Net area, not gross area — but don't over-deduct
Start from wall or floor area, then subtract openings. The trap is deducting too eagerly: a single small window is not worth taking out of a paint estimate, because the cutting-in around it wastes more than the glass saves. A sensible rule is to deduct any opening larger than about 1 m² and ignore the rest. For a room, wall area is perimeter × height, so a 4 × 3 m room at 2.4 m high has 33.6 m² of wall before deductions — and that is what a tin of paint has to cover, twice, because coverage figures are always per coat.
Coverage, coats and the second-coat lie
Paint coverage is typically 10–14 m² per litre per coat on a smooth, sealed, previously painted surface, and it collapses on anything porous — bare plaster, new drywall or a strong colour change can halve it. Always plan for two coats, and add a primer or mist coat on new plaster; the first coat on bare substrate soaks in and covers a fraction of the stated area. Dark colours over light ones, and reds and yellows in particular, often need three. Buy tins, not litres: a job that needs 6.4 litres means three 2.5 litre tins, and the leftover is what you use for touch-ups two years later.
Waste factors by material and pattern
Waste is not a single number — it is a function of how the material is laid. A straight-lay tile or plank floor in a square room wastes 5–10%. A diagonal or herringbone lay wastes 15–20%, because every board meets the wall at an angle and every off-cut is a triangle. A patterned wallpaper wastes according to its repeat: a 64 cm repeat on a 2.4 m drop means each drop is cut to a multiple of the repeat, and you can lose most of a roll to matching alone. Carpet, sold from a roll of fixed width, is worse still — the seam plan, not the floor area, decides how many linear metres you buy.
Drywall, plaster and the things you buy by the sheet
Divide the wall and ceiling area by the sheet area — a 1200 × 2400 mm board is 2.88 m², a 4 × 8 ft sheet is 32 ft² — and round up per surface, not for the job as a whole, because you cannot use half a sheet on two different walls. Then budget the consumables people forget: roughly 10 screws per square metre, one roll of tape per two or three boards, and about 0.5 kg of joint compound per square metre for a three-coat finish. Skim plaster covers roughly 10 m² per 25 kg bag at 2 mm; sand-and-cement render at 12 mm covers barely 1 m² per 25 kg. Insulation and BTU sizing then close the loop: get the R-value and the heat load right and the room is comfortable, not merely finished.
