How many cornice lengths do I need?

Count each wall separately, because every corner is mitred and a length can't turn a corner. A wall shorter than one length takes one length; a longer wall is joined. Offcuts from long walls can cover short ones. Dividing the perimeter by the length only gives the minimum.

How much cornice cement do I need?

About 12 kg of setting-type cornice cement per 100 metres of standard cove cornice, which is 0.12 kg per metre.

Should plasterboard go on horizontally or vertically?

Horizontally on walls, with the paper-bound edges across the studs. It cuts the joint length and the manufacturers' jointing figures are for horizontal sheeting. Ceiling sheets go at right angles to the joists.

Do doors and windows reduce the sheet count?

Usually not by much. Most fixers sheet straight over openings and cut them out, keeping joints at least 200 mm from the corners of openings, so the calculator counts full walls.

Free calculator

Plasterboard and cornice calculator

Lay each wall in horizontal rows of 1200 mm sheets using the stocked lengths that waste least. Cornice is counted wall by wall because every corner is mitred, at about 12 kg of cornice cement per 100 m.

Your job

Rooms

Length

Width

Height

wallsceilingcornice

Doors and windows are sheeted over and cut out, as most fixers do, so they don't reduce the sheet count.

Sheets

Sheet width

Lengths you can get

Walls are sheeted horizontally. Each row takes the mix with the least board, keeping butt joints at least 600 mm apart row to row.

Ceiling sheets run

Ceiling sheets go at right angles to the joists or battens.

Spare sheets

Cornice

Cove size

Length you're buying

Every corner is mitred, so each wall is its own run. Offcuts from long walls are used on short ones. Cornice cement at 12 kg per 100 m.

Fixing

Set this to your studs and joists. Walls go up to 600 mm; ceilings in 10 mm wall board need 450 mm.

Optional

Your price per sheet

Ask your plasterboard supplier. The greyed number is an example only and is never used in your result.

The room unfolded: striped sheets are cut. Below, each cornice length and what's cut from it.

Sheets to buy

11sheets

7 × 4.2 m + 4 × 3.6 m

Board area

50.9m²

Walls 36.5, ceilings 14.4

Cornice

4lengths

4.8 m lengths for 15.20 m, 4 corners mitred

Cornice cement

1.8kg

1 × 10 kg

Screws

316

With stud adhesive, 600 mm frames

Stud adhesive

1.8kg

Jointing compound

12.2kg

8.1 base + 4.1 finish

Paper tape

38m

The working, with your numbers

1. Sheetslaid wall by wall, 1200 mm rows= 11
2. Board areawalls 36.48 + ceilings 14.40= 50.88 m²
3. Cornice runs4 pieces packed into 4.8 m lengths (perimeter alone says 4)= 4 lengths
4. Cornice cement15.20 m × 0.12= 1.82 kg
5. Screws50.88 ÷ 100 × 620, rounded up= 316
6. Jointing50.88 ÷ 100 × (16 base + 8 finish)= 12.2 kg
7. Tape50.88 ÷ 100 × 75= 38.2 m

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The formula, in plain words

1Walls are sheeted horizontally: rows = wall height ÷ sheet width, rounded up, with the top or bottom row ripped to fit.
2Each row uses the stocked lengths that buy the least board, keeping butt joints at least 600 mm apart row to row. Ceiling sheets run at right angles to the joists.
3Cornice: every corner is mitred, so each wall is its own run. Runs longer than a length are joined; offcuts go to shorter walls.
4Cornice cement = cornice metres × 0.12 kg.
5Screws, stud adhesive, jointing compound and tape = board area ÷ 100 × the manufacturer's rate per 100 m².

Worked example

One room 4 × 3.6 m with a 2.4 m ceiling: walls, ceiling and 90 mm cove cornice in 4.8 m lengths. 1200 mm sheets, adhesive and screws at 600 mm.

