 × 8, 4 × 10 and 4 × 12 ft) and common metric sizes (1200 × 2400, 2700 and 3000 mm). Board thickness does not affect the sheet count, so it is not an input.
How to use it
- Enter the room perimeter (the sum of the wall lengths), or one wall length and the number of walls.
- Enter the wall height.
- Enter the number and size of doors and windows.
- Tick “Include the ceiling” and enter its length and width if you are boarding it too.
- Choose the sheet size and waste allowance. Optionally add the price per sheet.
Drywall formulas
Wall area = Perimeter × Wall height
Net wall area = Wall area − Doors − Windows
Total area = Net wall area + Ceiling area
Sheets = Total area × (1 + Waste % ÷ 100) ÷ Sheet area, rounded up
A 4 × 8 ft sheet covers 32 sq ft; 4 × 12 ft covers 48 sq ft; 1200 × 2400 mm covers 2.88 m².
Worked example
A 12 × 12 ft room with 8 ft walls, one door (3 × 7 ft), one window (3 × 4 ft) and the ceiling:
- Wall area: 48 ft × 8 ft = 384 sq ft
- Openings: 21 + 12 = 33 sq ft → net wall area 351 sq ft
- Ceiling: 12 × 12 = 144 sq ft → total 495 sq ft
- 4 × 8 sheets: 495 ÷ 32 = 15.5 → 16 sheets without waste
- With 10% waste: 544.5 ÷ 32 = 17.02 → 18 sheets
Using 4 × 12 ft sheets instead: 544.5 ÷ 48 = 11.3 → 12 sheets, with fewer joints to tape.
Metric: a room with a 14.4 m perimeter and 2.4 m walls, one 0.82 × 2.04 m door, one 1.2 × 1.2 m window and a 3.6 × 3.6 m ceiling totals 44.4 m². With 10% waste and 1200 × 2400 mm sheets: 48.9 ÷ 2.88 = 16.96 → 17 sheets.
Understanding the results
- Sheets needed – including your waste allowance, rounded up to whole sheets.
- Total area – net wall area plus ceiling.
- Without waste – the minimum number of sheets that could cover the area.
- Estimated offcuts – the board area you are buying beyond the measured area.
Tips for planning sheet layout
- Longer sheets mean fewer joints, but check you can carry them into the room and that they suit your wall height.
- Small window openings are often boarded over and cut out, so subtracting them may underestimate slightly.
- Ceilings are usually boarded before walls.
- Bathrooms, garages and shared walls may require moisture-resistant or fire-rated board; check local requirements.
What can change the sheet count
- Room shape – alcoves, bulkheads, sloped ceilings and stairwells add cuts and waste.
- Framing layout – sheet edges must land on studs or joists, which affects how efficiently sheets are used.
- Wall height – walls slightly taller than a sheet width or length can leave large offcuts.
Once the board is up, work out your paint with the paint calculator, which uses the same wall measurements. To total several rooms, the square footage calculator can help.