Free GIB and plasterboard calculator. 1.2 × 2.4m sheets for walls and ceilings plus the screws to hang them, with 10% wastage, or 15% in a wet area.
This calculator counts plasterboard sheets for a room's walls and, if you want it, the ceiling: plus the screws to hang them. Sheets are counted at 1.2m by 2.4m, which is 2.88 square metres each.
A 4 x 3 m room with 2.4 m walls has 33.6 square metres of wall. With CostMate's allowance for offcuts, the calculator shows 37.0 square metres of board, which is 13 sheets of 2.4 x 1.2 m.
The ceiling is only included if you tick it, and doors and windows are not taken off the wall area.
Wall area is the room perimeter multiplied by the wall height. Tick the ceiling option and the floor area is added to it.
Wastage is CostMate's allowance, 10% normally and 15% in a wet area, because a bathroom is a small room full of cuts (around a vanity, a shower tray, a window reveal) and the offcuts from a 2.4m sheet in a room that size mostly do not get used again.
The wet-area switch also changes what the page tells you to order: GIB Aqualine 13mm instead of GIB Standard 10mm.
Framing centres and screw spacing are your inputs, not CostMate's assertion. Fixing patterns differ between a screw-fixed system and an adhesive-and-screw one, and differ again where a sheet is doing bracing work.
The geometry is simple once you supply those two: one line of screws down every stud or batten, and one screw every screw-spacing interval along that line.
The screw count runs off the wastage-inclusive area rather than the net one, because screws fix the sheets you actually hang, not the theoretical area they cover.
Bracing elements, fixing patterns and wet-area linings are specification decisions. Check them against your plans and the board manufacturer's system before you order or fix.