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Litres needed = (Surface area m²) ÷ (Coverage rate m²/L) × Coats × Wastage factor
Surface area (room) = (2 × (Length + Width) × Height) − Door and window areas
Wastage factor = 1 + (Wastage % ÷ 100) — typically 1.10 to 1.15 (10–15%)
Worked example: Room 4m × 3m, 2.4m ceiling height. Wall area = 2 × (4+3) × 2.4 = 33.6 m². Subtract 2 doors (1.8 m² each) and 1 window (1.2 m²): net area = 33.6 − 3.6 − 1.2 = 28.8 m². Paint coverage 12 m²/L, 2 coats, 10% wastage: 28.8 ÷ 12 × 2 × 1.10 = 5.28 litres → buy 2× 2.5L tins (5L) or 1× 5L tin.