One gallon of paint typically covers 350 square feet. However, a gallon of primer usually only covers about 200 square feet. The exact amount needed depends on brand, paint type, the surface and the application method.
To calculate the amount of paint needed to cover a certain area, first determine the area of the surface to be painted. Measure its length and height, and multiply those figures to get the square footage. Since a gallon of paint covers 350 square feet, divide the total square footage by 350. Round up to the nearest gallon. For example, if the walls of the room are 60 feet long and 10 feet high, the area is 600 square feet. Divide 600 by 350, and the answer is about 1.7 gallons, so about 2 gallons are necessary. Calculate with greater accuracy by subtracting the area of windows and doors.