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.