How many plasterboard sheets do I need?

Divide the area by the board area and add for cuts. 46 m² of wall at 2400 × 1200 boards is 46 ÷ 2.88 = 16 boards before waste, so order 18. Buy the whole job at once — board thickness and edge profile vary between batches and suppliers.

2400 × 1200 is the standard for walls and gives the fewest joints, but it is heavy and awkward for one person and will not go up a tight stairwell. 1800 × 900 is the usual compromise for domestic work and refurbishments where access is the limiting factor.

Deduct the large openings and leave the small ones in. The cuts around a small window waste nearly as much board as the opening saves, so leaving them in acts as a sensible waste allowance rather than an error.

It depends on whether you are taping joints only or skimming the whole surface, and the coverage is printed on the tub. As a rough guide a 17 kg tub of ready-mixed goes a long way on taped joints and nowhere near as far on a full skim.