Gallons per room, with coats, doors and windows accounted for — at 350 sq ft per gallon.
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All free calculatorsWall area = room perimeter × ceiling height, minus ~21 sq ft per door and ~15 sq ft per window. One gallon covers about 350 sq ft per coat on smooth, primed walls. Plan on two coats for color changes, and buy a quart extra for touch-ups — paint from the same batch always matches best.