Drywall Calculator

Quadruple the floor area and you only double the plasterboard

Wall area depends on the perimeter, and perimeter grows with the square root of area. A 4 by 4 metre room at 2.5 m high has 40 m² of wall; an 8 by 8 room has four times the floor but only 80 m² of wall. Boarding a big room is cheaper per square metre of floor than boarding a small one, which is the opposite of most people's intuition.

How it works

• Computes wall area as the perimeter times the height, and adds the ceiling if you want it.
• Divides by the area of the sheet size you have chosen.
• Rounds up to whole sheets, since you cannot buy part of one.

wall area = 2 × (length + width) × height
total = wall area + (ceiling ? length × width : 0)
sheets = ⌈total ÷ (sheet width × sheet height)⌉

Worked example

Two square rooms, one four times the floor area of the other.

1. 4 × 4 m at 2.5 m high: 2 × (4 + 4) × 2.5 = 40 m² of wall
2. 8 × 8 m at 2.5 m high: 2 × (8 + 8) × 2.5 = 80 m² of wall
3. the floor area went from 16 m² to 64 m², a factor of four
4. the wall area only doubled, because the perimeter only doubled
5. adding the ceiling changes this: ceilings do scale with floor area

Forty square metres against eighty. Wall and ceiling behave completely differently as a room grows, which is worth knowing before estimating a whole house from one room.

Reading the result

• The calculation does not subtract doors and windows. That is deliberate for boarding, where offcuts around an opening are rarely reusable, but it does mean a room that is mostly glazing will be over-estimated — subtract the big openings by hand.
• Sheet size matters more than it looks. Fewer, larger boards mean fewer joints to tape and fill, which is the slow part of the job; smaller boards are easier to carry and fit through a loft hatch. The cheapest order is not always the fastest wall.
• Ceilings need more fixings and more hands than walls of the same area, and the board is often thicker. Treat the ceiling figure as a different job rather than more of the same one.
• Round up generously on anything with cuts. The arithmetic gives the theoretical minimum, and a room with reveals, a stairwell or an awkward corner will eat more than the number suggests.

Common questions

How many sheets of plasterboard do I need for a 5 × 4 m room?

18 sheets of 2400 × 1200 mm board for the walls, assuming a 2.5 m ceiling and a 10% waste allowance. The walls come to 2 × (5 + 4) × 2.5 = 45 m², which is 49.5 m² with waste, and each sheet covers 2.88 m². Board the ceiling as well and the total rises to 25 sheets.

How many screws, how much tape and how much joint compound per sheet of drywall?

The calculator uses common trade rules of thumb: about 36 screws per sheet, 1.3 m of joint tape and 0.9 kg of compound per square metre. For the walls of a 5 × 4 m room with 2.5 m ceilings that works out at 648 screws, 65 m of tape and 45 kg of compound. Rooms with many corners and cut edges use more tape and filler than this.

Should I buy 2.4 m or 2.5 m plasterboard for a 2.5 m ceiling?

Choose sheets at least as long as the wall is high if you can handle them. With 2500 × 1200 mm boards a 2.5 m wall is covered floor to ceiling in one piece, so there is no horizontal joint to tape and fill; 2.4 m sheets leave a 100 mm strip to piece in along every wall. For a 5 × 4 m room the count also drops from 18 sheets to 17.