Calculate exactly how many litres of paint you need for any room or surface. Enter room dimensions and number of coats — includes doors, windows, and wastage allowance.
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.
Calculate the total wall area (perimeter × height), subtract doors and windows, then divide by the paint's coverage rate (typically 10–12 m² per litre). Multiply by number of coats.
Coverage varies by product and surface texture, but most standard emulsion paints cover 10–12 m² per litre per coat on a smooth wall. Textured or porous surfaces may reduce this to 6–8 m².
A strong colour change usually needs 2 coats. White or light colours over white may need only 1. Dark colours or new plaster often require a primer plus 2 topcoats.
Yes, if you're painting the ceiling a different colour. Many calculators treat walls and ceiling separately. Ceiling paint typically has lower sheen and slightly different coverage.
Common sizes are 1L, 2.5L, 5L, and 10L. It's often more economical to buy one large tin than multiple small ones. Leftover paint can be resealed for touch-ups.