Measure your room once and get the exact amount of paint needed — walls, ceiling, doors and windows subtracted, with two-coat coverage and cost estimate.
Measure length × width × height, subtract doors (1.9 m² each) and windows (1.4 m² each). This calculator does all of it and adds 10% for wastage.
About 10 m² per coat for standard wall paint. Porous or dark walls may need more — the 10% wastage buffer covers this.
Two coats give even color and durability, especially over a different color. One coat is fine for same-color touch-ups.
1 US gallon = 3.785 liters. The calculator shows both so you can buy the right can size.
Yes — and this calculator does it for you. A standard door removes about 1.9 m² and a window about 1.8 m²; skipping this step overbuys paint by roughly 5–10% in a typical room.
About 10% beyond the calculated amount. It covers roller waste, cutting-in, and leaves paint from the same batch for future touch-ups — a new can mixed months later can differ very slightly in shade.
No. Primer seals porous surfaces and gives topcoats something to grip; it does not replace color coats. On new plaster or dark-to-light changes, one primer coat plus two topcoats is the professional standard.
Paint math has three steps. 1. Wall area: for a rectangular room, 2 × (length + width) × height. 2. Subtract openings: doors (~0.9 × 2.1 m) and windows — you do not paint them. 3. Divide by coverage: paint needed = paintable area × coats ÷ coverage per liter (most wall paints cover ~10 m² per liter per coat; check your can). Ceilings and trim are separate surfaces with their own area — a ceiling is simply length × width.
Wall area: 2 × (4 + 5) × 2.7 = 48.6 m². Subtract one door (0.9 × 2.1 = 1.89 m²) and one window (1.5 × 1.2 = 1.8 m²): 48.6 − 3.69 ≈ 45 m² of paintable wall. Two coats at 10 m² per liter: 45 × 2 ÷ 10 = 9 liters — so you buy a 10-liter can, leaving a comfortable margin for touch-ups.
Buy about 10% more than calculated — running out mid-wall risks a visible shade mismatch between batches. Prime bare plaster or dramatic color changes (it is cheaper than extra topcoats), and remember two thin coats always beat one thick coat for coverage and durability.
Coverage figures assume smooth walls with eggshell/matte finish. Textured or rough surfaces (popcorn, heavy stucco) can drink 20–30% more paint — multiply your result by 1.25. Gloss and semi-gloss cover slightly less per liter than flat paint but need fewer coats over primed surfaces. Dark-to-light color changes almost always need a third coat (or a tinted primer) regardless of what the can promises; paint companies test coverage over white. And ceilings: flat white ceiling paint is formulated for minimal splatter and maximum hide — do not substitute wall paint and expect the same one-coat result the label advertises.