1.4 m walls: two rows of one 4.2 m sheet each. 3.6 m walls: two rows of 3.6 m. Ceiling: three rows of 4.2 m.
2.Sheets: 7 × 4.2 m + 4 × 3.6 m = 11 sheets for 50.88 m² of board.
3.Cornice: runs of 4, 3.6, 4 and 3.6 m, each from its own 4.8 m length = 4 lengths, 4 corners mitred.
4.Cornice cement: 15.2 m × 0.12 = 1.8 kg. Screws: 50.88 ÷ 100 × 620 = 316.

Answer: 11 sheets (7 × 4.2 m + 4 × 3.6 m) and 4 lengths of cornice.

How to measure it

Measure each room's length, width and ceiling height.
Tick walls, ceiling and cornice for each room you're doing.
Check which sheet lengths your supplier stocks and untick the rest.
Note which way the ceiling joists or battens run: ceiling sheets go across them.

Where the numbers come from

Every default in the calculator is a number a supplier or manufacturer publishes. We list the source next to it so you can check.

What	Value	Source
Standard sheet width is 1200 mm; 1350 mm is also stocked	1,200 mm sheet width	Gyprock Red Book 2, Residential Installation Guide (CSR, 2020) (checked 2026-09-30)
Stocked sheet lengths: 2400, 2700, 3000, 3600, 4200, 4800 and 6000 mm (varies by state)	6,000 mm longest sheet	Gyprock Plus plasterboard sizes per state (updated October 2025) (checked 2026-09-30)
Maximum framing centres for walls (ceilings in 10 mm wall board: 450 mm)	600 mm max framing centres	Gyprock Red Book 2, Residential Installation Guide (CSR, 2020) (checked 2026-09-30)
Adhesive and screws at 600 mm centres: screws (per 100 m²)	620 screws	Gyprock Red Book 2, Residential Installation Guide (CSR, 2020) (checked 2026-09-30)
Adhesive and screws at 600 mm centres: stud adhesive (per 100 m²)	3.5 kg	Gyprock Red Book 2, Residential Installation Guide (CSR, 2020) (checked 2026-09-30)
Adhesive and screws at 450 mm centres: screws (per 100 m²)	750 screws	Gyprock Red Book 2, Residential Installation Guide (CSR, 2020) (checked 2026-09-30)
Adhesive and screws at 450 mm centres: stud adhesive (per 100 m²)	4.2 kg	Gyprock Red Book 2, Residential Installation Guide (CSR, 2020) (checked 2026-09-30)
Screws only, horizontal sheets, 600 mm centres (per 100 m²)	820 screws	Gyprock Red Book 2, Residential Installation Guide (CSR, 2020) (checked 2026-09-30)
Screws only, horizontal sheets, 450 mm centres (per 100 m²)	1,000 screws	Gyprock Red Book 2, Residential Installation Guide (CSR, 2020) (checked 2026-09-30)
Base coat (tape and second coat), horizontal sheeting	16 kg per 100 m²	Gyprock Red Book 2, Residential Installation Guide (CSR, 2020) (checked 2026-09-30)
Finish coat (a common topping compound), horizontal sheeting	8 kg per 100 m²	Gyprock Red Book 2, Residential Installation Guide (CSR, 2020) (checked 2026-09-30)
Paper joint tape	75 m per 100 m²	Knauf Plasterboard Installation Manual (AU), Table 10 (checked 2026-09-30)
Cove cornice (55, 75, 90 mm) in 3000, 3600, 4800 and 5400 mm lengths, subject to availability	4,800 mm cornice length	Gyprock Cove Cornice product page and data sheet (checked 2026-09-30)
Cornice cement: about 12 kg per 100 metres of standard cornice	0.12 kg per metre	Gyprock Red Book 2, Residential Installation Guide (CSR, 2020) (checked 2026-09-30)

Manufacturers named here are the source of the number, not a recommendation. Confirm quantities with your supplier.

Frequently asked questions

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