Paint Quantity from Coverage Rate

Coverage, not volume, is what the label tells you and what you should calculate with. A US gallon of wall paint traditionally covers about 400 ft², a 3.78 L can about 37 m², so the sum is area times coats divided by coverage. Two coats is the realistic default for anything but a repaint in the same colour, and going from a dark colour to a light one can mean three.

Treat the label figure as optimistic. It assumes a smooth, sealed, previously painted surface. Bare drywall drinks the first coat, textured or stucco walls can cut coverage by a third, and rough masonry by half. Spraying wastes more than rolling, especially outdoors. If in doubt, work out the area first and then buy to the next whole container, because a second trip for a half litre of a tinted colour risks a batch that does not quite match.

Measuring the area is where the real errors hide. Take wall length times ceiling height for each wall, and subtract only the doors and large windows. Trying to deduct every small opening is false economy, and the leftover is what you keep for touch-ups.